A DELTA ONCE MORE: RESTORING RIPARIAN AND WETLAND HABITAT IN THE COLORADO RIVER DELTA

Transcription

1 A DELTA ONCE MORE: RESTORING RIPARIAN AND WETLAND HABITAT IN THE COLORADO RIVER DELTA Daiel F. Luecke Jeifer Pitt ENVIRONMENTAL DEFENSE FUND Boulder, Colorado Chelsea Cogdo Boulder, Colorado Edward Gle, Ph.D. Evirometal Research Laboratory, Uiversity of Arizoa Tucso, Arizoa Carlos Valdés-Casillas, Ph.D. Istituto Tecológico y de Estudios Superiores de Moterrey Guaymas, Mexico Mark Briggs Soora Istitute Tucso, Arizoa Jue 1999

2 ISBN Evirometal Defese Fud The Evirometal Defese Fud is a leadig, atioal, New York-based, private, o-profit, research ad advocacy orgaizatio with more tha 300,000 members atiowide. EDF s staff icludes scietists, ecoomists, egieers, ad attoreys who seek practical solutios to a broad rage of evirometal ad huma health problems. Copies of this report may be obtaied from: EDF Publicatios 1875 Coecticut Aveue, NW Washigto, DC 20009

3 ACKNOWLEDGMENTS EDF gratefully ackowledges the cotributors who have geerously supported its work i the Colorado River Delta regio, icludig the Compto Foudatio, the Geeral Service Foudatio, the Charles Stewart Mott Foudatio, the Catto Foudatio, ad the Evirometal Protectio Agecy. Thaks to Elea Chavarria for her itelliget traslatio of the executive summary. I additio, credit is due to Osvel Hiojosa, Yamilett Carrillo, Jaquelie Garcia ad Fracisco Zamora for their cotributios to research i the Colorado River Delta. ABOUT THE AUTHORS: Daiel F. Luecke is a evirometal scietist ad regioal director with the Evirometal Defese Fud i Boulder, Colorado. Over the past two decades he has bee ivolved i a umber of riverie ad aquatic habitat protectio projects. Edward Gle is a professor i the Soil, Water, ad Evirometal Sciece Departmet at the Uiversity of Arizoa. He has studied the vegetatio ad water quality of the delta i collaboratio with Mexica scietists over the past decade. Jeifer Pitt is a seior resource aalyst with the Evirometal Defese Fud i Boulder, Colorado. From 1994 to 1998 she worked o river restoratio for the Natioal Park Service ad America Heritage Rivers. Chelsea Cogdo was a seior resource aalyst with the Evirometal Defese Fud for 11 years. From 1995 to 1998 she worked i EDF s Rocky Moutai office, leadig the orgaizatio s efforts i the Colorado River delta. She cotiues to work o delta restoratio as a cosultat ad documetary film producer. Carlos Valdés-Casillas is the director of the Ceter for Coservatio of Natural Resources (CECARENA), ad head of research of the Istituto Tecológico y de Estudios Superiores de Moterrey (ITESM), Campus Guaymas. He has published several reports o assessmet ad plaig of coastal areas i Mexico, i particular o the Sea of Cortez. Mark Briggs is the director of research at the Soora Istitute, Tucso, Arizoa. His area of focus is o evaluatig the health of riparia ecosystems ad developig strategies for improvig their overall ecologic coditio. iii

4 EXECUTIVE SUMMARY Historically, the Colorado River fed oe of the greatest desert estuaries i the world. The Colorado s delta cosisted of vast riparia, freshwater, brackish, ad tidal wetlads that covered 1,930,000 acres (780,000 ha) ad supported a legedary richess of plat, bird, ad marie life. Today, coditios are chaged. Decades of dam costructio ad water diversios i the Uited States ad Mexico have reduced the delta to a remat system of small wetlads ad brackish mudflats. Recetly, however, the delta has begu to make a comeback. I the last two decades, floodwater, releases from reservoirs i the Uited States, agricultural retur flows from both coutries, ad muicipal wastewater from Mexico have proved beeficial. Although flood flows are extremely ureliable ad irregular, ad wastewater is high i saliity ad pollutats, this water has begu to restore some areas of the delta. Curret coditios have allowed wetlads to flourish o about 150,000 acres (60,000 ha). The authors believe that key areas of the delta might be saved through more efficiet use of water that ow flows ito the delta. This could be accomplished without adverse effects o other Colorado basi water users. While demad for water by irrigators, cities, ad other importat costituecies makes it ulikely that a atural flow regime ca be restored to the delta, deliberate maagemet of existig water resources, such as agricultural draiage, wastewater, ad floodwater, could make a sigificat differece. Although the delta s ecosystems deserve greater cosideratio i the allocatio of Colorado River resources, the delta s miimum requiremets are surprisigly modest. This report outlies the delta s atural ad cultural history, documets recet scietific fidigs about the delta s partial recovery, reviews its curret political cotext, ad makes recommedatios for securig, assurig, ad maagig existig flows to further beefit ad sustai the delta s remat wetlad ecosystems. The authors assessed habitat values i the delta s vegetated riparia areas ad wetlads, ad foud relatively large areas of dese, woody vegetatio capable of supportig sigificat bird populatios. I additio, the authors measured the quatity ad rate of water flows eeded to sustai delta vegetatio, ad foud that aual flows of 32,000 acre-feet (4 x 10 7 m 3 ), supplemeted by periodic (oce every four years o average) flood flows of 260,000 acre-feet (3.2 x 10 8 m 3 ), should suffice. Research documetig flows eeded to support the fisheries of the delta ad the ear-shore marie eviromet of the Gulf of Califoria has ot yet bee coducted. Fially, the authors outlie recommedatios to maage existig flows for the beefit of delta ecosystems; chage iteratioal istitutios ad agreemets to support delta ecosystems; establish market mechaisms ad fudig sources for delta preservatio; icrease public participatio; coduct further research; ad implemet site-specific restoratio. iv

5 RESUMEN EJECUTIVO Por tiempos imemorables, el río Colorado alimetó uo de los mayores estuarios desérticos del mudo. El delta del Colorado cosistía e más de 781,060 ha. (1,930,000 acres) cubiertas por bastos humedales riparios de agua dulce, agua salobre y de mareas, que sustetaba ua riqueza legedaria de platas, aves y vida maria. Hoy las codicioes ha cambiado; décadas de costrucció de presas y desviació de agua e Estados Uidos y México, ha reducido el delta a u sistema remaete de pequeños humedales y lodazales salobres. Recietemete, si embargo, el delta ha iiciado su regreso. E las dos últimas décadas, la liberació de flujos de agua de las presas de Estados Uidos, las aguas residuales agrícolas de ambos países y el agua de desecho muicipal de México, ha probado ser beéficas. A pesar de que los flujos de iudació resulta ser extremadamete aleatorios e irregulares, y que las aguas residuales tiee elevada saliidad y cotamiació, esta agua ha empezado a restaurar alguas áreas del delta. Las codicioes actuales ha permitido el establecimieto de 54,725 ha. (150,000 acres) de humedales. Los autores cofía e que ciertas áreas del delta podría ser salvadas mediate u uso mas eficiete del agua que fluye ahora a través del él. Esto puede lograrse si efectos adversos sobre los otros usos del agua del cauce del Colorado. Dado que la demada de agua para irrigació, ciudades y otros usuarios de importacia, hace remota la posibilidad de que u régime de flujo atural pueda restaurarse para el delta, el maejo deliberado de los recursos hidráulicos, como drees agrícolas, aguas residuales y flujos de iudació, podría lograr ua diferecia sigificativa. A pesar de que los ecosistemas del delta merece mejores cosideracioes e la distribució de recursos del Río Colorado, los requerimietos míimos del delta so sorpredetemete modestos. Este reporte describe la historia atural y cultural del delta, documeta los hallazgos cietíficos mas recietes referetes a la recuperació parcial del delta, revisa el cotexto político actual y ofrece recomedacioes para asegurar y orietar el maejo de los flujos existetes hacia el beeficio y sustetabilidad de los ecosistemas de humedales remaetes del delta. Los autores evaluaro los valores del hábitat e la vegetació riparia del delta y ecotraro áreas relativamete extesas de desa vegetació de galería y boscosa capaz de soportar poblacioes sigificativas de aves. Del mismo modo, los autores realizaro medicioes sobre la catidad y tasa de flujos de agua requeridos para soportar la vegetació del delta y ecotraro que flujos auales de 4 x 10 7 m 3 (32,000 acres-pie), acrecetados por flujos de iudació de 3.2 x 10 8 m 3 (260,000 acres-pie) cada 4 años e promedio, será suficiete. Au o se ha llevado a cabo la ivestigacio para establecer los flujos ecesarios para las pesquerias e el delta y e el litoral del Golfo de Califoria. Fialmete, los autores elista recomedacioes para el maejo del flujo existete para beeficio de los ecosistemas del delta: cambiar istitucioes y acuerdos iteracioales para apoyar los ecosistemas del delta, establecer mecaismos de mercadotecia y recaudar recursos para la preservació del delta; icremetar la participació pública, avazar la ivestigació e implemetar restauració e sitios específicos. v

6 TABLE OF CONTENTS EXECUTIVE SUMMARY... iv RESUMEN EJECUTIVO... v 1. THE DELTA IN CONTEXT... 1 Overview... 1 The Delta of Yesterday... 2 The Colorado Basi Trasformed... 2 The Delta Today A Cotemporary Geography... 4 Recet Research Efforts... 7 Protectig the Iterests of the Delta s People... 7 Growig Iteratioal Iterest i the Delta WATER SOURCES IN THE DELTA The Law of the River Apportioed Freshwater Flows Tides ad Floods Agricultural Wastewater DELTA HABITAT AND RESTORATION POTENTIAL The Delta s Ecological Sigificace...17 The Evirometal Defese Fud s Research i the Delta Flood Flows ad Water Requiremets of Delta Vegetatio Delta Wetlads ad Riparia Vegetatio Threats to Delta Habitats Restoratio Objectives POLITICAL LANDSCAPE Values of the Colorado River Delta Preservatio Strategies Agets of Chage Iteratioal Boudary ad Water Commissio Iteratioal Authorities Natioal Agecies Tribes, Basi States, ad Local Commuities...36 Nogovermetal Orgaizatios...36 Other Orgaizatios ad Authorities Legal Mechaisms Related Efforts ad Opportuities Colorado River Etitlemets ad Califoria s 4.4 Pla...38 Salto Sea...38 Yuma Desaltig Plat...40 Lower Colorado Coservatio Plaig...40 All America Caal ad Delivery of Water to Mexico...40

7 5. RECOMMENDATIONS Coclusio Recommedatios Maage Existig Flows for the Beefit of Delta Ecosystems Chage Istitutioal Arragemets ad Agreemets to Support Delta Ecosystems Establish Market Mechaisms ad Fudig Sources for Delta Preservatio Icrease Public Participatio Coduct Further Research Implemet Site-Specific Restoratio Colorado River Delta Riparia Corridor (Zoes 2 4)...46 Colorado River Delta Riparia Corridor ad Wetlad (Zoe 4) Rio Hardy Wetlads (Zoes 4 & 5) La Ciéega de Sata Clara/El Doctor/El Idio (Zoe 7) BIBLIOGRAPHY APPENDIX A: SELECTED FLORA AND FAUNA OF THE COLORADO RIVER DELTA... A-1 APPENDIX B: MATERIALS AND METHODS... B-1 APPENDIX C: PUBLIC INVOLVEMENT... C-1 BOXES Box 1. A Related Issue: Wastewater Treatmet for Mexicali A New Source of Water for the Delta? Box 2. A Related Issue: La Cieega de Sata Clara Box 3. Vegetatio of the Colorado River Delta Box 4. A Related Issue: The Yuma Desaltig Plat Box 5. A Related Issue: The Salto Sea TABLE Table 1. Characteristics of the Colorado River Delta Flood plai i Mexico FIGURES Figure 1. The Colorado River Delta, Figure 2. The Colorado River Delta, Figure 3. Huma Settlemets Withi the Highest Ifluece Area of the Colorado River Delta Wetlads... 9 Figure 4. Water Flows at the Souther Iteratioal Boudary, Figure 5. Satellite Image of the Colorado River Delta, July 15, Figure 6. Vegetatio Zoes of the Colorado River Delta PHOTOS Photo 1. Rio Hardy... 6 Photo 2. Palmer s saltgrass i the Colorado River delta... 8 Photo 3. Aerial view of the Colorado River delta, ear where the Colorado meets the upper Gulf of Califoria Photo 4. La Ciéega de Sata Clara Photo 5. Cottowood trees ad willows i the Colorado River riparia corridor Photo 6. Dry mudflats i the Colorado River delta Photo 7. Gulls over the delta... 25

8 1 THE DELTA IN CONTEXT I have ever goe back to the Delta of the Colorado sice my brother ad I explored it, by caoe, i For all we could tell, the Delta had lai forgotte sice Herado de Alarcó laded there i O the map the Delta was bisected by the river, but i fact the river was owhere ad everywhere, for we could ot decide which of a hudred gree lagoos offered the most pleasat ad least speedy path to the Gulf. Aldo Leopold, A Sad Couty Almaac, 1948 OVERVIEW Prior to the costructio of major dams alog its route, the Colorado River fed oe of the greatest desert estuaries i the world. Spread across the orthermost ed of the Gulf of Califoria, 1 the Colorado River delta s vast riparia, freshwater, brackish, ad tidal wetlads oce covered 1,930,000 acres (780,000 ha) ad supported a legedary richess of plat, bird, ad marie life. Because most of the river s flow reached the delta at that time, its freshwater, silt, ad utriets helped create a complex system of wetlads that provided feedig ad estig grouds for birds, ad spawig habitat for fish ad crustaceas (Gle et al., 1996). The Gulf s estuary was a source of fertility reachig far from its shallows, ad the legedary richess of the etire Gulf ca be laid to the delta s productivity as well as its capacity to support marie ad bird life. I cotrast to the surroudig Soora Desert, the Colorado River delta s abudace was strikig. Today, coditios i the delta are chaged. Like other desert river deltas, such as the Nile (Staley ad Ware, 1993) ad the Idus (Sead, 1987; Leicheko ad Wescoat, 1993), the Colorado River delta has bee greatly altered by huma activity. Decades of dam costructio ad water diversios i the Uited States ad Mexico have reduced the delta to a remat system of small wetlads ad brackish mudflats. As reservoirs filled behid dams ad captured floodwaters, freshwater flows o loger reached the delta. Now that the reservoirs are full, the periodic flood flows that do reach the delta are costricted by levees. 2 I the 1970 s ad 1980 s, the popular ad scietific press referred to the delta as a dewatered or dead ecosystem (Spamer, 1990). 3 Sice 1981, however, the delta has begu to make a slow comeback. From 1980 to 1998, total water releases to the delta have amouted to a estimated 20 percet of the Colorado s total flows over the same period. While most of this is either floodwater or agricultural ad muicipal wastewater, these flows are provig beeficial. Although flood flows are extremely ureliable ad irregular, ad wastewater is high i saliity ad pollutats, this water has begu to restore some areas of the delta. Curret coditios have allowed wetlads ad riparia vegetatio to flourish o about 150,000 acres (60,000 ha). This report documets recet scietific fidigs about the delta s partial recovery ad makes recommedatios for maagig existig flows to further beefit the delta s ecosystems (see Chapter 3). The authors believe that key areas of the delta might be saved through deliberate maagemet of water that ow flows ito the delta. This could be accomplished without adverse effects o other Colorado basi water users. While demad for water by irrigators, cities, ad other importat costituecies makes it ulikely that predevelopmet flows ca be restored to the delta i the short term, the purposeful maagemet of existig water resources such as agricultural draiage, wastewater, ad floodwater could make a sigificat differece. Although the delta s ecosystems deserve greater cosideratio i the allocatio of Colorado River resources, the delta s miimum requiremets are surprisigly modest. Eve i its preset state, the delta is the most sigificat wetlad system i the America Southwest, ad a very productive estuary. Icreasig populatio throughout 1 The Gulf of Califoria is also kow as the Sea of Cortez. 2 For the purposes of this report, a flood is ay volume of water that crosses the Uited States Mexico iteratioal border, either as a result of releases from U.S. reservoirs for flood cotrol purposes (or other reasos), or directly as a result of floodig i the U.S. (e.g., floodig i the Gila basi), i excess of the U.S. delivery obligatio to Mexico, ad that is delivered at a rate that exceeds Mexico s diversio capacity ad iudates lad (either withi the levees or beyod) that is ormally dry. 3 Reports of the delta s demise iclude accouts i the popular press such as Philip Fradki s A River No More (1981), as well as scietific publicatios.

9 A DELTA ONCE MORE the regio ad growig pressures o water, lad, ad other resources will itesify the strai o the delta. At some poit, water users both orth ad south of the border may be forced to make some difficult choices about Colorado River allocatios. The authors logterm view is that the delta s ecological, social, ad ecoomic values will come to be widely recogized i future deliberatios over the allocatio of surplus waters, ad that i-stream flows may be dedicated to sustai it. THE DELTA OF YESTERDAY The Colorado River meets the Gulf of Califoria i Mexico, where the states of Baja Califoria ad Soora share a border. The delta oce covered over 3,000 square miles (7770 km 2 ), a area the size of Rhode Islad. Highly variable flood cycles o the Colorado created a dyamic ad ustable delta populated by a rich array of adaptable ad resiliet plat ad aimal species ad huma commuities that lived off this bouty. For eos, as much as 70 percet of the Colorado River s silt load was carried to the delta, 4 brigig utriets ad spreadig the delta ever wider ito the upper Gulf. The delta s richess is further icreased by the actio of tides typically 13 feet (4 m) or higher, a uusually high ebb ad flow that exteds the tidal estuary 34 miles (56 km) or more upriver (Paye et al., 1992). The iteractio of these tidal flows with freshwater from the Colorado River creates a rich breedig groud for the Gulf s marie life, amog other beefits. The delta oce supported a estimated 200 to 400 species of vascular plats (Ezcurra et al., 1988). Early explorers reported jaguars, beavers, deer, ad coyotes i additio to the legedary abudace of waterfowl, fish, ad other marie ad estuary orgaisms (Spamer, 1990; Leopold, 1948). Early explorers also ecoutered local people kow as the Cucapá, or the people of the river. The Cucapá are descedats of the Yuma-speakig Native Americas ad have ihabited the delta for early a thousad years. Spaish explorer Herado de Alarcó made the first recorded cotact with the Cucapá i 1540 ad reported seeig may thousads. The Cucapá used the delta floodplai extesively, for harvestig Palmer s saltgrass, a wild grai, ad for cultivatig cor, beas, ad squash. Other foods icluded mesquite groud ito a meal or made ito a drik deer ad wild boar, wild geese ad ducks, doves, quail, ad fish. They lived i rectagular ad roud huts, ad relied o dugouts ad rafts for river travel (Williams, 1983). THE COLORADO BASIN TRANSFORMED By the ieteeth cetury, the delta was ope for avigatio, ad steamboats traveled from Yuma, Arizoa, through the delta to the Gulf, i a active river trade. This trade ceased i 1877, whe the Souther Pacific rail lie reached the Colorado River (Sykes, 1937; Williams, 1983). By the early 1900 s, farmers i the Mexicali Valley had begu to irrigate their fields, ad the desert bloomed. Water s power to trasform the dry desert ladscape ad its power to create electricity would make Colorado River water a irresistibly valuable resource through the etire twetieth cetury. [See Figure 1.] As the West s populatio ad eed for water have grow, the Colorado River has bee tapped through a system of dams ad diversios that begi close to its source i the moutais of Colorado ad Wyomig. Over its 1400-mile (2300 km) course, from its headwaters to the ed of its mai chael at Motague Islad i the Gulf of Califoria, the Colorado is iterrupted by more tha 10 major dams. More tha 80 major diversios carry water away from the river for agriculture ad other uses. The costructio of Hoover Dam i the 1930 s marks the begiig of the moder era for the Colorado delta. For six years, as Lake Mead filled behid the dam, virtually o freshwater reached the delta. Eve sprig floodig was captured. This ecologically devastatig evet was repeated from 1963 to 1981 as Lake Powell filled behid the Gle Cayo Dam (Gle et al., 1996). With these reservoirs ow filled, the dams are used to regulate flow so that water ca be reliably apportioed amog users ad its use maximized. Most flood flows ca be cotaied, regulated, ad added to the river s capacity to sustai the West s urba ceters ad agriculture. Floodwaters are released oly whe the Bureau of Reclamatio, the agecy maagig the dams, predicts flows that exceed the system s capacity for use ad storage. 4 Betwee 45 millio ad 455 millio metric tos of silt per year were trasported through the Grad Cayo betwee 1922 ad 1935 (Mickley, 1991). 2

10 ENVIRONMENTAL DEFENSE FUND Figure 1. The Colorado River Delta, 1933 (Valdés-Casillas et al., 1998a) Imperial Dam Colorado River Cities Gulf of Califoria New River Lagua Salada Calexico Mexicali Volcao Lake levee Las Abejas River Pescaderos River Saiz Levee Yuma Yuma Levee Ockerso Levee Bacaora Caal Colorado River Delta Basi Railroad Levees Hardy River Medao Iteratioal Boudary Itermittet Streams Pereial Streams Agricultural Drais Colorado River Riíto Gore Islad Motague Islad Sata Clara Slough N W E S Kilometers Source: Sykes Map of the Colorado Delta Regio, Approximate Status of Scale 1:750,000. 3

11 A DELTA ONCE MORE Today, the Colorado River irrigates more tha 3.7 millio acres (1.5 millio ha) of farmlad i the southwester Uited States ad Mexico, ad supplies water to early 30 millio people. It is oe of the most highly regulated ad diverted rivers i North America: virtually every drop is accouted for i the allocatio of water amog 9 states (7 i the Uited States ad 2 i Mexico) ad 27 ative tribes that have rights to use it (Potius, 1997). While irrigated agriculture tops the list of Colorado River water uses i the Uited States ad Mexico, the secod largest cosumptio of water is evaporatio from reservoirs. 5 Diversios out of the Colorado basi, such as water piped to Los Ageles, are the third largest draw, ad are followed by muicipal ad idustrial uses. Federal hydroelectric plats alog the Colorado have a total geeratig capacity of about 4425 megawatts (MacDoell ad Driver, 1996). THE DELTA TODAY A CONTEMPORARY GEOGRAPHY Where the Colorado River meets the Uited States Mexico border, the Morelos Dam stads as the last major structure i the river s maistem. [See Figure 2.] For about eight miles, the border follows the river, after which the river is fully i Mexico. Water maagers refer to two referece poits alog the reach of the Colorado that forms the border: the Norther Iteratioal Boudary (NIB) at Morelos Dam, ad the Souther Iteratioal Boudary (SIB) where the river leaves the Uited States. The Colorado River delta is surrouded today by a area kow as the Mexicali ad Sa Luis Rio Colorado agricultural valleys. At preset, the delta ecompasses approximately 150,000 acres (60,000 ha), i a basi surrouded by early 500,000 acres (200,000 ha) of irrigated agriculture lad. Two rivers form its core: the Colorado, ad also the Rio Hardy, a tributary to the orthwest. I additio, water from the Wellto-Mohawk Caal empties ito the easter delta. These freshwater sources support delta vegetatio, foud i riparia areas ad wetlads. Although costraied by levees, the delta is broadly boud by the Cucapá Moutais to the west, ad the Soora Mesa to the east. Two islads, Motague ad Pelicao, mark where the delta meets the Gulf of Califoria to the south. Aother sigificat feature is the Lagua Salada, a dry depressio west of the delta ito which Colorado River waters drai whe flows are high. Although the Colorado basi drais 244,000 square miles (632,000 km 2 ), icludig 2000 square miles (5200 km 2 ) i orther Mexico, most of its water does ot reach the delta. Durig the twetieth cetury, river flows ito the delta have bee reduced early 75 percet, from a aual average betwee 1896 to 1921 of 16.7 millio acrefeet (maf) (20.7 x 10 9 m 3 ) (Fradki, 1981), to a aual average betwee 1984 ad 1999 of 4.2 maf (5.2 x 10 9 m 3 ) (Gle et al., 1999). This reductio i water has resulted i major chages to the delta: less silt, fewer utriets, higher saliity, ad higher cocetratios of pollutats. 6 Erosio rather tha accretio is ow the domiat physical process i the delta (Thompso, 1968), a highly uusual coditio for a river delta. Like other river deltas at risk, such as the Nile s, the Colorado s delta has actually begu to decrease i size (Staley ad Ware, 1993). The loss of freshwater flows to the delta over the past cetury has reduced delta wetlads to about 5 percet of their origial extet, ad oative species have compromised the ecological health of much of what remais. Stress o ecosystems also has allowed ivasive plats to choke out ative species alog Colorado River riparia areas. Native forests of cottowood ad willow have yielded to sad ad mudflats domiated by the oative tamarisk (also kow as salt cedar), arrowweed, ad iodiebush, a trasformatio that has decreased the habitat value of the riparia forest (Briggs ad Corelius, 1997). The Colorado River delta ad its estuary ecosystems may bear the worst effects of the river s heavy upstream use ad developmet, ad developmet i the delta further compromises its health. Much of the upper delta has bee coverted to irrigated farmlad, ad levees ad chaels have chaged the physical delta sigificatly. I the lower delta, where saliity makes agriculture impossible, the effects of upstream water diversio ad developmet are clearly see. 5 Allocatios made uder the laws ad compacts that make up the Law of the River (see Chapter 2) do ot accout for 1.5 maf i aual evaporative losses from maistem reservoirs (Potius, 1997). 6 The atural ecology of most of the world s large river systems has bee disrupted by dams, flow diversios, chaelizatio of the riverbed, ad alteratio of riparia zoes by agricultural activities which i tur reduce flows, silt accretio, ad utriet loads to their deltas. 4

12 ENVIRONMENTAL DEFENSE FUND Figure 2. The Colorado River Delta, 1999 (Valdés-Casillas et al., 1998a) A LAGUNA SALADA B WELLTON-MOHAWK CANAL A - Norther Iteratioal Boudary B - Souther Iteratioal Boudary 5

13 A DELTA ONCE MORE Nevertheless, despite its dimiished state, the delta plays a sigificat ecological role that goes far beyod its bouds. For migratory birds, the delta is a key stopover alog the Pacific Flyway (Paye et al., 1992), ad it supports large umbers of witerig waterfowl. Although residet ad migratory bird desities have ot bee studied extesively, the delta is cosidered a key elemet of the flyway, ad the oly sigificat freshwater wetlad amog the Mexica Pacific Coast marshes (See Appedix A for a list of birds recorded i the delta). Species uder threat elsewhere i the Colorado basi still fid refuge i the delta, which is also home to the largest kow populatios of two edagered species, the desert pupfish ad the Yuma clapper rail (Gle et al., 1996). Delta marshes still have the capacity to provide ursery habitat for marie life that, i tur, supports other marie life across the etire upper Gulf. Sigificat riparia areas ad wetlads iclude: riparia areas alog the Colorado River from the border to the delta (82,000 acres [33,000 ha]), which have recovery potetial, i part because gallery forests of cottowood ad willow have show a capacity for self-restoratio durig recet floods; the Rio Hardy/Rio Colorado wetlads, a area that fluctuates with floods ad was recetly measured at 23,719 acres (9600 ha), i the wester delta ear the cofluece of the Rio Hardy ad the Colorado, supported by flows from the Rio Hardy ad Colorado River flood flows; Photo 1. Rio Hardy 6

14 ENVIRONMENTAL DEFENSE FUND about 20,000 acres (8,000 ha) of itertidal wetlads, which ca be foud up to 34 miles (56 km) upstream from the Gulf supported by high tides; ad la Ciéega de Sata, a wetlad i the easter delta that was uitetioally created by agricultural retur flows from the U.S. ad Mexico that arrive via draiage caals, ad the adjacet El Doctor ad El Idio wetlads, both of which are supported by artesia sprigs these wetlads ecompass some 44,000 acres (18,000 ha). RECENT RESEARCH EFFORTS Durig fieldwork i 1997 ad 1998, scietists supported by the Evirometal Defese Fud (EDF), the Evirometal Research Laboratory (ERL) i Arizoa, the Istituto Tecológico y de Estudios Superiores de Moterrey (ITESM), ad the Soora Istitute documeted the extet of survivig ecosystems i the Colorado River, studied their habitat values, ad assessed their overall ecological sigificace. They also determied the quatity ad quality of water i the delta. These measuremets play a critical role i defiig what is eeded to secure the viability of these remat delta ecosystems. This report will detail these fidigs. Researchers also examied the size ad periodicity of floods i the delta. Floods are kow to have high value for riparia ad estuary ecosystems (Grimm et al., 1997). The cottowood for istace, a importat riparia species, depeds o floods for seed germiatio. Flood flows that reached the delta from 1980 to 1987 were up to te times higher tha ormal, ad average excess flows across the border from 1980 to 1993 were three times higher tha Mexico s legal etitlemet (Gle et al., 1996). 7 These flows have reestablished a active floodplai, revegetated may areas of the floodplai withi irrigatio ad flood cotrol levees, ad helped to reestablish riparia forests. May marie species deped o freshwater to keep saliity levels low i estuaries so they ca breed ad lay eggs, ad flood flows have improved delta shrimp fisheries (Galido-Bect,.d.). Flood releases to the delta ca be expected to cotiue if reservoirs alog the Colorado remai full, but a strig of dry years could elimiate the fragile ecosystems that have bee reestablished. PROTECTING THE INTERESTS OF THE DELTA S PEOPLE People livig i the delta regio cotiue to use ad deped o the delta, from fisherme i the Gulf to farmers alog its orther margis. Approximately 207,000 people live i the 1127 small settlemets withi 5 km of the delta. Most of these settlemets (92 percet) have fewer tha 100 ihabitats (Valdés-Casillas et al., 1998a). [See Figure 3.] The two largest muicipalities i the delta regio are Mexicali (pop. 696,034) ad Sa Luis Rio Colorado (pop. 133,140). 8 The ecoomic promise of these border cities has led to high rates of immigratio from other regios of Mexico. Sa Luis Rio Colorado s growth rate exceeds 3.5 percet aually (Carrillo, 1999) Nearly 2.5 millio acres (1 millio ha) aroud the delta are used for agricultural productio, relyig o water from the Colorado ad its tributaries. The irrigatio district that icludes Mexicali ad Sa Luis Rio Colorado some 440,000 acres (178,000 ha) was recetly assessed a value of early $3 billio i aual crop productio (Valdés-Casillas et al., 1998a). The delta regio attracts migrats from cetral Mexico with the ecoomic promise of proximity to the iteratioal border ad the predomiace of irrigated agriculture. Cocetrated i wheat, cotto, ad alfalfa productio, it is the regio s largest idustry, employig upward of 60 percet of the workig populatio. The delta geerates sigificat ecoomic activity i additio to irrigated agriculture. Three commuities El Golfo de Sata Clara, Sa Filipe, ad Puerto Peñasco cotiue to rely o fishig as the basis for their culture ad ecoomy. Sixtee tourist camps located ear the cofluece of the Rio Hardy ad the Rio Colorado are used by visitors from Mexicali ad the Uited States for fishig, hutig, ad other water-based recreatio, ad local residets work as guides for these visitors. May commuities i the delta rely o riparia forests for fuel wood. Oe commuity produces catfish i a aquaculture facility (Valdés-Casillas et al., 1998a). Today, approximately 600 Native Americas live i the delta regio. Of these, some 200 are Cucapá, who ow 353,000 acres (143,000 ha) alog the Rio Hardy ad Rio Colorado (Valdés-Casillas et al., 1998a). No loger able 7 Delivery requiremets to Mexico are discussed at legth i Chapter 2. 8 Populatio figures are based o 1995 data. 7

15 A DELTA ONCE MORE to practice their traditioal subsistece of harvestig Palmer s salt grass (Distichlis palmeri) which has limited reproductive capability without regular floodig to disperse seeds the Cucapá have looked to other harvests the delta supports. Several Cucapá commual agricultural settlemets, or ejidos, lie the Pescaderos River, a tributary of the Colorado ow coverted to agricultural draiage, ad these commuities use the Pescaderos for fishig ad hutig. Dimiished flows i the river have forced may Cucapá to truck their boats miles to reach the earest waterways, ad may travel farther to fid work i the agricultural fields of the Mexicali Valley (Boyer, 1998). The Cucapá have the oe licesed commercial fishig operatio i the delta the Cucapá Fishig Productio Uit which has tribal rights to fish for Gulf corvia ad shrimp. Although subsistece fishig, as well as hutig ad gatherig, oce were commo, Cucapá commuities ow work as hutig ad fishig guides, ad sell their arts ad crafts to tourists (Valdés-Casillas et al., 1998a). Other residets fish commercially i the southermost Colorado River betwee Cucapá El Mayor ad the river s mouth for bigmouth bass, carp, mullet, catfish, tilapia, ad crayfish. Commercial fisheries i the upper Gulf, all depedet o the delta for breedig grouds ad/or utriets, iclude shrimp, shark, milkfish, ad corvia (Valdés-Casillas et al., 1998d). Photo 2. Palmer s saltgrass i the Colorado River delta. The success of ay effort to preserve delta ecosystems will deped o its ability to idetify ad iclude the iterests ad cocers of local people ad commuities. As part of the research discussed i this report, social scietists have approached the people of the delta to begi the process of uderstadig their eeds ad to provide outreach ad educatio cocerig the research effort. I additio, locals are beig asked to help scietists uderstad the delta s ecology ad the ways i which it has chaged i recet decades. (Appedix C cotais a more complete summary of this outreach effort.) GROWING INTERNATIONAL INTEREST IN THE DELTA The past decade has brought greater scietific uderstadig of the Colorado River delta, ad it also has brought icreased political iterest from both sides of the border. I 1993, the delta ad the upper Gulf were declared a Biosphere Reserve by the Mexica govermet. This desigatio, sactioed by the Uited Natios, is desiged to protect world-class ecosystems while ecouragig cotiued sustaiable ecoomic activity i surroudig buffer areas. 9 The Biosphere Reserve of the Upper Gulf of Califoria ad Colorado River Delta covers a total of 2,309,782 acres (934,756 ha) 407,218 acres (164,779 ha) i the core ad 1,902,564 acres (769,976 ha) i the buffer. The core area icludes la Ciéega de Sata Clara ad el Doctor wetlads. The Reserve is desiged to protect a estimated 19 percet of the plat species foud i Mexico, 22 of the 37 saltwater fish species edemic to the Gulf of Califoria, ad the desert pupfish, the oly survivig ative freshwater fish species of the delta (Cetro de Ivestigacioes Cietificas y Tecológicas de la Uiversidad de Soora,.d.). 10 The delta was recogized as part of the Wester Hemisphere Shorebird Reserve Network i I 1994, Mexico joied the U.S. ad Caada i the North America Waterfowl Maagemet Pla, ad listed the delta as cotietally importat habitat. I 1996, delta wetlads were listed as a Ramsar site whe Mexico became a party to the Covetio o Wetlads (also kow as the Ramsar Covetio) ad thereby agreed 9 Biosphere Reserves, desigated by the Mexica govermet uder the authority of the Geeral Law of Ecological Equilibrium ad Evirometal Protectio, ca be created to protect areas greater tha 25,000 acres (10,000 ha) that cotai edemic, threateed, or edagered species (Cetro de Ivestigacioes Cietificas y Tecológicas de la Uiversidad de Soora,.d.). 10 The Biosphere Reserve s Maagemet Pla cites sigificat evirometal threats to the regio such as reduced flows, illegal fishig, agricultural pollutio, illegal hutig, illegal extractio of plats, ad overfishig. 8

16 9 ENVIRONMENTAL DEFENSE FUND Figure 3. Huma Settlemets Withi the Highest Ifluece Area of the Colorado River Delta Wetlads (Valdés-Casillas et al., 1998a) N E W S Lagua Salada Ciéega de Sata Clara Mexicali Sa Luis Río Colorado Colorado River Hardy River Colorado River Cities Iteratioal Boudary Railroad Rivers Roads Levees Huma Settlemets Gulf of Califoria Modified from INEGI ad T.M. Satellite Images Kilometers ITESM Campus Guaymas. CECARENA/UIB

17 A DELTA ONCE MORE to place a high priority o wetlad coservatio. As early as 1988, Mexico had joied Caada ad the Uited States i sigig the Tripartite Agreemet o the Coservatio of Migratory Birds, opeig the door to fudig from the U.S. North America Waterfowl Coservatio Act. To date, several ivetory projects i the delta have received these fuds. I the past decade, coservatio orgaizatios i Mexico ad the Uited States have become icreasigly ivolved i efforts to assess threats to the ecosystems of the delta ad upper Gulf, ad to idetify opportuities for protectig ad ehacig these resources. 11 I 1998, the Colorado River Delta was listed as oe of the top te edagered rivers i North America i a aual press release used to geerate political iterest by the advocacy group America Rivers. Public agecies, resource maagers, ad water users i both coutries also have begu to recogize the importace of a coservatio ageda for the regio. Mexico s Natioal Istitute of Ecology (INE) maages the Biosphere Reserve ad is promotig the creatio of a biatioal istitutio charged with sustaiable water use i the delta. The Iteratioal Boudary ad Water Commissio (IBWC) is coveig a Delta Taskforce. The U.S. Evirometal Protectio Agecy (EPA) Iteratioal Office has begu fudig projects i the border regio. New istitutios created as part of the North America Free Trade Agreemet may also be able to weigh i o delta issues. 12 Oe otable exceptio to this tred is the U.S. Multi-Species Coservatio Program for the Lower Colorado River (MSCP). Participats have bee ecouraged ad have refused to cosider the delta regio i their ivestigatio of mitigatio measures for the lower river habitats. 13 Iterest i the delta is likely to icrease as U.S. policy makers recogize its value. The delta cotais thousads of acres of habitats that have become rare elsewhere i the Colorado basi (Gle et al., 1996; Valdés-Casillas et al., 1998a), ad may be the oly viable repository for several edagered species. Also, coservatio efforts i the delta may prove more productive tha elsewhere i the lower basi, ad the delta could become a focus for efforts to mitigate the impact of developmet further orth. I ay case, the delta is a key compoet of the lower Colorado riparia zoe, which exteds from the Grad Cayo to the Gulf of Califoria. Ay viable recovery pla for the river s riparia zoe must iclude the delta, regardless of the iteratioal border that divides the regio. At some poit, U.S. evirometal policy may obligate the Uited States to help protect the delta. 14 U.S. water maagemet agecies are directly resposible for the well-beig of the delta ad orther Gulf fisheries, as the delta ecosystems are almost etirely supported by wastewater flows or flood releases the agecies cotrol. 11 The Evirometal Defese Fud, Ducks Ulimited, the Nature Coservacy, Coservatio Iteratioal, America Rivers, Defeders of Wildlife, the Soora Istitute, the Pacific Istitute, Itercultural Ceter for the Study of Desert ad Oceas (CEDO), ad PRONATURA have a ogoig iterest i the regio. 12 See Chapter 4 for a discussio of NAFTA-related istitutios. 13 MSCP icludes represetatio from the lower basi states, ad water ad power users. Represetatives of evirometal orgaizatios resiged from the MSCP i 1998 whe the represetatives of the states ad water users refused to iclude the delta i the scope of their program. 14 For a discussio of U.S. law ad evirometal policy, see Legal Mechaisms i Chapter 4. 10

18 2 WATER SOURCES IN THE DELTA ENVIRONMENTAL DEFENSE FUND Curtailmet of the surface water supply ad its restrictio to the cultivated areas ad a arrow chael will evetually result i the reversio to the coditio of the surroudig deserts of much of the regio which is at preset occupied by luxuriat vegetatio. Godfrey Sykes, The Colorado Delta, 1937 THE LAW OF THE RIVER Use of Colorado River water is govered by a complex set of legal ad admiistrative agreemets kow collectively as the Law of the River. This body of agreemets gives highest priority to cosumptive uses of water, ad lowest priority to public good uses such as maitaiig i-stream flows to support fish, wildlife, ad habitat. 15 To date, the Law of the River cotais o provisio for allocatig water to support the ecological health of the Colorado s riparia zoe or the delta ad upper Gulf. However, recet reforms ad ogoig egotiatios to amed existig water maagemet istitutios suggest a potetial for securig a dedicated water supply for the delta at some time i the future. Eve withi the costraits of the Law of the River, the U.S. federal govermet ad several states have successfully secured flows for habitat ad edagered species protectio i the basi. I 1987, the Recovery Implemetatio Pla for the Upper Colorado River Basi was developed to protect ad improve i-stream flows, restore habitat, ad reduce the adverse effects of oative fish species. 16 I the lower basi, water users represetig irrigatio, muicipal, ad power iterests lauched the Lower Colorado River Multi-Species Coservatio Program (MSCP) i 1994 to mitigate water developmet impacts o threateed ad edagered species ad their habitat. 17 I 1996, the Bureau of Reclamatio released a flood of stored water from behid Gle Cayo Dam i a effort to redistribute sedimets i the Grad Cayo ad re-create eroded beaches. 18 These efforts suggest a growig awareess of the importace of the river s ecological health ad show the willigess of water users ad their represetatives to reform water maagemet practices. The authors believe, however, that a sigle-focused effort to gai additioal water for the delta could lead to coflicts with U.S. water users ad a breakdow of cooperatio, as competitio for Colorado River water is already high i this fast-growig regio. Lower basi states i the U.S. are ow or soo will be usig their full etitlemet of water from the river ad are workig to secure access to additioal supplies. 19 Mexico has logstadig cocers over the quatity ad quality of water delivered to the border. 20 I additio, Mexico views its Colorado River etitlemet as additioal to 15 For detailed discussio of the laws ad decisios that comprise the Law of the River, see Charles Meyers s ad Richard Noble s articles i the Staford Law Review (1966 ad 1967). The provisios for delivery of Colorado River water to Mexico are set forth i the Treaty with Mexico Respectig Utilizatio of the Waters of the Colorado ad Tijuaa Rivers ad of the Rio Grade, February 3, The Law of the River gives priority to 1) the delivery of water to Mexico, 2) preset prefected rights (or, water rights that were exercised prior to 1922, icludig the rights of Idia tribes), 3) delivery of water to the lower basi for cosumptive uses, 4) cosumptive uses i the upper basi, 5) ecoomic, ocosumptive uses (e.g., power geeratio), ad 6) oecoomic, ocosumptive uses (e.g., evirometal protectio). Provisios pertaiig to the quality of water the U.S. must deliver to Mexico are the subject of yet aother agreemet, datig to 1964, Miute 242 to the 1944 Treaty. 16 The Recovery Implemetatio Program for Edagered Fish Species i the Upper Colorado River Basi (RIP) is a cooperative effort ivolvig the U.S. Fish ad Wildlife Service, Bureau of Reclamatio, Wester Area Power Admiistratio, the states of Utah, Colorado, ad Wyomig, water users, ad evirometalists. The recovery program, which is expected to require 15 years, cotais five major elemets: 1) habitat maagemet desiged to idetify ad acquire i-stream flows, icludig the chage i operatio of federal reservoirs i the basi; 2) habitat developmet based o the developmet of research methods for creatig, protectig, ad improvig habitat; 3) stockig ative fish based o a geetic maagemet pla; 4) oative species cotrol; ad 5) research, moitorig, ad data maagemet programs desiged to study various meas of recoverig fish, moitor log-term populatio treds, recommed flows, evaluate geetic differeces betwee populatios, recommed refugia (facilities to hold ad protect rare fish), evaluate differeces betwee hatchery ad wild fish, establish brood stock, ad develop ad maage a cetralized database. EDF is oe of the evirometal participats i the RIP ad has focused its attetio o i-stream flow issues. 17 See Chapter 1, Growig Iteratioal Iterest i the Delta, for further discussio of the MSCP. 18 The 1996 flood helped icrease the sadbar volume of 50 percet of the campig beaches measured betwee Gle Cayo ad Hoover dams. The flood bypassed the dam s turbies, ad cost approximately $2.5 millio i lost hydropower reveues. (Harpma,.d.) 19 Califoria already uses more tha its allocated share, ad Nevada is close to usig its allocated share. 11

19 A DELTA ONCE MORE groudwater supplies that it ca pump i the border regio, while Califoria ad Nevada have proposed a actio the liig of the All America Caal i Califoria that would substatially capture water that ow recharges this groudwater basi. Give the relative scarcity of water i the Colorado River basi, it is more likely that the delta s salvatio will occur through some level of protectio for flows that are presetly occurrig but are ot madated. This report focuses o steps that ca be take to improve the maagemet of existig flows, rather tha steps to icrease trasboudary water deliveries. I the short term, delta ecosystems will cotiue their recovery if flows occur at levels recorded i recet years. However, some assurace of these flows, as well as dedicated tras-boudary water deliveries, may be part of the log-term solutio. APPORTIONED FRESHWATER FLOWS Whe Colorado River waters were apportioed, first by the 1922 compact ad subsequetly a Upper Basi compact, court decisios, federal law, ad iteratioal treaty, the river was overallocated. The problems arisig from this overallocatio are compouded by the fact that there are very differet iterpretatios of the defiitio of cosumptive use, treatmet of evaporatio from reservoir surfaces, ad the water delivery obligatios of the Upper Basi states uder the treaty to Mexico (Getches, 1985) If the Colorado is already overallocated because water etitlemets were based o optimistic estimatios of average aual flow, it may be all the more difficult to secure additioal water allocatios dedicated to delta ecosystems. This reiforces the importace of fidig ways to improve maagemet of existig flows that ow reach the delta. I ay case, cooperatio, accommodatio, ad creativity will be essetial, especially as demads for water icrease. Before 1980, while major reservoirs o the Colorado River were still fillig, flood flows were oexistet. The riparia zoe of the river from Morelos Dam to the juctio with the Rio Hardy was a dry ecosystem, domiated by widely spaced mesquite trees (Valdés-Casillas et al., 1998a). 21 Below the juctio of the two rivers, the chael was pereial, due to the discharge of agricultural drai water from the Mexicali Valley ad tidewater eterig from the Gulf of Califoria. I the years sice 1980, flood flows have bee released from Lake Mead (the last major storage o the Colorado River i the U.S.) whe flow exceeds storage capacity ad upstream use. I years without floodig, the oly Colorado River water to reach Mexico is its 1.5 maf (1.8 x 10 9 m 3 ) treaty allotmet, about 10 percet of the river s average aual flow. 22 The U.S. delivers early all of Mexico s water allotmet to the Norther Iteratioal Boudary at Morelos Dam. Mexico diverts this water to the Mexicali ad Sa Luis Rio Colorado irrigatio districts by way of the Cetral Caal, 23 which has a capacity sufficiet to divert Mexico s etire allocatio. Water i the Cetral Caal ot used for irrigatio is routed to Mexicali ad Tijuaa for muicipal use (Ybarra, 1999). The relatively small portio of Mexico s allocatio that is delivered at the Souther Iteratioal Boudary at Sa Luis Rio Colorado is also diverted for irrigatio. Durig flood-free years, o Colorado River water reaches the remat delta wetlads below the irrigated farmlad (Gle et al., 1996). 24 The oly flows that cotiue past Mexico s irrigatio diversios ad ito the delta are flood flows (see ote 2 ad Tides ad Floods below). Durig dry years, the oly water reachig the delta comes from groudwater seeps, agricultural draiage, ad tidewater. [See Box 1.] 20 Mexico has objected to the quality of this water, statig that it is so salie (averagig 1500 parts per millio [ppm]) that it reduces yields o the 94,000 acres (38,000 ha) where it is used for irrigatio, causig soil deterioratio ad icreasig groudwater saliity. Additioally, Mexico has suggested that this water is possibly cotamiated with pesticides. Mexico has requested that its etire etitlemet be delivered to the Norther Iteratioal Boudary (NIB) at Morelos Dam o the river s mai chael. The U.S.-Mexico agreemet cocerig water quality i the Colorado River is foud i Miute 242 to the 1944 Treaty. It requires that the average aual saliity of water delivered at the NIB ot exceed 115 +/-30 ppm over the aual average saliity of water arrivig at Imperial Dam i Califoria (Potius, 1997). A portio of Mexico s treaty water is set to the Souther Iteratioal Boudary (SIB), where agricultural wastewater collected by several drais, ad at times augmeted by pumped groudwater, is delivered, 2 miles east of the river chael. Water quality at the SIB is ot govered by the treaty ad has a average saliity of 1500 ppm. Approximately 140,000 acre-feet (1.7 x 10 8 m 3 ) of Mexico s 1.5 maf (1.8 x 10 9 m 3 ) etitlemet is delivered to the (SIB). The saliity of seawater is 35,000 ppm. 21 These observatios are based o ispectio of 1972 aerial photographs ad iterviews with residets. 22 The average flow (over the historic log term) of the Colorado River is 15 maf (1.8 x m 3 ). Flows as low as 6 maf (7.4 x 10 9 m 3 ) ad as high as 24 maf (3 x m 3 ) have bee recorded (Potius, 1997). 23 Approximately 2 maf (2.4 x 10 9 m 3 ) per year are used for irrigatio i the Mexicali ad Sa Luis Rio Colorado valleys, with Colorado River water makig up the majority of this supply (Valdés-Casillas et al., 1998a). 24 While the Law of the River gives clear priority to cosumptive uses of water, the Colorado River also provides cosiderable value i terms of recreatioal ad fish ad wildlife beefits. However, the ecological eeds of the Colorado River have oly recetly gaied legal recogitio ad protectio. Some of these coservatio efforts i the U.S. are discussed i Chapter 1. 12

20 ENVIRONMENTAL DEFENSE FUND BOX 1. A Related Issue: Wastewater Treatmet for Mexicali A NEW SOURCE OF WATER FOR THE DELTA? Each year Mexicali, a Mexica border city, discharges about 40,000 acre-feet (4.9 x 10 7 m 3 ) of effluet ito the Iteratioal Boudary Drai, which empties ito the New River. The New River origiates 22 miles (35 km) south of the iteratioal boudary ad flows orth through Mexicali, crossig the border ito Califoria s Imperial Valley. About 45 miles (70 km) to the orth, it empties ito Califoria s Salto Sea, a closed basi, where evaporatio teds to cocetrate its pollutats [see Box 5.]. Although some of Mexicali s effluet is treated, raw sewage ad idustrial waste ofte flow directly ito the New River through storm drais ad other outlets. The New River is widely regarded as oe of the most polluted rivers i the Uited States, ad it has log bee the subject of egotiatios betwee the Uited States ad Mexico. Recetly, Mexico ad the Uited States agreed to build a biatioal wastewater treatmet plat to be called Mexicali II. O completio i 2015, the plat will treat more tha 37 millio gallos per day (mgd) (1645 liters/sec) ad serve a projected populatio of more tha half a millio people (IBWC, 1996). A key questio i the plat s desig is where to discharge its treated water (Eberhardt, 1996). This water could empty ito the New River (ad could possibly improve water quality coditios i the Salto Sea), or it could be piped to the Rio Hardy basi. Disposal i the Rio Hardy wetlads would help maitai importat ecosystems i the Colorado River delta, ad the wetlads might eve serve as a fial step i the treatmet process (Reteria ad Luecke, 1997). TIDES AND FLOODS Two sources of water tides ad floods cotiue to sustai parts of the delta much as they have for ceturies. Tides are a daily give i the delta, ad the topography of the log ad arrow Gulf creates a exceptioally high tidal swig of 10 feet (3 m) or greater at its orther ed. This allows high tides to flow more tha 34 miles (56 km) ilad i some places (Gle et al., 1996) ad spread over a total of 81,500 acres (33,000 ha) (Thompso et al., 1968). Tides have sustaied vast areas of the delta through the last several decades, although without freshwater flows to dilute the seawater, tides ca have a deleterious effect as well. Sice 1980, floods have oce agai reached the delta, though they are o loger a guarateed sprigtime occurrece as they oce were. The Colorado s system of dams ca regulate much of the variatio betwee wet ad dry years, but extraordiarily wet years will probably cotiue to brig floodig to the delta whe releases exceed the capacity of users i the Uited States ad Mexico to divert the water. Sice the fillig of the Colorado River s reservoirs, these releases have reestablished a active floodplai from Morelos Dam to the tidal zoe i the Gulf of Califoria. Near-record flood releases i the witer of 1983 were at first cosidered a aberrat evet, but occasioal floodig has cotiued, coicidig with El Niño evets (Gle et al., 1996). From 1980 to 1993, average aual flood flows across the border (cross-border flows mius Mexico s treaty allotmet) were 3.9 maf (4.8 x 10 9 m 3 ). This is early three times Mexico s 1.5 maf (1.8 x 10 9 m 3 ) treaty allotmet, ad 25 percet of the historic flow ito the delta before dam costructio (Gle et al., 1996). The largest releases occurred i the early 1980 s, with flows after 1986 more sporadic ad smaller i volume. I , flows exceedig 1.5 maf (1.8 x 10 9 m 3 ) were released to the delta (Gle et al., 1999). 25 [See Figure 4.] 13

21 A DELTA ONCE MORE Photo 3. Aerial view of the Colorado River delta, ear where the Colorado meets the upper Gulf of Califoria These floods are sigificat, sustaiig the delta s ecosystems through the periodic iudatio of its riparia areas ad wetlads. Satellite images show a marked declie i summer vegetatio durig five dry years from 1988 to Floods i 1993 were followed by three more dry years, but i 1997 scietists observed that midstory trees that appeared to have germiated i 1993 domiated may parts of the delta (Gle, 1998b). These trees survived three years without river flows. This ad other evidece lead researchers to believe that flood flows at three- or four-year itervals could maitai vegetatio. These floods also could help sustai upper Gulf shrimp fisheries ad other marie species depedet o the delta. Future research o Gulf aquatic species may yield more specific iformatio cocerig their depedece o freshwater flows from the delta. Give the apparet importace of floods, oe possible way to support delta ecosystems would be to deliberately maage flood releases for maximum beefit. Water maagemet agreemets o the Colorado River iclude provisios for allocatig water uder shortage ad surplus flow coditios. 26 These agreemets could be revised to esure that a portio of surplus flood flows are stored for, or delivered to, water-depedet ecosystems i amouts ad rates, ad at times, that would be most beeficial. 25 It should be oted that aual flow averages ad flood release figures do ot always reflect the amout of overbak floodig the iudatio of riparia lads that occurs i the delta. This is more accurately reflected i rate of flow as measured i cubic feet or meters per secod. 26 The U.S. Bureau of Reclamatio, the agecy resposible for maagemet of the Colorado River dams ad flows, reports expected flow coditios to Mexico through the Iteratioal Boudary ad Water Commissio (Johso, 1999). 14

22 ENVIRONMENTAL DEFENSE FUND Figure 4. Water Flows at the Souther Iteratioal Boudary, 1997 (Valdés-Casillas et al., 1998a) Cubic mts. per secod J F M A M J J A S O N D Date (1997) AGRICULTURAL WASTEWATER Delta wetlads with sigificat coservatio iterest also survive o agricultural wastewater. Sevetee agricultural drais from the Mexicali Valley flow ito the Rio Hardy/Colorado River system, carryig a average aual volume of 51,000 acre-feet (6.33 x 10 7 m 3 ) (Valdés-Casillas et al., 1998a). Aother 125,000 acre-feet (1.5 x 10 8 m 3 ) of mildly salie (3000 parts per millio) agricultural wastewater pumped from Arizoa s Wellto-Mohawk Irrigatio District is delivered to Mexico at the Souther Iteratioal Boudary. This water is disposed i the easter delta after travellig 48 miles (77 km) i a cocrete caal called the Mai Outlet Drai Extesio (MODE). MODE water jois about 25,000 acre-feet (3.1 x 10 7 m 3 ) of agricultural wastewater (from the Riito drai) to support la Ciéega de Sata Clara. La Ciéega de Sata Clara was oce part of a Colorado River chael that ra alog the edge of the Soora Mesa. As the chael shifted westward, this shallow depressio (formed by a brach of the Sa Adreas fault) early dried up. I 1973, before MODE water arrived, the wetlad covered oly 490 acres (200 ha), fed by artesia sprigs ad the Riito drai (Gle et al., 1996). After 1977, whe the MODE water started emptyig ito the Sata Clara depressio, flows supported wetlads of up to 50,000 acres (20,000 ha). I 1993, floods o the Gila River required the MODE to be closed for repairs for about eight moths. As a result, wetlads i la Ciéega de Sata Clara decreased i size to 2700 acres (1100 ha). With the restoratio of flows, the marsh retured to its former dimesios withi five moths (Gle et al., 1996; Zegel et al., 1995). [See Box 2.] Agricultural wastewater ca chage ecosystem health sice it teds to affect the cocetratio of pollutats, salts, ad mierals. High levels of seleium, for example, are foud i may delta areas that receive wastewater, ad seleium is kow to affect birds ad other wildlife. Although agricultural wastewater has bee a fairly costat source for la Ciéega de Sata Clara, over the years the MODE caal has carried less water ad its saliity has declied. This is due to lower pumpig rates ad lower groudwater saliity. Wastewater flows from 15

23 A DELTA ONCE MORE the Mexicali Valley aually carry 70,000 tos (64,000 metric tos) of fertilizer ad 110,000 gallos (400,000 liters) of isecticide (Direccio Geeral de Ecologia, i Valdés-Casillas et al., 1998a). While agricultural wastewater may ot be a ideal source of water, its beefits may for the preset outweigh its liabilities, particularly sice there are few other potetial sources for restorig delta ecosystems. Oe of the chief goals of recet research efforts i the delta has bee to examie survivig ecosystems ad lear about icidetal water supplies, such as wastewater, that support wetlads. I fact, wastewater from geothermal wells forms the headwaters of the Rio Hardy. If wastewater ca be deliberately maaged, may areas of the delta might be sustaied without ay ew dedicated flows. BOX 2. A RELATED ISSUE: LA CIÉNEGA DE SANTA CLARA La Ciéega de Sata Clara (ciéaga meas wetlad i Spaish) is the major marsh wetlad i the easter regio of the Colorado River delta. At 50,000 acres (20,000 ha), domiated by cattails, it is home to perhaps the largest remaiig populatios of the Yuma clapper rails ad desert pupfish. It is a major stopover for migratory waterfowl o the Pacific Flyway, ad supports guidig, hutig, fishig, ad limited ecotourism activities for local commuities. I 1992, la Ciéega de Sata Clara was icluded i the core zoe of the Biosphere Reserve of the Upper Gulf of Califoria ad the Delta of the Colorado River (see Chapter 1). Historically, a arm of the Colorado River was located where la Ciéega de Sata Clara is today. The baks of the river were covered with willow ad cottowood. Over time, the river chael shifted westward, ad after the costructio of Hoover Dam the river o loger etered the shallow depressio (formed by a brach of the Sa Adreas fault) that defies the wetlad. I 1973, the oly water flowig ito la Ciéega was supplied by local artesia sprigs ad agricultural draiage water from the Riito drai. La Ciéega de Sata Clara was reduced to a mere 500 acres (200 ha) (Gle et al., 1996). Photo 4. La Ciéega de Sata Clara. Startig i 1977, brackish agricultural draiwater from Arizoa has flowed 50 miles south ito Mexico via the MODE caal, ad draied ito the Sata Clara depressio. These flows created a wetlad of up to 50,000 acres (20,000 ha) of water surface, of which 11,200 acres (4,500 ha) were thickly vegetated. This is cosidered to be the mature size of the wetlad. Flow from the caal was iterrupted for eight moths i 1993 (followig floods), ad the vegetated wetlad dimiished agai to 2,750 acres (1,100 ha). The resiliecy of the delta marsh systems was demostrated whe flows retured to the MODE caal ad the vegetated area assumed its former dimesios (Gle et al., 1996). The flows through the MODE were origially iteded to be temporary. The U.S. Bureau of Reclamatio made other plas for this water as a source for the Yuma Desaltig Plat, completed i [See Box 4.] However, the plat is ot yet operatig. Ay viable pla to operate the Yuma Desaltig Plat must provide replacemet water supplies ad other assuraces to protect ad sustai the remat wetlads of la Ciéega de Sata Clara which ow deped o MODE draiage outflows. 16

24 3 DELTA HABITAT AND RESTORATION POTENTIAL ENVIRONMENTAL DEFENSE FUND The still waters were of a deep emerald hue. A verdat wall of mesquite ad willow separated the chael from the thory desert beyod. At each bed we saw egrets stadig i the pools ahead. Fleets of cormorats drove their black prows i quest of skitterig mullets; avocets, willets, ad yellow-legs dozed oe-legged o the bars; mallards, widgeos, ad teal sprag skyward i alarm. At every shallow ford were tracks of burro deer. We always examied these deer trails, hopig to fid sigs of the despot of the Delta, the great jaguar, el tigre. We saw either hide or hair of him, but his persoality pervaded the wilderess. Aldo Leopold, A Sad Couty Almaac, 1948 THE DELTA S ECOLOGICAL SIGNIFICANCE Productivity ad diversity i the delta have declied over the last cetury, but the delta ecosystem remais a importat biological resource evertheless. It remais a oasis of life i the midst of the arid Soora Desert. Although reduced flows ad the costructio of levees have trasformed the delta, floodwaters, agricultural draiage, muicipal wastewater, ad seawater i the tidal zoe cotiue to support large riparia areas ad marshes. The size of these areas teds to vary dramatically from oe seaso to the ext: durig the period from 1973 to 1993, freshwater ad brackish wetlads raged from 2300 to 25,500 acres (5800 to 63,000 ha). I 1997, flood releases reestablished ative vegetatio alog the delta s Colorado River floodplai ad riparia zoes as well as i southeaster delta wetlads (Valdés-Casillas et al., 1998c). The delta supports a variety of wildlife, icludig several threateed ad edagered species. Mexico s Evirometal Regulatios o Edagered Species lists the followig edagered species foud i the terrestrial ad aquatic regios of the delta (Diario Officiel, 1994): the desert pupfish, also listed as a edagered species i the U.S. the largest remaiig populatio aywhere is i La Ciéega de Sata Clara; the Yuma clapper rail, also listed as a edagered species i the U.S.; the bobcat; the vaquita porpoise, the world s smallest marie mammal, listed as a species of special cocer by the U.S. Marie Mammal Commissio; ad the totoaba, ow virtually extict, a steel-blue fish that grows up to seve feet (2 m) ad 300 pouds (136 kg), ad oce supported a commercial fishery that closed i 1975 (Postel et al.,.d.). I additio, Mexico lists five threateed species: the yellow-footed gull, Heerma s gull, elegat ter, reddish egret, ad peregrie falco; three species for special protectio: the brat, house fich, ad mockigbird; ad oe rare species: the great blue hero. Although ot extesively studied, the delta s sigificace for migratory birds is idisputable, as it is the priciple freshwater marsh i the regio. From 1980 to 1985, some 45,000 ducks ad 200 geese witered i the delta (Paye et al., 1992). A 1992 witer survey foud more tha 160,000 birds i the delta, of which some 9000 were avocets ad 8000 were willets (the remaider beig smaller species such as sadpipers) (Morriso et al., 1992 i Mellik et al., 1997). A series of delta surveys i documeted 21 seabird species, with more tha 16,000 idividuals; 6 hero species, ad more tha 220 idividuals; 20 shorebird species, ad early 150,000 idividuals (Mellik, et al., 1997). The delta also provides estig, breedig, ad ursig sites for egrets, sadpipers, avocets, cormorats, ducks, pelicas, gulls, ad ters. (See Appedix A for a selected species lists.) I additio to bird couts, there is evidece that delta habitats are of greater value to birds tha riparia habitat upstream (i the Uited States) o the Colorado River (see discussio of Delta Wetlads ad Riparia Vegetatio below). Tree species compositio is kow to be critical i avia habitat selectio, particularly i desert, riparia habitats (Rice et al., 1984). O the lower Colorado River, birds prefer gallery forests of cottowood ad willow, both ative species, over screw bea ad mes- 17

25 A DELTA ONCE MORE quite. More cottowood-willow habitat exists i the delta tha i the riparia forests upstream (Ohmart et al., 1988). Delta wetlads provide habitat for a umber of mammals, icludig raccoos, skuks, bats, coyotes, bobcats, muskrats, rabbits, jackrabbits, desert rats, gophers, ad squirrels (Valdés-Casillas et al., 1998a). The tidal zoe ad ear shore marie habitats of the Gulf of Califoria also support edagered species ad importat fisheries. Fish species iclude catfish, carp, tilapia, mullet, ad largemouth bass, ad the last remaiig populatios of desert pupfish, which still survive i backwaters ad lagoos (Valdés-Casillas et al., 1998a). The delta is a egative estuary (where the saliity is greater tha the ocea s due to evaporatio that exceeds precipitatio ad river flow) that is a rich breedig groud for marie species ad has a sigificat ifluece o fish populatios, possibly throughout the etire Gulf. Reductio of freshwater flows ito the Gulf has reduced the trasport of utriets ad chaged the characteristics of this critical ursery habitat. Nutriet cocetratios are higher tha most of those reported i the literature for estuaries ad egative estuaries, perhaps due to the strog mixig caused by tides of great amplitude (Heradez-Ayo et al., 1993). The shrimp fishery has dropped off steeply ad other fisheries are i declie. The totoaba is ow virtually extict, ad the vaquita porpoise is thought to umber oly a few hudred (Marie Mammal Commissio, 1996). The loss of upper Gulf fisheries may be the most costly effect of reduced flows to the delta. Overfishig certaily cotributes to the problem, but scietists have oted a correlatio betwee shrimp catches ad flood flows to the delta. 27 This corroborates aecdotal evidece ad reports from local fisherme that idicate treds such as a temporary icrease i the umber of fish species observed i the mid-1980 s after high flood flows reached the delta (Postel et al.,.d.). THE ENVIRONMENTAL DEFENSE FUND S RESEARCH IN THE DELTA I 1997 ad 1998, staff from the Evirometal Defese Fud, the Evirometal Research Laboratory (ERL), the Istituto Tecológico y de Estudios Superiores de Moterrey (ITESM) ad the Soora Istitute coducted fieldwork to evaluate the potetial for restoratio of delta habitat. This was the first serious attempt to study delta habitat sice the restoratio of flows. The team quatified the effects of a flood release of kow magitude, ad ivetoried vegetatio. They were able to observe the respose of vegetatio to flows as a basis to defie the amout of water eeded to sustai delta ecosystems. I additio, they assessed the potetial for existig vegetative cover to support wildlife, 28 particularly i compariso to the upstream stretches where curret coservatio efforts are focused. 29 The fidigs also serve as a baselie for evaluatig the quality ad extet of habitat that could be restored ad maitaied with a dedicated supply of water ad a program for maagig that water. [Appedix B details the methods used i this study.] To coduct its ivetory, the research team used satellite imagery, low-level aerial videography, ad groud surveys to map chaels ad plat life. [See Figure 5.] The mai (avigable) course of the river was foud by explorig its chaels i a small boat durig floods. The team also surveyed the marshlads supported by agricultural drai water. They assessed the relative potetial ad importace of restorig ad maagig wetlads i the delta, basig its assessmet o habitat values, degree of evirometal threat, ad the importace of each area to local people who use the wetlad resources ad may be willig to help protect them. 30 The results of this fieldwork are summarized below, followed by a umber of restoratio objectives. 27 Data correlatig shrimp ladigs at Sa Felipe, Baja Califoria, the earest shrimpig statio to the delta, with discharges from the Colorado River to the orther Gulf of Califoria show a sigificat correlatio sice the resumptio of flows (Galido-Bect et al.). 28 Previous research has ivetoried the extet ad habitat values of the Ciéega de Sata Clara wetlad i the southeast portio of the delta ad the wetlads of the Rio Hardy/Rio Colorado cofluece. See Gle et al., 1992, ad Gle et al., Coservatio efforts o the Colorado River are discussed i Chapter These scietific fidigs are i mauscript, ad will be published (Valdés-Casillas et al., 1998c), ad are preseted i a report the North America Wetlad Coservatio Coucil prepared by scietists from ITESM ad others (Valdés-Casillas et al., 1998a). 18

26 ENVIRONMENTAL DEFENSE FUND Figure 5. Satellite Image of the Colorado River Delta, July 15, 1997 (Valdés-Casillas et al., 1998a) The colors i this Ladsat image reflect sesitivity to heat (red depicts vegetatio). 19

27 A DELTA ONCE MORE Flood Flows ad Water Requiremets of Delta Vegetatio Water deliveries below Morelos Dam sice 1980 have bee extremely variable i frequecy ad volume. Furthermore, kowledge of the vegetatio is based o a sigle sapshot ispectio i Hece, it is ot possible to determie the vegetatio respose to a particular flow regime without further study. However, ispectio of the vegetatio that exists i the delta ow ad the recet flow history allows some ifereces about the water requiremets. Sice 1980, there have bee two prologed periods without flow: a four year period from 1988 to 1993, ad a three-year period from 1994 to However, the 1997 ispectio showed cottowood ad willow trees datig back to both the 1980 s ad 1993 flow evets. It appears that germiatio correlates with flood flows. It also appears that aual flood evets are ot ecessary for survival of the ative tree species. They are capable of survivig at least three-to-four-year itervals betwee major flow evets i the delta floodplai. It is ot clear whether their survival depeds o local agricultural retur flows or other sources that may recharge the riparia zoe durig periods i which water does ot flow from the Uited States. Apparetly, delta riparia vegetatio ca survive a period of several years without water deliveries from the Uited States, oce flood flows have allowed the seeds to germiate. The vegetatio aalysis (see Delta Wetlads ad Riparia Vegetatio below) was based o a satellite image from July 1997, plus field ispectios. The aalysis was coducted followig flood releases i Jauary April 1997 of approximately 260,000 acre-feet (3.2 x 10 8 m 3 ) of water at flow rates from approximately 3500 to 7000 ft 3 per secod (100 to 200 m 3 per secod) below Morelos Dam. A February 21, 1997, satellite image, plus low-level aerial ad groud surveys durig February flows, showed that the 1997 witer flood was sufficiet to cause overbak floodig of the Colorado River chaels throughout the floodplai betwee the levees. I additio, this flood produced ruoff from the floodplai ito the Gulf of Califoria ad Lagua Salada, a dry depressio i the delta. Durig these flows, ocea saliity was diluted to less tha half the saliity of seawater at the orther tip of Motague Islad. It appears that flow rates of 3500 to 7000 ft 3 per secod (100 to 200 m 3 per secod) are sufficiet to iudate the floodplai. I additio, aual volume releases totalig 260,000 acre-feet (3.2 x 10 8 m 3 ) i witer ad sprig are sufficiet to produce a vegetatio respose i summer. The extet to which the July 1997 vegetatio respose was due to early 1997 water releases, ad how much might have occurred without floodig, is ot kow. However, copious emergece of seedligs followig the floods was observed, so it is possible to coclude that this volume of water was sufficiet to support the existig vegetatio ad stimulate ew growth alog the floodplai. The etire floodplai betwee the levees is 150,000 acres (60,000 ha), of which about 25 percet supported high-desity vegetatio i July Potetial evapotraspiratio by wetlad ad riparia vegetatio i the floodplai is as high as 8 feet (2.5 m) per year. The high-desity vegetatio, which cosists maily of cottowood, willow, mesquite, salt cedar ad cattail, ca use approximately 304,000 acre-feet (3.75 x 10 8 m 3 ) of water per year. This is greater tha the flows recorded durig Jauary through April The 1997 vegetatio respose may idicate that the floodplai is recharged by local aquifers i additio to river flows from the Uited States. Based o these observatios, it is apparet that irregular flows sice 1980 have cotributed to revegetatio of the floodplai despite three-to-four-year itervals of o cross-border flows. The flood releases from Lake Mead by the Bureau of Reclamatio 31 produce sufficiet flows to iudate the remaiig floodplai area i the delta ad produce a vegetatio respose the followig summer. Prelimiary observatios suggest that large, cotiuous flows of water i the river are ot ecessary to support the remaiig delta riparia habitats. Coservatio ad restoratio goals might be achieved through two assuraces: 1) that whe surpluses (as defied by the curret capacity for use) arise i the Colorado River system, they will be delivered as flood flows to the delta; ad 2) that agricultural waste flows will cotiue to be coveyed there. 31 The Bureau of Reclamatio terms these floods Stage 1 ad Stage 2 space-buildig releases. 20

28 ENVIRONMENTAL DEFENSE FUND Delta Wetlads ad Riparia Vegetatio South of the iteratioal border, the Colorado River s riparia zoe arrows as it passes through a system of earthe levees built to protect irrigated agriculture from floods. 32 The Rio Hardy also is chaeled, ad at their cofluece, the combied Colorado ad Hardy rivers wide oce agai. Dowstream the river divides ito umerous subsidiary chaels, which the recombie ito a sigle chael before reachig the sea. Plat cover i the floodplai varies i itesity, species compositio, ad habitat value accordig to its positio i the floodplai. I this study, researchers divided the floodplai ito seve zoes based o the domiat plat species associatios idetified through groud surveys. [See Figure 6.] The itesity of biomass (lad cover) was determied by spectral aalyses of satellite images with the umber 1 referrig to the highest itesity ad the umber 4 referrig to the lowest itesity. (For example, R1 refers to riparia vegetatio with high biomass Photo 5. Cottowood trees ad willows i the Colorado River riparia corridor 32 Uless otherwise oted, the fidigs i this sectio are summarized from a report by Carlos Valdés-Casillas ad others (1998c) that has ot yet bee published. 21

29 A DELTA ONCE MORE Figure 6. Vegetatio Zoes of the Colorado River Delta (Valdés-Casillas et al., 1998a) ZONE 1 ZONE 2 Hardy/Colorado Wetlads ZONE 4 ZONE 5 Colorado River Delta Riparia Corridor ZONE 3 ZONE 7 Lagua del Idio R1 R2 R3 R4 W1 W2 DIST. WATER Ciéega de Sata Clara El Doctor Wetlads ZONE 6 Itertid Wetlads Classificatio of vegetatio commuities, usig spectral aalysis of a satellite image (July 15, 1997). R1-R4 iclude riparia vegetatio, with R1 havig the highest biomass level. W1 ad W2 iclude marsh vegetatio, with W1 havig the higher biomass level. DIST refers to the areas covered with salt grass (Distichlis palmerii), ad WATER refers to ope water areas. 22

30 ENVIRONMENTAL DEFENSE FUND itesity; W1 refers to wetlad vegetatio with high biomass itesity.) [See Table 1 ad Box 3.] Geerally, the team foud three types of wetlad ecosystem i the study area: 1) riparia deciduous forest ad woodlad i areas subject to periodic river floodig (Zoes 1 5) domiated by the mesophytic cottowood ad willow trees i the orth (Zoes 1 3), ad by the phreatophyte tamarisk ad other salt-tolerat shrubs as the river approached the tidal zoe to the south (Zoes 4 & 5). 2) maritime, submerget mud flats domiated by the edemic Palmer s salt grass i the tidal portio of the river (Zoe 6). 3) brackish marshlads domiated by cattails ad other emerget hydrophytes i areas flooded with agricultural draiage water i the easter side of the delta (Zoe 7). Dese gallery forests of cottowood ad willow i the ortheaster delta (Zoes 1 3) are cosidered amog the most valuable habitat types i the lower Colorado River regio (Ohmart et al., 1988). These trees have prevailed through droughts ad itese floods. 33 Above Photo 6. Dry mudflats i the Colorado River delta. 33 Zoe 1 is otable for its dese willow stads, which are ow so rare they are o loger listed as a habitat class alog the river above Morelos Dam. Zoes 2 ad 3 cotai approximately 3700 acres (1500 ha) of cottowood ad willow gallery forest, while oly 250 acres (100 ha) of gallery forest remai o the Uited States stretch of river. 23

31 A DELTA ONCE MORE Morelos Dam, these ative trees are rarely domiat. Oly about 250 acres (100 ha) of cottowood-willow forests remai i the Colorado s floodplai i the Uited States. The Rio Hardy/Rio Colorado complex (Zoes 1 5) provides critical habitat for wildlife [see Appedix A]. Zoes 4 ad 5, the Rio Hardy/Rio Colorado wetlads, are the largest brackish wetlad i the delta (23,719 acres [9600 ha]). At the tur of the cetury, this area was described as a gallery forest of cottowoods ad willow, trasitioig to a tidal plai of salt grass ad other halophytes iterspersed with screw bea ad mesquite trees (Gle et al., 1996). By 1977, with the elimiatio of freshwater flows ad their replacemet with brackish irrigatio retur flows, the gallery forest was goe ad salt-tolerat plats domiated the vegetatio. The Rio Hardy/Rio Colorado wetlads have udergoe sigificat chages i the last several decades. From 1947 to 1983, the wetlads covered approximately 45,000 acres (18,000 ha) ad were sustaied by geothermal sprigs ad agricultural wastewater that backed up behid a atural sad dam i the chael. Floods i 1983 icreased the size of the wetlads to 156,000 acres (63,000 ha) but fially broke through the dam, ad the wetlads shruk early i half to 79,000 acres (32,000 ha). I 1986, Mexico bega to improve flood cotrol systems i the Mexicali Valley, buildig up the levees alog the mai Colorado ad creatig draiage caals. These improvemets further reduced the wetlads to oly 2900 acres (1175 ha) of scattered marshlads. Floods o the Gila River i 1992 restored part of the orther portio of the wetlads, but oly temporarily. Zoe 7 (which icludes la Ciéega de Sata Clara) is separate from the mai chael of the Colorado, ad its wetlads are fed mostly by agricultural draiage from the MODE ad Riito caals ad small artesia sprigs. Overall, Zoe 7 cotais 14,350 acres (5808 ha) of emerget, hydrophytic vegetatio ad 5620 acres (2274 ha) of R1 vegetatio. The W1 vegetatio cosists maily of dese cattail stads, while the W2 vegetatio cosists of sparse stads of cattail, bulrushes, ad Palmer s salt grass o the salt-marsh friges. A large area occupied by the low-itesity R3 ad R4 lad-cover classes cosists maily of stuted tamarisk ad iodie bush that have coloized large flats of wet, salie soil i the supralittoral zoe. Zoe 7 also cotais 932 acres (377 ha) of Palmer s salt grass i the tidal area below la Ciéega de Sata Clara, which received both agricultural drai water exitig the marsh ad tidewater eterig from the Gulf of Califoria. Based o this fieldwork, researchers believe that the delta ca potetially support 68,000 residet ad 49,000 oresidet summer birds i the R1 vegetatio of Zoes 1 to I the Uited States, the etire Colorado River is estimated to support fewer tha half as may birds (Ohmart et al., 1988). Because the research team did ot iclude other delta habitat classes, this estimate of the delta s capacity to support bird life is almost certaily a uderestimate. 35 At 150,000 acres (60,000 ha), the Colorado s vegetated floodplai i Mexico is early twice the size of the river s vegetated floodplai i the Uited States (84,000 acres [34,096 ha]) (Balogh, 1996). 34 This calculatio is based o the work of Aderso ad Ohmart (1986). 35 The compariso betwee river reaches i the Uited States ad Mexico is made to emphasize the importace of the delta regio to the overall lower Colorado River ecosystem. 24

32 ENVIRONMENTAL DEFENSE FUND Photo 7. Gulls over the delta 25