Characterisation of river bed and suspended sediments in the Rio Madeira drainage basin (Bolivian Amazonia)

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1 I PERGAMN Journal of South Amercan Earth Scences 12 (1999) Journal of South Amercan Earth Scences Charactersaton of rver bed and suspended sedments n the Ro Madera dranage basn (Bolvan Amazona) Jean LUP Guyota9*, Jean Mare Jouanneau b, Jean Gabrel WassonC I "IRD, CP 7091, Lago Sul, CEP , Brasla DF, Brazl bdepcrtmet Ghologe et c&"rcr(,he, URA 197 CNRS, Urwsté Bordenrs 1, Avetre des Facrrlt&s, Talence, Frnce 'CEMAGREF, Laboratore Hydro-Ecologe Qrarttctve, BP 220, Lyon Cedes. 09, Frace Abstract The gran sze dstrbuton of rver bed sedments n the Ro Madera dranage basn shows rapd deposton of the coarser materal (> I mm) as the rver flows out of the Andean range to enter the Amazon plan (Llanos). In the Andes, the suspended sedment gran sze vares between 0.02 and 0.10 nm n dameter, whle n the Llanos the suspended load s domnated by fne slts, between 7 and 13 pm n dameter. In the Llanos, nether the bed sedment nor the suspended sedment vared along a 800 km reach Elsever Scence Ltd. All rghts reserved. Resumen La granulometra de los sedmentos transportados sobre et lecho de los cursos de agua (arrastre) demostró que el materal ms grueso (> I mn) se deposta rpdamente en la zona de pe-de-monte. En los Andes, et tamaño del materal en suspensón vara de 0.02 a 0.10 nm, mentras que en los Llanos el sedmento transportado es lmo fno, con una granulometra entre 7 y 13 pn. En los Llanos, no es perceptble cualquer evolucón en el tamaño de los materales en suspensón, o de los sedmentos de fndo, de aguas arrba hasta agtas abajo, sobre 800 km Elsever Scence Ltd. All rghts reserved. 1. Introducton Knowledge about the gran sze dstrbuton of the rver bed and suspended sedment s fundamental to the study of geomorphologcal process along a fluval hydrosystem. Modellng sedment load by means of transport equatons, estmatng the major and trace element fluxes n the fne fracton transport, understandng sedment transfer processes, dentfyng sedment deposton areas, calculatng sedmentaton/suspenson and waterlsedment dscharge ratos, all requre good granulometrc data. Moreover, the bologcal component of rver ecosystems s strongly dependent on * Correspondng author. E-na1 address: jean.guyot@aps.com.br (J.L. Guyot). t"-&sj ' u /99/$ - see front matter Elsever Scence Ltd. All rghts reserved. PII: S (99) _ the structure and stablty of the bed sedment, as well as water turbdty. Thus sedment dynamcs are a key factor n understandng the ecologcal functonng of many rvers, especally n the Amazonan area (Sol, 1984)..The Ro Madera s the most mportant southern trbutary of the Amazon wth a mean annual dscharge of 31,200 m3 s-l (Molner et al., 1997), and s the rver that drans the Andean range, the Brazlan sheld and the Amazon plans (or "Llanos"). Despte the geographcal mportance of ths trbutary, data on gran sze dstrbuton are extremely lmted for bottom sedment, and totally lackng for suspended sedment matter. The present study ams (1) to provde the results of numerous granulometrc observatons carred out n the Bolvan part of the Ro Madera dranage basn,

2 ~ ~ J.L. Guyot et al. 1 Journal of South Amercan Earth Scences 12 (1999) r.-q Fg. 1. PHICAB gaugng statons (0) n the Amazonan dranage basn of Bolva, and sedment samplg ponts along the (see Tables 1 and 2 for staton codes). and (2) to pont out salent features of sedment structure characterstcs and dynamcs along ths poorly known hydrosystem. Most of the data were collected between 1983 and 1992 wthn the scope of the Hydrologcal and Clmatologcal Program of the Bolvan Amazon Dranage Basn-PHICAB; RSTM: Insttut Franps de Recherche Scentfque pour le Développement en Coopératon; SENAMHI: Servco Naconal de Meteorología e Hdrología de Bolva; UMSA: Unversdad Mayor de San Andrés, La Paz, Bolva (Guyot, 1993), and durng hydrobologcal studes (Wasson and Marn, 1992). 2. Study area The Bolvan part of the Ro Madera dranage basn (Fg. 1) covers, 903,500 km2, extendng over the Andes (25%), the Brazlan sheld (27%), and the Llanos (48%). At the Brazlan fronter (Vllabella), mean annual dscharge of the Ro Madera s 18,000 1n3 s-l, wth a mnmum low value of 2000 m3 sl1 and a flood maxmum of 52,000 m3 s- (Bourges and Hoorelbecke, 1995). At Vllabella, the sedment load s estmated at 230 x 10 t yr- ; 72% of the sedment load s suppled by the and 28% by the Ro Mamore, the two man Andean trbutares of the Ro Madera, orgnatng n the eastern Peruvan and Bolvan mountan ranges. The regonal sedment budget of the Madera rver bsn shows that more than 50% of the suspended yeld exported by the Andes wll sedment n the Amazonan floodplan (Guyot et al. 19S8, 1989). Ths large and relatvely prstne basn presents a wde range of clmatc and geomorphologcal features. Alttude vares from 6500 to 120 m, and ranfall ranges from 200 to more than 6000 mm yr-. In the Andes, ndvdual watersheds are heavly contrasted; they range from sem-ard, hghly erodble basns ncsed n the Altplano Tertary-Quaternary sedmentary seres (such as the Ro La Paz watershed), to densely vegetated hyper-humd basns on Palaeozoc rocks (such as the Yungas valleys), or Tertary sedmentary seres

3 J.L. Guyot et al. I Jolrtal of South Atnercan Earth Scences I2 (1999) Fg. 2. Sedment samplng ponts localsaton n the Andean Ro Alto-Ben drgnage basn (see Tables 1 and 2 for staton codes). n the Alto-Ben foothlls. The Bolvan Llanos s a flat plan of recent alluval sedments, whose level s controlled by the Precambran Brazlan Sheld outcrops upstream of Vllabella. About 150,000 km are flooded annually n ths regon (Roche and Fernandez, 19SS). Sedments have been sampled n these three man valley types, and wll be referred to n the followng text as (1) sem-ard for the Ro La Paz and trbutares, and (2) Yungas for the Ro Huarnlla- Coroco-Kaka stem; the Ro Alto-Ben drans both types and (3) Llanos for the Amazonan floodplan rvers (see Fg. 2). All the rvers sampled orgnate n the Andean range, whch s the man source of the sedment yeld. 3. Methodology 3.1. Bed sedments The granulometry of coarse bed sedment n the Andes was determned n stu by a rope method derved from Wolman (1954). A graduated rope s placed randomly wthn (f possble) or nearby the stream bed

4 404 J.L. Guyot et al./ Journal of South Amercat Earth Scences 12 (1999) Table 1 Rver staton characterstcs and rver bed sedment medan dameter (dso) Code Rver staton Alttude (m) Dstance from Vllabella (km) d50 ("1 c1 c2 c3 c4 CS CG c7 C8 KK LI L2 L3 L4 BA AB B2 B3 B4 B5 BG B7 BS B9 PC Ro Huarnlla near the Cumbre Ro Huarnlla at the Inca brdge Ro Huarnlla at'choro Ro Huarnlla at Charo Ro Coroco at San Pedro Ro Coroco at IBTA Staton Ro Coroco at Santa Fe Ro Coroco at Alcoche Ro Kaka at Puerto Pando Ro Achuman at La Paz Ro La Paz at Puente Lpar Ro Palca at Tahuapalca Ro La Paz at La Plazuela Ro Alto-Ben at Chepte at Angosto del Bala Ro Madd at Portachuelo I I at low flow. 'Space between rope graduatons s about twce the dameter of the largest elements.of the sedment. Each element found under a graduaton s measured by passng t through a granulometer, whch s analogous to a seve seres n geometrc progresson accordng to the Ph scale (wth half-ph graduaton). The gran sze of the fnest elements s referred to as fne gravel (2-8 mm), coarse sand (0.5-2 mm), fne sand ( mm), or slt (< mm) by a vsual comparson wth seved sedment. 50 elements are measured by sample (Kellerhals and Bray, 1971) and samplng s replcated 2-5 tmes accordng to the apparent heterogenety of the substrate. In the larger Llanos rvers wth sandy substrate, the gran sze was obtaned by the classcal method of sevng and weghng of a rver bed sedment sample, n the SENAMHI Laboratory (El Alto, Bolva). In stu granulometrc measures and rver bed sedment samples were collected from the dranage basn durng the 1988 and 1989 low flow perods Susyetded matter At the PHICAB statons n the Llanos (Fg. l), suspended sedment samples were collected near the surface, n the mddle of the rver. Measurements made wth depth ntegratng samplers at gaugng statons exhbted good homogenety of suspended sedments over the whole cross-secton (Guyot, 1993). The sedments collected on flters were extracted by passng through an ultrasound tank. Partcle-sze dstrbuton analyses were then performed on the suspenson, usng a laser gran-sze measurng devce (3600E Malvern model) n the Bordeaux Unversty Laboratory (France). The suspended sedment samples of Ben and Mamore rvers n the Llanos were provded by the PHICAB Network durng the perod. For the Andean rvers, suspended sedments were collected durng the March 1988 flood perod. 4. Results and dscusson 4.1. Bed sednents A notceable dfferkce appears between the bed sedments of the two man valley types (Table 1). In the Yungas, along the Huarnlla-Coroca-Kaka rver transect (from 4000 to 380 m - C1 to C&, KK - Fg. 2), d50 values decrease normally fro& 200 to 60 mm (Fg, 3), ndcatng the progressve settlng of largest partcles. In the sem-ard valley, alopg the La Paz-Palca-Alto Ben rver stem (from 3550 to 300 m - - L1 to L4, LU, AI - Fg. 2), no sgnfcant ds0 varaton s observed downstream, suggestng a mass transport of unsorted sedment. Ths s due to the partcular granulometrc composton of the sedments n these basns. Some samples (L3 and L4) showed two man modes, one of whch corresponds to a stock

5 J.L. Guyot et al. 1 Jornal of Soìtl Antercat Earth Scemes 12 (1999) % Ro Coroco - Kaka Andes mm - CI ---_ c2... c3 c4 c5 -D- C6 --A-- c7.... Ca.- KK ? -., * ;... Ro La Paz - Alto Ben Andes % t o I L4 -.._ BA I0 % 50 - AB ---o o--- EM * e6 07 I o I mm Fg. 3. Rver bed sedmentary granulatory of the dranage basn (see Table 1 for staton codes).

6 ~ 406 J.L. Guyot et al. 1 Journal of South Amercan Earth Scences 12 (1999) I ANDES 1 Ro Coroco \ I Kaka o0 $,.. _ lo c loo Dstance from Vllabella (km), s,o o tl' I... U :A La Paz - Alto Ben y Andes lmt I %....!. a... A A a I : , d Dstance from Vllabella (km).... r 1. FI ~... o o. I... Andeslmt 0 :......_....> r d.., 0.1 n E u) 3 ua b Dstance from Vllabella (km) Fg. 4. Longtudnal secton, rver bed and suspended sedment medan dameter (d50) of the dranage basn.

7 J.L. Guyot et al. I Journal of Soth Anercar Earth Scetces I2 (1999) Table 2 Rver staton characterstcs and suspended sedment medan dameter (d5") Code' Rver staton Alttude (m) Dstance from Vllabella (km) Number of samples dso (pm) dgo (pm) dto (pm) LI Ro Achuman at La Paz L4 Ro La Paz at La Plazuela LU Ro Lurbay at Lurbay C Ro Consata at Consata AI Ro Alto-Ben at Angosto Incua AB at Rurrenabaque PC at Portachuelo CE at Cachuela Esperanza AP Ro Grande at Abapo PV Ro Ichlo at Puerto Vllarroel PG Ro Mamore at Puerto Ganadero PS Ro Mamore at Puerto Sles GM Ro Mamore at Cuayaramern ' L1, L4, LU, C and AI data from Barragan, of fne sand (10.1 mm) and the other, to a coarse stock (> 10 nm). Such bed sedments are hghly unstable, and scoured very frequently due to the energy of these hgh gradent streams (slope >3%), as evdenced by the scarcty of the benthc fauna (Wasson and Marn, 1992). Ths partcularty s due to the fne sedment load of the Andean rvers (ros La Paz, Sapahaqu and Lurbay) whch dran the hghland quaternary basn of the Altplano. ther data obtaned from bed load samplng n the same regon seems to corroborate these fndngs. In the Yungas valley, the d50 of the materal transported s 26 nm for the Ro Unduav at Srupaya (alt m - ENDE, 1980), lower than the bed sedment d50 values for the same alttudnal range n our data (Table 1). Conversely, bed load samplng on the Archuman and Huayllan rvers, two trbutares of the Ro La Paz, resulted n d50 values rangng from 3 to 80 mm durng flood events, wth an average of 25 mm (Bourges et al., 1995), very close to the bed sedment d50. The most abrupt dscontnuty n the bed sedment composton occurs at the transton between the Andes and the Llanos. Stes located before the outflow of the Andes (BA and AB - Fg. 2) have a bed sedment made of gravel and pebbles wth d50 near 30 mm. As the enters the Llanos, the same granulometrc composton of the bed sedment s observed on the frst 20 km (B2 and B3), but then the medan dameter decreases drastcally over 10 km (Fg. 4), to reman remarkably stable thereafter along the next 800 km wth a djo rangng from 0.09 to 0.15 mm. The same stablty over long dstances has been observed n other large tropcal rvers, such as the Amazon and the rnoco (Nordn et al., 1980; Mertes and Meade, 1985). In fact, partcle-sze dstrbuton of sedments vares from one ste to another on a gven stream, but there does not seem to be an overall trend of downstream fnng. The mean value of the medan gran sze of the bed sedments n the Llanos s 0.11 mm; ths value s consstent wth Taborga's data (1964) for the Ben rver. The samples of the Ro Mamore and ts trbutares, from the Andean pedmont to Guayaramern, show a more regular decrease of partcle szes. The d50 of the Mamore sedments n the Llanos range from 0.02 to 0.08 mm, wth an average of 0.04 mm (Taborga, 1963). The d5o of the Ro Mamore bed sedments s three tmes smaller than that of the. Such sedment characterstcs are explaned by the dfference n gradent between the two streams, as well as the extent of ther flood plans, whch are seven tmes larger n the case of the Ro Mamore basn (Roche and Fernandez, 1988). The medan ldameter of bed sedment partcles n the Bolvan Llanos rvers flowng out from the Andes (around 0.1 mm) appears to be lower than n other rvers n the Amazonan range: 0.25 mm for the Amazon, 0.4 mm for the rnoco, and 0.34 mm for the Ro Guapore-Itenez (Taborga, 1967), a trbutary of the Ro Mamore whch drans the Brazlan sheld. The coarser sedments of the Ro Itenez may be explaned by ancent hydraulc condtons, snce present day current suspended matter concentratons are always very low (an average 30 mg/l, whch made gran-sze analyss very dffcult) Suspended nutter In the Andes, the gran sze dstrbuton of suspended sedment has been determned by means of samples taken from some trbutares of the durng the March 1988 flood perod (Barragan, 1990; Guyot et al., 1993). Suspended sedments seem to be made up of fne sands and slts, although the clay fracton always exceeds 10%. The medan gran sze n such suspensons vares between 0.02 and 0.10 mm

8 J.L. Guyot et al. Jorlrnal of South Amerc n Earth Scences 12 (1999) IW Suspendend Sedment Gan sze (pm) Suspended Sedmant Gran Sze (p) W Suspended Sedment Gran Sze (pm) M 1MM suspended Sedment Gan sze (pm) low Suspended Sedment Gran Sze (pm) IW 1MM suspended Sedment Gran Sze (pm)...:i.: Suspended Sedment Gran Slze (pm) I w loo0 suspended sednent Gran sre (pm) Fg. 5. Suspended sedment granulometrc curves of the PHICA gaugng statons n the Llanos (see Table 2 for staton codes). (Table 2), whch s the samle order of magntude of the bed sedments n the Amazon plans. From upstream to downstream, suspended sedment d50, progressvely decreases (Fg. 4), except for the Ro Achuman (L1 - Fg. 2), whch has a low d50 value at 3550 m. Ths partcularty s due to the fne fracton of the Altplano quaternary sedments. In the Llanos, the results obtaned durng the hydrologcal year ndcate that, over the whole basn, suspended matter s essentally slt and the per-

9 J.L. Guyot et al. 1 Journal of South Amercan Earth Scences 12 (1999) \ A \ Y._ N al v1 I- E 25 o d50 - & 0 dl Dscharge (m3.s-1) E 50 n a Y d)._ N u).- C E 25 o 75,.."".." ì o... 18,. o o," o d10 o -_ - -I I Suspended sedment (mg.1-1) Fg. 6. Suspended sedment gran sze vs dscharge and suspended sedment load, Ro Mamore et Puerto Ganadero (PG). centage of fne sands s always low. The suspended sedment d50 value n the Amazonan plan presents no sgnfcatve varaton from upstream to downstream. The transton from the Andes to the Llanos s not as marked as for the bed sedments (Fg. 4). Systematc gran-sze analyses of suspended sedment for the complete hydrologc year have been carred out usng samples taken at 10-day ntervals at two statons on the Ro Mamore (PG and PS - Fg. 1). Results show lttle varaton throughout the year (Fg. 5) and no relaton between dstrbuton of gransze fractons and dscharge or suspended sedment load (Fg. 6). The mean medan gran sze of suspended sedment was smlar at all statons n the Llanos, and vared from 7 to 13 pm (fne slts). The medan gran sze of suspended matter vared from 2 to 20 pm n the Amazon (Meade, 1985) and from 6 to 16 pm n the Ro Paraná (Drago and Amsler, 1988), whereas, n the Congo, t vared from 9 to 14 pm (Jouanneau et al., 1990). Lke rver bed sedments, suspended sedment partcle sze does not vary much from upstream to downstream. The suspended sedment's dameter n the two large rvers ( at Cachuela Esperanza and Ro Mamore at Guayaramern - CE and GM - Fg. 1) s smlar (d50 = 9 pm), equvalent to approxmately one-tenth of the bed sedments dameter. 5. Concluson Ths frst nsght nto the granulometrc composton of rver bed and suspended sedments n the Bolvan Ro Madera dranage basn has shown a clear dstncton and a very sudden transton between the Andes and the Llanos. Emphass must be gven to the transton zone at the Andean pedmont; a strongly subsdent area, whch s

10 ,, 410 J.L. Gtfyot et al. 1 Journnl of South Amercan Earth Scences 12 (1999) s the preferental accumulaton zone for the coarse materaí of Andean orgn. nly the fne partcles are transported towards the Amazon, wth the flood plans playng a regulatng role n the transfer. Ths explans the homogenety of bed and suspended sedment gran sze over several hundreds of klometers, n both the Ben and the Mamore rvers, and the lack of relatonshp between rver dscharge and gran sze dstrbuton n the lower part of these rvers. Wthn the Andean range, the two man geomorphologcal valley types referred to as sem-ard and Yungas sampled n our study also exhbted dstnct granulometrc characterstcs, correspondng to dfferent sedment structure and transport processes. Such dfferences are reflectsed n substratum stablty and water qualty, and have mportant bologcal mplcatons. Thus geomorphologcal valley type appears to be a relevant descrptor of rver functonal types n aquatc ecology. Acknowledgements We acknowledge H. Calle (SENAMHI, El Alto, Bolva) for granulometrc determnatons on rver bed sedments, G. Hanaut (Unv. Bordeaux, France) for granulometrc determnatons on suspended sedments, and J.F. Dumont (RSTM, La Paz, Bolva) for rver bed sedments n stu measures. References Barragan, M.C Estado sedmentológco de la cuenca andna del Río Ben. Tess de grado, Unv. La Paz, Bolva, 267 pp. Bourges, J., Hoorelbecke, R., Varaton du régme des écoulements dans le système ando-amazonen de Bolve. In: Servat, E., LeBarbe, L. (Eds.), Régonalsaton en hydrologe. RSTM, Pars, pp Bourges, J., Rbsten, P., Detze, C., Guyot, J.L., Hoorelbecke, R., Flux et crues sngulères d un pett cours d eau andn ou les effets pervers de l urbansaton. Revue de Géographe Alpne 1995 (l), Drago, E.C., Amsler, M.L., Suspended sedment at a cross secton of the mddle Paraná rver: concentraton, granulometry and nfluence of the man trbutares. In: Bordas, P., Wallng, D.E. (Eds.), Sedment budgets. IAHS, Wallngford, UK, pp ENDE, Empresa Naconal de Electrcdad SA, Proyecto hdroeléctrco Sakhahuaya, Hdrologa y sedmentologa. ENDE, Cochabamba, Bolva, 121 pp. Guyot, J.L Hydrogéochme des fleuves de l Amazone bolvenne. Coll. Etudes et Thèses, RSTM, Pars, France, 262 pp. Guyot, J.L., Bourges, J., Hoorelbecke, R., Roche, M.A., Calle, H., Cortes, J., Barragan, M.C., Exportaton de matères en suspenson des Andes vers l Amazone par le Ro Bén, Bolve. In: Bordas, P., Wallng, D.E. (Eds.), Sedment budgets. IAHS, Wallngford, UK, pp Guyot, J.L., Bourges, J., Calle, H., Cortes, J., Hoorelbecke, R., Roche, MA., Transport of suspended sedments to the Amazon by an Andean rver: the Rver Mamore, Bolva. In: Rver Sedmentaton. IRTCES, Bejng, Chna, pp Guyot, J.L., Jouanneau, J.M., Quntanlla, J., Wasson, J.G., Les flux de matères dssoutes et partculares exportés des Andes par le Ro Bén (Amazone Bolvenne), en pérode de crue.. Geodnamca Acta 6 (4), Jouannesu, J.M., Lapaquellere, Y., Latouche, C., Tastet, J.P., Résultats prélmnares de la campagne ubangu-congo de Novembre Bull. des Scences Géologques 43 (l), Kellerhals, R., Bray, D.I., Samplng procedures for coarse fluval sedments. Amercan Socety of Cvl Engneers 97, Meade, R.H., Suspended Sedment n the Amazon Rver and ts Trbutares n Brazl durng , US Geologcal Survey pen Fle Report , Denver, USA, 39 pp. Mertes, L.A.K., Meade, R.H., Partcle szes of sands collected from the bed of the Amazon Rver and ts trbutares n Brazl durng , US Geologcal Survey pen Fle Report , Denver, USA, 16 pp. Molner, M., Guyot, J.L., Call.de, J., Gumarges, V., lvera E, Flzola, N., Hydrologe du bassn amazonen. In: Théry, H. (Ed.), Envronnement et développement en Amazone bréslenne. Beln, Pars, pp Nordn, C.F., Meade, R.H., Curts, W.F., Bóso, N.J., Landm, P.M.B., Sze dstrbuton of Amazon Rver bed sedment. Nature 286, Roche, M.A., Fernandez, C., Water resources, salnty and salt yelds of the rvers of the Bolvan Amazon. Journal of Hydrology 101, Sol, H., The Amazon. W. Junk, The Hague, 763 pp., Taborga, J., El transporte fluval en Bolva. Tomo I: Cuenca de los ros Ichlo y Mamore. Fuerza Naval Bolvana, La Paz, Bolva, 169 pp. Taborga, J., EI transporte fluval en Bolva. Tomo II: Cuenca de los ros Ben, Madre de Dos y rton. Fuerza Naval Bolvna, La Paz, Bolva, 169 pp. Taborga, J., El transporte fluval en Bolva. Tomo III: Cuenca des Ro Itenez. Fuerza Naval Bolvana, La Paz, Bolva, 77 pp. Wasson, J.G., Marn, R., Tpología y potencaldades bológcas de los ros de altura en la regón de La Paz (Bolva). Memora de la Socedad de Cencas Naturales La Salle 48, Wolman, G., A method of samplng coarse rver bed materal. Trans. Amer. Geoph. Unon 35 (6),