Luis Noriega

SALINITIES AND SEDIMENT TRANSPORT IN THE BOLIVIAN HIGHLANDS

Guyot, J.L., Roche, M.A., Noriega, L., Calle, H. and Quintanilla, J., 1990. Salinities and sediment transport in the Bolivian highlands. J. Hydrol., 113: 147-162. Salinities and sediment loads of the rivers of the entire interior drainage basin of the Lake Titicaca, Rio Desaguadero, Lake Poopo and salars, as well as their evolution from upstream to downstream, have been characterized on the basis of the results of four samplings during runoff periods. On the other hand, the regimes of dissolved and suspended matter transport are characterized with perioclic measurements and samplings at hydrometrical stations over several years. The junction of Lake Poopo and the Salar of Coipasa, since 1985 is noted, which recalls that an unique terminal lake of large area existed during some parts of the Quaternary. Mechanical and chemical erosion rates have been calculated for the basins that drain the Western (Rio Mauri) and Eastern (Rio Suches) Cordilleras of the Bolivian Andes. Most of the sediment comes from the Western Cordillera, with a mechanical erosion rate of 640 t kmz yr-I for the basin of the Rio Mauri. On the other hand, Lake Titicaca is the main source of dissolvea matter to Lake Poopo via the Rio Desaguadero.

Hydroclimatology of the Upper Madeira River basin: spatio-temporal variability and trends

ABSTRACT Rising in the Andes, the Madeira River drains the southwestern part of the Amazon basin, which is characterized by high geographical, biological and climatic diversity. This study uses daily records to assess the spatio-temporal runoff variability in the Madeira sub-basins. Results show that inter-annual variability of both discharge and rainfall differs between Andean and lowland tributaries. High-flow discharge variability in the Andean tributaries and the Guaporé River is mostly related to sea surface temperature (SST) in the equatorial Pacific in austral summer, while tropical North Atlantic (TNA) SST modulates rainfall and discharge variability in the lowlands. There also is a downward trend in the low-flow discharge of the lowland tributaries which is not observed in the Andes. Because low-flow discharge values at most lowland stations are negatively related to the SST in the tropical North Atlantic, these trends could be explained by the warming of this ocean since the 1970s. EDITOR A. Castellarin ASSOCIATE EDITOR A. Viglione

River Mixing in the Amazon as a Driver of Concentration-Discharge Relationships

Large hydrological systems aggregate compositionally different waters derived from a variety of pathways. In the case of continental-scale rivers, such aggregation occurs noticeably at confluences between tributaries. Here we explore how such aggregation can affect solute concentration-discharge (C-Q) relationships and thus obscure the message carried by these relationships in terms of weathering properties of the Critical Zone. We build up a simple model for tributary mixing to predict the behavior of C-Q relationships during aggregation. We test a set of predictions made in the context of the largest worlds river, the Amazon. In particular, we predict that the C-Q relationships of the rivers draining heterogeneous catchments should be the most “dilutional” and should display the widest hysteresis loops. To check these predictions, we compute 10 day-periodicity time series of Q and major solute (Si, Ca2+, Mg2+, K+, Na+, Cl-, SO42-) C and fluxes (F) for 13 gauging stations located throughout the Amazon basin. In agreement with the model predictions, throughout the Amazon Basin C-Q relationships of most solutes shift from fairly a “chemostatic” behavior (nearly constant C) at the Andean mountain front and in pure lowland areas, to more “dilutional” patterns (negative C-Q relationship) towards the system mouth. More prominent C-Q hysteresis loops are also observed at the most downstream stations. Altogether, this study suggests that mixing of water and solutes between different flowpaths exerts a strong control on C-Q relationships of large-scale hydrological systems.