Hydrological and geochemical characteristics of the Jamari and Jiparana river basins (Rondonia, Brazil)

The authors investigate the hydrological and geochemical characteristics of the Jamari (30430 km 2) and Jiparana (60350 km 2) river basins (Amazonia), during the period 1978-1984. A spectral analysis of Fourier is applied to time series of mean monthly river discharges, in order to assess the contribution (7 to 8%) of the surface runoff to the total river flow. The mean annual runoff coefficient calculated for the Jiparana river basin (36%), is higher than for the Jamari (32%), and this coefficient increases during the study period, only for the Jiparana. The total specific suspended sediment discharge calculated for both rivers shows the same value 13 t/km2/y, and the estimated suspended sediment concentration in the surface runoff is slightly superior for the Jiparana river (0.3 g/ 1) than for the Jamari one (0.2 g/l). The river suspended sediments are mainly composed of kaolinite, quartz and feldspar, but the Jiparana is more enriched in quartz. For both rivers, the dominant clay mineral is the kaolinite which is in agreement with the rock weathering type determined for both basins using the Tardy's weathering index: the monosiallitisation. The total chemical erosion rate calculated after correction for the atmospheric inputs (ions and CO2), is higher for the Jiparana (10.11 t/km2/ y) than for the Jamari river basin (7.75 t/km2/ y). These values are lower than the mechanicald enudation rate calculated previously for both river basins