Abstract:We introduce here a model for calculating, for the first time, the annual material fluxes exported by the main tributaries of the Congo River. This model is based on a knowledge of the functioning of the main physiographic systems of the Congo River basin. For the Upper Congo, Kasaï and major right-bank tributaries, we obtained a water flow values of 11Ð2, 10 and 8Ð8 l s 1 km 2 respectively, suspended matter values of 7Ð6, 8Ð9 and 5Ð1 t km 2 year 1 and total dissolved solid values of 16Ð4, 10 and 8Ð6 t km 2 year 1 respectively. We investigated the role played by the main physiographic units in the dynamics of material transfers within this huge concentrically structured basin. The belt of high ground covered by shrubby savanna supplies most of the suspended matter (SM D 24 mg l 1 at Mongoumba on the Ubangi River-200 km above the confluence with Congo River and 32 mg l 1 at Ouesso on the Sangha River) and of the total dissolved solids (TDS D 42 mg l 1 at Mongoumba on the Ubangi River and 41 mg l 1 at Ouesso on the Sangha River). The concentration of suspended matter is reduced by the waters of the 'Cuvette Centrale', a vast basin covered by rain forest (SM D 8 and TDS D 16 mg l 1 for the Likouala aux Herbes basin). Material export fluxes at the sub catchment outlets range from 4 to 9 t km 2 year 1 for suspended matter and from 11 to 31 t km 2 year 1 for total dissolved material, i.e. TDS C DOM (dissolved organic matter). The basin acts as a gigantic organic matter reservoir, which can produce 25 t km 2 year 1 of organic matter in the central flooded zone. Finally, the paper presents the first map of specific mechanical and chemical denuidation rates for the main sub catchments of the Congo basin.
We report 2 yr of monthly data from the Congo River (the second largest river in the world) on dissolved silicon concentrations, biogenic silica particle concentrations, isotopic signatures of dissolved silicon, and biogenic siliceous particle counts. Diatoms predominated in the biogenic silica fraction, especially during low flow; phytoliths and sponge spicules were more abundant than diatoms only during the rainy season, when biogenic silica fluxes are low. Biological processes dominated the seasonal variations of the dissolved d 30 Si ratio that are superimposed on a constant abiotic d 30 Si value of +0.70% 6 0.05% throughout the year. The measured biogenic silica concentration is less than the amount required to explain the monthly variations of dissolved d 30 Si signatures. We use these signatures and a Rayleigh isotopic fractionation model to calculate that 82% 6 7% of the diatoms produced each month would be exported out of the water column, probably through settling in the Malebo Pool or further upstream. The uptake of dissolved silicon by diatoms during low water flow periods could explain the absence of the dilution effect observed for the other major elements. Annual Si export to the estuary is 1.17 3 10 10 mol yr 21 in the form of biogenic silica and 2.23 3 10 11 mol yr 21 in dissolved Si form, with a mean dissolved d 30 Si of +0.96% 6 0.27%. Phytoliths make only a minor contribution to the annual biogenic silica flux, and dissolved Si fluxes predominate over biogenic Si fluxes.
This study compares the total suspended sediment (TSS), total dissolved solids (TDS), and dissolved organic carbon (DOC) dynamics of two major inter-tropical rivers, the Congo in Central Africa and the Orinoco in South America, focusing on sampling undertaken during the period 2006-2010. Both rivers are characterized by similar mean annual discharges into the Atlantic Ocean, on its eastern and western margins, respectively. For both rivers, the results presented are placed in the context of the available longer-term hydrological time series (Congo: 1903(Congo: -2010 Orinoco: 1926 Orinoco: -2010. The key features of the recent records of material flux for both rivers were established and compared with other published data from the 20th century. The available discharge time series show that the rivers are characterized by similar maximum monthly discharges (Congo: 75 500 m 3 s -1 ; Orinoco: 85 500 m 3 s -1 ) and similar interannual variability (Congo: 1.69; Orinoco: 1.66). However, contrasts in low-flow regime (minimum low flows: Congo: 23 000 m 3 s -1 ; Orinoco: 2300 m 3 s -1 ) and seasonal variability (Congo: 3.3; Orinoco: 37.2), as well as in material fluxes were identified. Specific suspended sediment yields for the Orinoco (89 t km -2 yr -1 ) were very significantly higher than those for the Congo (9.4 t km -2 yr -1 ). These differences are mainly explained by contrasts in the physiographic characteristics of the two river basins, such as their contrasting relief. The differences between the TSS fluxes calculated for the Orinoco in this study and those indicated by published data from the second half of the 20th century can be explained by the impact of the hydroelectric dams built in the foothills of the Andean Cordillera. Changes in the TSS fluxes of the Congo are mainly explained by the impact of climatic change on total runoff.
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