The hydrochemical influence of the Branco River on the Negro River and Anavilhanas archipelago, Amazonas, Brazil.

Abstract

The influence of ion- and sediment-rich water from the Branco River on the dilute, acidic water of the Negro River and its tributaries from the confluence of the two rivers to the Anavilhanas archipelago was studied for 6 months to determine the hydrochemical dynamics that occur in the lower Negro River and evaluate a possible mechanism responsible for the formation of the archipelago. The ionic sum of pure Branco River water was a factor of three and four times higher than that of the Negro River at high and low water, respectively, and the dominant anions in the Branco and Negro rivers was bicarbonate and organic anions, respectively. In mixing experiments of pure Branco and Negro river waters, no large deviations from linearity were observed for any of the major solutes. In situ sampling indicated that most solutes from individual tributaries acted conservatively upon mixing along the main channel of the Negro River. In contrast, DOC, the N and P fractions, and SO 4 2- showed more variability. The latter was due to proportionally larger SO 4 2- concentrations to those of mineral weathering products in the Branco River than the Negro River and its tributaries. Trilinear plots and estimates of the runoff from subbasins indicated that lateral discharge from tributaries and small streams (igarapes) had a small hydrochemical influence at isolated points of the Negro River. Hence, the dominant process influencing the chemical signature of the Branco River along the study transect was its dilution by Negro River water. No evidence of flocculation was observed in situ or in the mixing experiments, indicating that the deposition of sediments in the archipelago is not due to the flocculation of kaolinite from the Branco River with humic acids from the Negro River, as has been previously suggested.