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

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 conflu ence of the two rivers to the Anavilhanas archipelago was studied for 6 mon ths to determine the hydrochemical dynamics that occur in the lower Negro R iver and evaluate a possible mechanism responsible for the formation of the archipelago. The ionic sum of pure Branco River water was a factor of thre e 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 b icarbonate and organic anions, respectively. In mixing experiments of pure Branco and Negro river waters, no large deviations from linearity were obse rved for any of the major solutes. In situ sampling indicated that most sol utes from individual tributaries acted conservatively upon mixing along the main channel of the Negro River. In contrast, DOG, the N and P fractions, and SO42- showed more variability. The latter was due to proportionally lar ger SO42- concentrations to those of mineral weathering products in the Bra nco River than the Negro River and its tributaries. Trilinear plots and est imates of the runoff from subbasins indicated that lateral discharge from t ributaries and small streams (igarapes) had a small hydrochemical influence at isolated points of the Negro River. Hence, the dominant process influen cing the chemical signature of the Branco River along the study transect wa s its dilution by Negro River water. No evidence of flocculation was observ ed in situ or in the mixing experiments, indicating that the deposition of sediments in the archipelago is not due to the flocculation of kaolinite fr om the Branco River with humic acids from the Negro River, as has been prev iously suggested.