Summary
Isotopic concentration was detected in different spots of Rio Negro, Rio Solimões and Rio Amazonas at regular intervals, for 12 consecutive months. Values as found in Rio Negro waters were significantly different. Difference between isotopic concentration found in Rio Negro and Rio Solimões was considered as useful information for identifying the spot where Rio Amazonas, formed by the confluence of Negro and Solimões, presents a homogenous vertical and horizontal structure after complete mixing of both rivers. A perfect mixture was not found up to 120 km below the confluence. Systematic sampling was found to be a method for the determination of relative outflow of both rivers. Data as obtained are close to the results of direct measurements performed by Oltman et al (1963).