Rivers continuously discharge dissolved material to the oceans. Dissolved compounds partially result from water–rock interactions, which produce a large range of water chemical and isotopic compositions. These waters are collected by rivers, that are commonly assumed to be well-mixed with regard to their different tributaries, as a result of turbulent dispersion. In this paper, we test this hypothesis on the Solimões River (at Manacapuru), the largest tributary of the Amazon River, by analyzing the sodium concentration and strontium isotopic composition of river water on a transverse section at different depths. High-precision measurements reveal lateral heterogeneities. This reflects poor mixing between two main river masses, that have distinct chemical and isotopic signatures, a hundred kilometers downstream from their confluence: the Solimões mainstream and the Purús River. Using sodium concentration data, the transverse dispersion coefficient is estimated for the studied Solimões reach (the Earth's largest river on which such an estimate now exists), and is found to be 1.8 ± 0.2 m2/s. Comparison with previously reported data highlights the potential role of bed morphology and islands in the efficiency of lateral mixing in large rivers. We finally demonstrate that the characteristic length of lateral mixing downstream from confluences in large rivers is at least of several tens of kilometers.