Hydrology

All mixed up?

Credit: FLAVIO GSOUZA

Large rivers are fuelled by waters originating from distinct regions. These different water masses each carry their own chemical fingerprint, but they are thought to blend into one when they reach the main river owing to turbulent mixing.

The rate of turbulent mixing — and thus the degree to which river waters mix — is important for determining the fate of different water masses and the spread of pollutants, but is difficult to measure and hence uncertain. Equally uncertain is the influence of riverbed morphology on mixing.

Now Julien Bouchez, of the Institut de Physique du Globe de Paris, France, and colleagues show that water in the Solimões River, the largest tributary of the Amazon River, is poorly mixed (Earth Planet. Sci. Lett. doi:10.1016/j.epsl.2009.11.054; 2009). They examined the sodium concentration and strontium isotopic composition of river water in a cross-section of the Solimões River. Lateral heterogeneities revealed the presence of two distinct water masses originating from two different sources.

One of the water masses contained high levels of the radiogenic strontium isotope and low levels of sodium, and probably originated from a lowland source. They suggest that the Purus River, which discharges into the Solimões River around 100 km upstream of the sampling site, is the source of this radiogenic water. The other water mass was defined by lower levels of radiogenic strontium and higher concentrations of sodium, and probably reflected the Solimões mainstream, which originates in the Andes.

Using the sodium concentration data they estimated the efficiency of turbulent mixing in the Solimões River; despite the presence of distinct water masses, efficiency was high compared with other rivers. They attribute this to the presence of islands and sand dunes on the riverbed, which are thought to increase the efficiency of lateral mixing.

The above results, together with previous studies, demonstrate that chemical heterogeneities can extend up to tens of kilometres downstream from confluences in large rivers; the distance could be even greater in the presence of smooth riverbeds, which are common in engineered channels.

If these findings are representative of other large rivers, estuaries around the world could contain the chemical signatures of several tributaries.

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Armstrong, A. All mixed up?. Nature Geosci 3, 76 (2010). https://doi.org/10.1038/ngeo769

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