Hydrodynamics and mixing at the confluence of rivers: Influence of density and tide contrasts

Abstract

River confluences are critical points in river networks where strong physical and chemical gradients develop, resulting in a wide range of distinctive environmental conditions (habitats) for biological growth. Large variations in water temperatures, organic matter, nutrients, for example, and in general, in water chemistry have been reported to occur at these sites. As a consequence of their high spatial and temporal heterogeneity of habitats and resources, river confluences behave as biological hotspots, where the number of species appears to increase very significantly in comparison with other river reaches. The effects of river confluences persist downstream, therefore, affecting biological communities and ecological processes at scales of river reaches and channel networks. The spatial extent of the reaches downstream of river junctions where heterogeneous habitat conditions persist, largely depends on the rate at which mixing between the mainstream and tributary waters occurs. The literature on mixing in river confluences is extensive, but still, our understanding of flow and mixing dynamics in these sites is far from complete. In particular, the effect of density contrast between the confluent streams on mixing has traditionally been neglected, which has been justified by differences in the inertia of the confluent flows being much higher than density differences. However, as the scale of the confluent channels increases, the probability of draining different geological terrains also increases which results in an increasing potential for significant differences in density. In this work, we present results of a series of field experiments carried out in a confluence in Northern Spain where the presence of density contrast is important for both the spatial arrangement of the rivers once at the confluence and river mixing.