Satellite and in-situ observations of a river confluence zone

Abstract

River confluence is a ubiquitous phenomenon which plays an important role in river dynamics, mixing processes, pollution transport etc. It can be often visually observed that two converging rivers continue to flow as two parallel weakly mixing streams separated by a relatively thin transition region – a mixing zone (MZ), which can reach out for some distance downstream the junction apex. A typical example of a river confluence is the merging of the Volga and Oka rivers (Russia). This paper presents some new results on the Volga/Oka rivers confluence based on both ‘in situ’ measurements of hydrological characteristics in the MZ and on satellite observations. During the ‘in situ’ measurements a large set of data regarding velocity fields, chlorophyll-a concentration, water temperature, turbidity, parameters of organic surface films etc. was obtained. It is found that significant differences between the hydrological characteristics of the Volga and Oka flows are observed at sufficiently large distances downstream the junction apex (about 10 river widths or more); and the mixing zone remains quite narrow. Film sampling and further analysis of the surfactant films in the MZ were carried out. It is shown that the surface tension decreases and the film elasticity grows in the foam/slick bands separating the Volga and Oka flows thus indicating the increased concentration of surfactants in the bands. Satellite images of the Volga-Oka MZ are collected indicating that the confluence area is clearly seen in satellite optical imagery (MSI Sentinel-2 satellite) due to strong difference in color between the flows. The radar imagery shows the mixing zone in the form of a slick/foam band (SAR Sentinel-1) which manifests on the water surface due to enhanced damping of short wind waves.