Manifestation of the Oka-Volga confluence zone during the ice melting in radar and optical satellite images

Abstract

Investigation of the mixing processes in river confluence zones is an important problem, particularly in application to ecological monitoring of rivers and inland waters. Such processes can be very complex and can affect riverbed deformation, mixing and pollution transport, etc. A typical example is a confluence zone (CZ) of the Volga and Oka rivers (Russia). In this paper, the Volga/Oka CZ signatures in satellite optical (Sentinel-2 MSI) and radar (Sentinel-1 SAR) images are analyzed for the period of ice/snow melting in early spring. First, the ice melting appears immediately downstream of the meeting point of the rivers. At the early stages of the ice cover melting the CZ is seen as a narrow dark band in the optical images and as a bright band in radar images because of wet snow/ice cover within the CZ. The faster melting in the CZ than outside can be associated both with the emissions of warm water from factories/thermal power plants and with water transportation towards the CZ by weak transverse currents in the river flows. The enhanced radar backscatter in the CZ can be associated with an increase of the snow/ice moisture and thus an increase of snow/ice cover roughness. With air temperature increase, an open water patch with fragmented ice appears transformed later into a long polynia. The polynia is manifested as a dark band in radar imagery presumably due to the suppression of wind waves associated with fragmented ice. This happens even at moderate wind velocities normally exceeding the threshold of wind wave excitation.