Formation of the density currents in the zone of confluence of two rivers

•3D modeling and in situ measurements of mixing near the river confluence are held.•Strong vertical stratification of water composition is found for low flow season.•Vertical stratification is found both downstream and upstream of the confluence.•Vertical stratification of water composition is formed due to the density currents.

SummaryThe peculiarities of the formation of density currents in the zone of confluence of two rivers with strongly different hydrochemical regimes are studied numerically and experimentally. The three-dimensional numerical simulation shows that the water of the river of higher mineralization and density flows under the water of the river of lower mineralization and density and vice versa. And besides, such overlapping of the water streams is observed both upstream and downstream of the confluence of two rivers. The results of numerical simulation are supported by the data of expedition observations and in situ measurements. A similar phenomenon, namely, a flow of two overlapped oppositely directed water streams was previously discovered in the mouth zone of the rivers flowing in the sea. Our study reveals the existence of a new type of the hydrological systems, in which such a phenomenon occurs.