Abstract

The paper investigates the transformation of longitudinal profile of rivers under the impact of anthropogenic changes in the river-mouths, with the case study of the rivers of Lake Sevan basin.

The anthropogenic changes in the level of Lake Sevan took place during the last 70 years after the decision was made to deepen the channel of the only river flowing out of the lake, the Hrazdan, with the aim of using the lake water for irrigation and energy generation.

In the world limnology, there has been no other case when a lake level would be artificially lowered by 18 meters during 3-4 decades (1930-70s), and by another 2 meters in the last decade (1990-2000).

The decrease of base level of erosion caused the activization of channel processes of flowing into the lake rivers. It brought about the violation of the balanced profiles of river valleys, formed within thousands of years. The active down-cutting erosion destroyed the foundations of bridges and led to their collapse.

Although these processes, as a result of a relative stabilization of the lake level, were left behind, their influence passed to the other components of the ecosystem like a chain reaction.

Today a new problem has emerged for Lake Sevan. In the recent five years the water level of the lake has increased by more than two meters, which is the effect of the growth of surface flow and decrease of outflow from the lake. The level growth is a threat of new wave of eutrophication to the lake.

Thus, the river-mouths of rivers, which flow into the lake, have shifted (now, in the reverse direction, i.e. up the stream). And the earlier riverbeds formed at the bottom of the lake get filled with young loose river-drift sediment.

Keywords: Lake Sevan basin, base level of erosion, channel processes, longitudinal profile of rivers, down-cutting erosion, river-drift sediment