ИСТИНА

Oka River is one of the largest rivers in European part of Russia, draining the East European Plain’s central part. Watershed area is 245.000 km2, mean long-term water discharge is 1300 m3/s. The river is as long as 1500 km. The river flows through the most populated and economically developed part of Russia, there are many cities and industrial facilities in its water basin. The river underwent the changes of channel morphology, river and floodplain ecosystems, due to human activity. Oka River in its upper and middle course is used as a source of sand and gravel materials for constructing purpose greatly required in the region as long as since the early 1950ies. More than 100 mln. m3 sand and gravel was mined from the riverbed during 6 decades. Taking into account the current bed sediment yield, river could transport such amount of material for 500 years. Meanwhile, no more than 12-15% of removed material was compensated on account of river sediment yield. Riverbed is substantially transformed. It consists of shallow ripples and overdeepened pools sequence. The last are the former mines which are as deep as up to 13 m and as long as several tens kilometers. Mean bed degradation is about 1,9 m. Bed degradation within mines occurred because of manmade material removing while vertical erosion developed in the sections between the mines. Erosion rate reached as much as 3-5 cm year-1. Longitudinal water surface profile became stepwise. Riverbed transformation has continued. It reveals itself in widespread low-water level drop which has the linear trend and exceeds at present 2 m. Level drop is the most serious effect to ecosystem. Riverbed recovery is probably impossible. To recover the riverbed, the increase of sediment yield from bank erosion and tributary influx is needed along with complete mining stop. Both sediment sources give little possibility to refill sediment yield. Alternative way to restore the water level and ecological situation could be low-head dams construction.