n natural floodplains, the grassed areas usually entrap fine suspended sediments (wash products rich in nutrients) of the floodwater, thus contributing to the process of self-purification. However, hydroelectric power plants (HPPs), built with the purpose of employing the renewable energy, can worsen this process, it occurs if they pond the floodplains and decrease these grassed areas. In the present paper, the sediment deposition in the floodplains with existing weir heights is investigated. The mathematical-hydraulic model is used for it. The 12 km length interval of the river Virvyte, having three ponds was investigated. The average volume of sedimentation in this region is 500-2000 t/year, i.e. quite large. In the investigated region of the river Virvyte, the sediment deposition decreased by about 50%, due to the impact of a too high dam (H=5.5 m) of the Kapėnai HPP. © 2013 Copyright Vilnius Gediminas Technical University (VGTU) Press.