PURPOSE: To prevent the deposition of sludge in dam as well as raise the quality of water in river by a method in which a water-stopping or permeable layer is provided on the slope upstream of dam, a float to collect sunk wood and deposit sludge is provided at the inlet of the dam, and a float to deposit sludge is provided in river downstream of the dam.

CONSTITUTION: A water-stopping or permeable sheet or layer A is provided on the surface of slope upstream of a dam, and a float body with a float 8 having a flap 7 is provided to the inlet of the dam in a multiply-lapped form B to collect sunk wood and deposit sludge. A float body is also provided to river downstream of the dam in a multiply-lapped form C and a sludge deposit trench is provided on the bottom of river to deposit sludge. The deposited sludge is removed out by dredging work. The quality of river water can thus be improved and deposition of sludge in dam can be prevented.