To provide a water management device of a dam capable of not only utilizing the water to be discharged in power generation without wasting it but also discharging sands efficiently to prolong the service life of the dam, increasing the quantity of the water stored in the dam substantially, and adjusting the quantity of the stored water in accordance with weather easily by letting a detour tunnel constructed at the same time when constructing the dam remain and making use of it.

When constructing the dam for damming up a river in a valley to obtain head, the detour tunnel is also constructed by excavating the bottom of the river or a mountain from the upstream side of the construction spot and arranging a pipe down to the downstream side to detour the river water in order to prevent it from becoming an obstacle in the construction and is made to remain even after the construction of the dam. A discharge selection device capable of switching the discharge of sediment on the bottom of a lake and the discharge of the water including no sediment in addition to cutting off water is provided in a suction port section where the detour tunnel is opened on the bottom of the lake on the upstream side of the constructed dam, and a device for utilizing the water to be discharged in the detour tunnel is provided on the downstream side.