To suppress changes in water qualities on the downstream in a water use control system and method discharging water from a reservoir to the downstream by a weir.

This water use control system and method stores water by the dam 5 constructed across a river, controls intake gates provided in the weir 5, and controls the water in the downstream to a prescribed water quantity and not to cause water quality changes such as a water temperature. This system is provided with the multiple intake gates 6-1, 6-2, and 6-3 provided in the weir 5 in positions different heights from the bottom of the lake, a gate control part 7 selectively controlling the opening/ closing of the intake gates, a stored water quality detecting part 9 detecting the water qualities such as the water temperature corresponding to the water depths of the reservoir 4, a downstream water quality detecting part detecting the discharged water qualities in the downstream, and a control processing part 13 finding an intake depth by comparing water quality detection information corresponding to the water depth detected by the stored water quality detecting part 9 with water quality detection information detected by the downstream water quality detecting part 15 and selectively controlling the intake gate corresponding to the intake depth.