A spillway water system comprising at least one adjustable barrier sluice gate of one watercourse and defining: one upstream stretch and one downstream stretch of the watercourse arranged upstream and downstream of the sluice gate respectively; one spillway point arranged at a spillway height and at which a spillway water flow rate skims which flows from the upstream stretch and flows into the downstream stretch; the sluice gate comprising adjustment device/unit adapted to raise or lower the spillway height; a first measurement device/unit for measuring the level of water flowing along the downstream stretch; a second measurement device/unit for measuring the level of water of the upstream stretch; and a command device/unit of the adjustment device/unit operatively connected to the first and to the second measurement device/unit and configured to raise or lower the spillway height depending on the level measured by the first and the second measurement device/unit.