We claim : 1. A system for controlling water flow in a reach in an irrigation canal, comprising an upstream gateand a downstream gate defining said reach between them, an outlet in said reach, and a control device connected to said upstream gate and having sensitive members located to simultaneously detect changes in the water levels in such reach at a first place adjacently downstream of said upstream gate and at a second place in the region of said outlet, said device being constructed and arranged to combine the indications of said members to provide resultant indications comparable to estimated changes in the water level at a predetermined point of said reach, and to utilize such resultant indications to control said upstream gate so as to maintain a practically constant given water level at such point.
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