A device allows a liquid to flow between two open structures with different liquid surface elevations separated by a wall. The device has a conduit with an inlet on one side of the wall and an outlet on the other side of the wall separated by a high point above the expected liquid surface elevation. The conduit also has two valved openings, one connected to a source of suction that can evacuate air from the conduit and the other connected to a vent. In one example, the conduit is formed between a pair of spaced transverse walls attached to an arched cover. The transverse walls each have a slot allowing the device to be placed on top of a dividing wall between two tanks with surfaces of the dividing wall defining part of the conduit. In an example with multiple conduits, one or more interior walls are provided between a pair of end walls and a cover over the interior and end walls may be provided in segments. To initiate flow, a conduit is evacuated of sufficient air to allow a siphon to develop. To stop the flow, sufficient air is allowed to enter the conduit through the vent valve to break the siphon. To control the rate of flow in a device with multiple conduits, flow can be permitted in just some of the conduits. Flowrate through a conduit can also be varied by controlling the volume of air at the top of a conduit.