Migrating fish that swim upstream in rivers for reproduction need to overcome obstructions, such as hydropower plants. If a fishway is used to help the fish pass such an obstacle, water needs to be taken from the dam without first passing through the turbines. Also, the fish may have difficulties finding the fishway, due to the dominating flow from the turbine tailrace. A fish lock that uses turbine tailwater to entice the fish into the lock and further on to a fishway is studied. The fish lock is a shallow open channel that uses a small fraction of the tailwater. A local acceleration of the flow is created by changing the cross sectional area of the lock channel. Measurements are concentrated on how to design the lock so enough water passes through and a sufficient velocity increment is reached. This is investigated in a lab-scale model using laser Doppler velocimetry. A full- scale prototype will then be tested at the Sikfors hydropower plant in the Pite river in Sweden.