AbstractDownstream migrating smolt must be guided around hydropower plants to avoid fish mortality due to the turbines. In Piteå River, which is already regulated, open spillways serve this purpose but few fish find this route. Hence, action must be taken to enhance downstream fish migration. One way to attract the fish to the spillways is to direct the surface flow towards them by means of a guiding device. The hydrodynamic design of one such device is outlined using numerical calculations of the flow upstream the spillways and by the assumption that the fish moves near the surface of the water. A number of geometries are evaluated by starting from a straight impermeable barrier that extends 2 m down from the water surface and stretches over a part of the river. A major result is that it is possible to redirect the surface water towards the spillways at very low spilling rates which means high energy efficiency. Another finding was that the device should stretch over a large part of the river. For optimal functionality, the spilling should match the guiding device geometry. High spilling implies that the guiding has a low impact while for low spilling the geometry is crucial for successful downstream migration. Copyright © 2009 John Wiley & Sons, Ltd.