Many fish that spend their entire life within a river still need to move between different types of habitat at different stages of their life-cycle. This behaviour is something that most would associate with salmon or perhaps sea trout - but might be more surprised by the lengths that other fish will go to in a desperate attempt to reproduce.
Fish that try to move upstream often encounter barriers (both natural and man-made) and their naturally evolved tactic is to find the point at which the majority of the flow passes over or through the structure. This flow often gives away the location of the lowest point of a barrier and is called "attraction flow" since migrating fish are drawn towards it.
For many constructed weirs - the instincts of the fish can lead them to repeatedly throw themselves against immovable stonework. Injuries, exhaustion and perhaps the loss of the opportunity or ability to spawn are the unfortunate consequences.