The harnessing, for hydro-electric generation, of Scottish rivers, which support stocks of migratory salmonids has been debated for many years. This paper examines the manner in which engineers and scientists in Scotland have attempted to alleviate the biological disadvantages caused by hydro dams and associated facilities and procedures in affected rivers. In particular, the effects of these installations and the control of flow regimes on the upstream and downstream migration of salmon (Salmo salar L.) and, to a lesser extent, sea trout (Salmo trutta L.) are addressed.