Runoff from a land area of approximately 490,000 km(2) enters the Gulf of Bothnia. This runoff is of essential importance for the flushing of the Gulf. a change in the volume of runoff effects the residence time. There are many natural as well as man made changes in the runoff, both in the form of long-term changes over many years and those occurring within one year. The most significant man made changes come from hydropower regulation. This report describes the effect on runoff from the development of the hydropower plants in Sweden and Finland by means of comparing recorded regulated runoff and simulated natural runoff. A recent time period, 1980-91, and a time period before regulation, 1925-36, were simulated. The monthly magnitudes of the redistributed flows were found to be on average 1,700 m(3)/s. The maximum redistributed monthly flow in May - June reached 5,000-6,000 m(3)/s.