Impact Assessment of Hydropower Development on Dissolved Oxygen at the Byllesby Dam

Extensive monitoring was carried out both in the Byllesby reservoir and downstream from the powerplant during summers of 1987 and 1988. In the summer of 1987 the lake was drawn down and then refilled, thus exposing the old flood plain and allowing dense terrestrial vegetation to grow. Upon refilling of the reservoir the vegetation began to die and decay creating an extremely high dissolved oxygen (D.O.) demand in the lower layers and causing very low D.O. concentrations, sometimes extending up to the water surface. Since the powerplant was operated during summer 1988 outflow was passed through the turbines, withdrawing water at approximately 4-6 m depth. No water was passed over the spillway. As a result downstream concentrations were low, especially in the morning. Mitigation of the observed low D.O. concentrations is necessary. The installation of a weir would reduce the percentage of diversion required, thus improving the power production possible under the D.O. restrictions, and also increase the average downstream D.O.