TEMPORAL VARIABILITY IN THE IMPACT OF RIVER REGULATION ON THERMAL REGIME AND SOME BIOLOGICAL IMPLICATIONS

1. Ten years of hourly water temperature data for three river sites in south-west England were used to investigate the longer-term thermal effects of river regulation. The impact of reservoir impoundment on downstream invertebrate and trout development were simulated using daily mean temperature data in conjunction with published biological models. 2. The main effects of regulation on water temperature were to increase the mean value, eliminate freezing conditions, depress summer maxima, delay the annual cycle and reduce diel fluctuation. These impacts persisted over a distance of at least 5 km below the dam but declined downstream, especially for seasonal and diurnal variations. 3. Simulations suggested that regulation is likely to have had a greater impact on the development of brown trout (Salmo trutta) than on the development of one Ephemeroptera and four Plecoptera species. Trout fry were predicted to emerge up to 57 days earlier, and to weigh up to 67% more by the end of the year following swim-up, in the regulated compared with the unregulated river. 4. Marked inter-annual contrasts in the physical and biological consequences of impoundment were evident, indicating that long-term studies are required to define properly the effects of river regulation.