The long-term thermal impact of reservoir operation and some ecological implications

Abstract A long-term study of the thermal impact of a regulating reser­voir in southwest England is reported. Detailed records collected for the regulated River Haddeo and the neighbouring unregulated River Pulham in a 13-year period following the attainment of top water level in the reservoir (Wimbleball Lake) reveal that the main thermal effects of impoundment and regulation have been to raise mean water temperature, eliminate freezing conditions, depress summer maximum values, delay the annual cycle and reduce diurnal fluctuation. Long-term records also reveal pronounced year to year contrasts in the impact of reservoir construction, which can be largely explained by fluctuations in the volume of runoff released from the reservoir in the summer period, or passing the spillway of the dam in the winter period. Combination of data on daily mean water temperatures with published biological models derived from laboratory studies suggests that the thermal modification associated with reservoir construction has had a greater impact on the life cycle and growth of brown trout than on the development of selected mayfly and stonefly species. Considerable inter-annual variability in the extent of the predicted biological impacts indicates the need for long-term, as well as detailed, records in order to define rigorously the physical and ecological consequences of impoundment. INTRODUCTION There have been very many studies of the impact of reservoir construction and asso­ciated river regulation on downstream thermal regime and the consequences for the ecology of water courses below impoundments. However, although these investigations have encompassed schemes which differ greatly in environmental setting, in purpose and in position and number of dams within the river system (e.g. Neel, 1963; Williams, 1968; Nishizawa & Yambe, 1970; Collings, 1973; Ward & Stanford, 1979; Edwards & Crisp, 1982; Petts, 1984; Rader & Ward, 1988; Brittain & Saltveit, 1989; Marchant, 1989; Tuch & Gasith, 1989; Voeltz & Ward, 1989; O'Keeffe et al, 1990; Saltveit, 1990; Liu & Yu, 1992; Gippel F Tvede, 1994), few of the conclusions concerning water temperature behaviour and its biological implications have been based on long-term studies. For example, the major published investigations of water tempera­ture in regulated rivers of northern England and Wales (Lavis & Smith, 1972; Cowx et al, 1987; Crisp, 1987) have relied on between one and five years of data. Although these and many other studies have provided valuable insights into the effects of