Pergamon zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFED Applied Geography, Vol. 16, No. 3. pp. 21 l-223, 1996 Copyright Q 1996 Elsevm Science Ltd Printed in Great Britain. All rights reserved 0143-6228/96 $15.00 + 0.00 zyxwvut SO143-6228(%)00007-0 Long-term variability in the thermal impact of river impoundment and regulation B W Webb and D E Walling Department of Geography, University of Exeter; Amory Building, Rennes Drive, Exeter EX4 4RJ, Devon, UK A long-term study of the thermal impact of a reservoir in southwest England is reported. Detailed records collected from the regulated River Haddeo and the neighhouring (unregulated) River Puiham in a 1Cyear period following the attainment of top water level in the reservoir (Wimhlehall Lake) show 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. However, the study also highlights pronounced year-to-year contrasts in the thermal impact of river impoundment and regulation. These can largely be explained with reference to inter-annual variations in both reservoir operation and weather conditions, which primarily control the amount of water released in summer months, and the volume of flow passing the spillway in the winter period, respectively. Copyright 0 1996 Elsevier Science Ltd Reservoir construction and associated regulation of the downstream water course are a major way by which human activity can modify the thermal regime of rivers, often with significant ecological repercussions (see, for example, Sylvester, 1963; Ward and Stanford, 1979; Petts, 1984). In consequence, there have been many investigations of thermal impacts associated with impoundments; schemes varying greatly in size and purpose, in environmental setting, and in position and number of dams within the river system have been studied (Neel, 1963; Nishizawa and Yambe, 1970; Lavis and Smith, 1972; Collings, 1973; Ward and Stanford, 1983; Crisp, 1987; O’Keefe et al., 1990; Liu and Yu, 1992; Gippel and Finlayson, 1993; Tvede, 1994). Although these and other investigations have provided much invaluable information on the factors controlling the impact of impoundment on thermal regime (Mackie et al., 1983; Petts, 1986; Cowx et al., 1987), on downstream persistence of the effects (Webb and Walling, 1988a; Palmer and O’Keeffe, 1989), and on biological consequences (Brooker, 1981; Voeltz and Ward, 1989; Saltveit et aE., 1994), many of the conclusions have been drawn from studies of limited duration. Results based on detailed records extending for more than 5 years, such as the investigation of temperatures in the Green River below Flaming Gorge Reservoir, USA (Bolke and Waddell, 1975), are rare. A short-term perspective may be limiting because, in the absence of long-term information, it is impossible to assess how the thermal impact of a reservoir varies in response to inter-annual fluctuations in hydrometeorological conditions, or to changes in the operation schedule of a regulation scheme in order to meet evolving demands for its water supply or other functions.