Man's Influence on Freshwater Ecosystems and Water Use (Proceedings of a Boulder Symposium, July 1995). IAHS Publ. no. 230, 1995. 245 The long-term thermal impact of reservoir operation and some ecological implications B. W. WEBB & D. E. WALLING Department of Geography, University of Exeter, Amory Building, Rennes Drive, Exeter EX4 4RJ, Devon, UK Abstract A long-term study of the thermal impact of a regulating reser­ voir 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 asso­ ciated 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 &Finlayson, 1993; 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 tempera­ ture 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 246 B. W. Webb & D. E. Walling impoundment, a short-term perspective may be limiting. In the absence of long-term records, it is impossible to assess how the physical and biological impacts of regulation vary in response to factors such as the maturation of water quality within reservoirs, significant inter-annual variability in hydrometeorological conditions and changes in the operation schedule of regulation schemes. The present study reports an investigation of a reservoir in southwest England, which has spanned more than 17 years and has provided the opportunity not only to quantify the long-term impact of impoundment and river regulation, but also to assess and explain inter-annual variability in the effects on water temperature and on selected aspects of the freshwater biota. THE WIMBLEBALL STUDY Wimbleball Lake, which is located in the River Haddeo on the eastern margins of the Exmoor upland in southwest England (Fig. 1A), was constructed as a dual-purpose reservoir to regulate the mainstream of the River Exe and to provide a direct supply for part of Somerset (Battersby etal., 1979). Impounding began in mid-December 1977 and the first overflow was recorded in November 1980 (Fig. IB). The catchment feeding the reservoir has an area of 29 km2 and mean annual rainfall and runoff of 1330 and 910 mm, respectively. Details of the Wimbleball Scheme and its operation are given in Webb & Walling (1988a). A compensation release of 9.1 Ml day"1, which is required at all times, is made up by seepage from springs lying just downstream of the dam, as well as by water released directly from the reservoir. The latter is mainly taken from the top 25 m of the water body. In order to assess the effects of Wimbleball Lake, the water temperature monitoring network established in the Exe Basin by the University of Exeter (Fig. 1A) includes a site on the River Haddeo situated 0.4 km below the dam (Upper Haddeo) and a station on the River Pulham (Pulham), which is a neighbouring tributary similar to the River Haddeo in all respects except that it is unregulated. Continuous measurements have been made at these sites with commercially-available mercury-in-steel thermographs during a period extending from Spring 1976 until the present day. Measuring bulbs are anchored to the stream bed in order to prevent exposure to the air and thermographs are checked against weekly readings taken with a standard laboratory-calibrated thermo­ meter. The latter procedure rarely reveals discrepancies exceeding 0.2°C. For the purposes of data analysis, water temperature information has been abstracted as hourly records from the thermograph charts. Work reported previously (Webb & Walling, 1988a; 1988b) has demonstrated that closure of Wimbleball Dam had an immediate effect on the thermal regime of the River Haddeo, which continued during the three-year filling phase of the reservoir. The present study is based on 13 years of detailed data following the attainment of top water level and covers the period from 1 January 1981 to31 December 1993. The effect of the Wimbleball Scheme on water temperature is assessed by comparing hourly records for the regulated River Haddeo and unregulated River Pulham, while the ecological implica­ tions of regulation are ascertained by combining water temperature data with published biological models derived from laboratory studies (cf. Webb & Walling, 1993). Long-term thermal impact of reservoir operation 247 Altitude in metres ES 200 300 400 500 600 Weeks since 1st January 1978 Fig. 1 The study reservoir and location of the study sites (A) and weekly variation in drawdown of Wimbleball Lake (B). LONG-TERM THERMAL IMPACTS It is clear from 13 years of detailed records that the main effect of the construction of 248 B. W. Webb & D. E. Walling Wimbleball Lake on the thermal regime of the River Haddeo has been to increase the mean water temperature value, eliminate freezing conditions, depress summer maxima, delay the annual cycle and reduce diurnal fluctuation (Fig. 2). Average temperature at Upper Haddeo was 0.6°C higher than at Pulham and temperatures in the regulated river Water Température {°C} A ) 1 Maximum ,_ Pulham 2 Mean (°C 3 Minimum e Upper~^"v 4 Maximum diurnal range Haddeo ^**>>«^_ 5 Mean diurnal range 6 Minimum diurnal range peratur E 11- <u 9- H 7- Water Temperature (°C) ^ B 5- 1 2 3 4 5 6 ~^>—— UH 19.6 10.3 1.6 6.3 1.2 0.1 x-—, ~~ v, £ 1- PU 21.3 9.7 -0.9 10.1 2.6 0.1 1 1 1 1 1 1 i 11 l|! Percentage of time U O 16 - c o 14 IÏ Pulham / / \ \ S 3 '3 S 12 / / \\ H io- Kl O \, S g* // Upper Haddeo Pulham - C 10 5 <" a -1 1 1 1— I I I i I i I I 1 1 J F M A M J J A M J J SOND Month Month D U U 16' o Upper Haddeo l¥l4- Upper Haddeo c a g 3 04 | g.H B 10 3 § I S Pulham ID H C Pulham £ 6- MAM J J A SOND M J J A S O N D Month Month Fig. 2 Water temperature statistics and duration curves (A) and the average annual cycle of mean (B), mean maximum (C), mean minimum (D) and mean diurnal range (E) values based on the period 1 January 1981 to 31 December 1993 for the regulated (Upper Haddeo) and unregulated (Pulham) river stations. Long-term thermal impact of reservoir operation 249 did not rise above 20°C, nor fall below 1.5 °C, during the 13-year study period. Tempe­ rature duration curves for the regulated and unregulated rivers (Fig. 2A) reveal that the Wimbleball Scheme has had a greater effect in increasing temperatures in the low to intermediate range than in reducing high values. It is also clear that impoundment and river regulation have reduced the extent of diurnal water temperature fluctuation in the River Haddeo. The average diurnal range at Upper Haddeo is less than half that at Pulham, while the maximum daily fluctuation recorded between 1981 and 1993 ex­ ceeded 10°C in the unregulated river but was ca 4°C less in the regulated water course. The delay in the annual cycle of water temperature fluctuation at Upper Haddeo compared with Pulham is evident from study-period mean values for individual months (Fig. 2B). The peak in the annual march occurs in July for the unregulated river, but in August for the regulated water course. Lowest mean water temperature is recorded in February for both catchments, but the spring rise and autumn fall are delayed at Upper Haddeo, with particularly strong differences in temperature occurring during the period from September to December. Similar contrasts between Upper Haddeo and Pulham are evident for the annual cycle of mean maximum water temperature, although values are more markedly lower in the spring months (Fig.