COLD WATER DISCHARGES FROM IMPOUNDMENTS AND IMPACTS ON AQUATIC BIOTA Publication SR3 February 2004 1. INTRODUCTION like nature of hydroelectric power generation releases. This publication provides an overview of factors influencing thermal stratification in dams, 2. THERMAL STRATIFICATION temperature depression effects below large impoundments and the impacts on downstream When the surface water in an impoundment absorbs aquatic biota. solar energy for extended periods, the water may The construction of large dams on the majority of become thermally stratified. This stratification Australia’s large rivers has resulted in extensive commonly occurs in summer and results in the changes to natural flow regimes and temperature formation of distinct temperature bands. These patterns downstream of dam releases. Many of bands are known as the epilimnion (surface layer), these large dams have been constructed to deliver thermocline (middle transition layer) and water for irrigation farmers and hydroelectric power hypolimnion (bottom layer) (Figure 1). The stations, and for flood mitigation. epilimnion is the upper band of water that receives the full impact of solar radiation. As a result of There have been a number of published studies on thermal dynamics and density, warm water has a the effects of dams on fish migration, in particular, tendency to rise and remains at the top of the water the impacts of changed flow regimes and the column, being further warmed by continual physical barriers that large impoundments create. exposure to solar radiation. The narrow transition To date however, little research has been conducted layer between the epilimnion to the hypolimnion on the ecological implications of cold-water (thermocline) is characterised by rapid changes in discharges in flow regulated systems. Temperature temperature with depth. The bottom layer of water pollution impacts have predominantly been studied (hypolimnion) is isolated from the warming effects in the northern hemisphere, on warm water of the sun by reflection, refraction, natural turbidity, discharges to river systems associated with industry and the presence of algae in the epilimnion which and power station cooling waters. reduces the depth that light and radiant heat can Numerous researchers have suggested that cold- penetrate (Abel, 1996). water pollution may be having effects on species During summer, the temperature of the surface layer distribution and abundances in Australia. in an impoundment can be several degrees warmer Conclusions have been speculative, due to than would normally be expected in the natural river confounding factors such as changes in flow system. This is due to continued solar absorption regimes, increased water velocities and the pulse- COLD WATER DISCHARGES FROM IMPOUNDMENTS AND IMPACTS ON AQUATIC BIOTA and heat cycling (Cazaubon and Giudicelli, 1999). At this time, the release of water from any depth in Conversely, due to insulating effects and heat the impoundment can be warmer than expected to cycling, water temperature in the hypolimnion can occur naturally. Hence, thermal extremes tend to be be substantially cooler than in the natural river retarded by the impoundment, and a ‘winter warm, system (Gaillard, 1984). summer cool’ scenario is the product (Figure 2) (Gore, 1977; Ward and Stanford, 1979; Walker, 1980; In winter, different heat cycling processes are Cowx et al. 1987; Webb and Walling, 1988; Saltveit observed. During the preceeding autumn months, et al., 1994). air temperatures and levels of solar energy begin to decrease. The result is a cooling of the epilimnion, Thermal stratification, although more common and usually to a point where the surface layer becomes dramatic in large impoundments, may also occur in substantially cooler and denser than the insulated weir pools. Gundigera Weir on the Namoi River in hypolimnion below. A process known as “overturn” New South Wales (NSW) showed a 5OC change in then results. The warmer less dense waters that are water temperature from the surface to a depth of 7m now in the bottom layer of the impoundment rise, during the summer period of 1999. Temperature and the cooler denser waters of the top layer sink. differences of 15OC between the surface and a depth The three water layers are consequently mixed of 10m have been also recorded in Pindari Weir on which eliminates the thermal gradient created in the Severne River (DLWC, 2000). summer. The impoundment is now termed isothermal, meaning that the temperature at all water levels is approximately the same. Solar Energy Refraction, reflection and absorption by alga and sediment particles 0 Epilimnion 10 ) 20 m Thermocline ( h pt 30 e r e u t a D r e 40 p m Hypolimnion e T r e 50 t a W Temperature oC 5101520 Figure 1: Thermal stratification occurring in impoundments during summer periods (Adapted from Abel, 1996). EPA Victoria 2 COLD WATER DISCHARGES FROM IMPOUNDMENTS AND IMPACTS ON AQUATIC BIOTA 25 O 20 C re u 15 t 10 ra pe 5 m 0 Te Jan March MayJulySeptNov Regulated stream Unregulated stream Figure 2: ‘Winter warm – Summer cool’ effect of impoundments on release waters compared to unregulated rivers. Studies conducted by Gaillard (1984) indicated • size of the impoundment differences of 20oC between normal river • location of the dam in the river continuum temperatures and temperatures of hypolimnion • river size (bottom) waters. • influence of downstream tributaries and their characteristics (flow and temperature) 3. TEMPERATURE RECOVERY • atmospheric conditions The release of water from an impoundment at a temperature different to that of the river into which it • air temperature is released will alter the temperature in the river. • degree of shading by riparian (stream bank) The recovery distance is the length of river vegetation downstream of an impoundment that is required for • bed substrate the return to temperatures that would normally be found in an unregulated river. This distance may • stream morphology vary, between different impoundments and between • ground water influences different seasons for an individual impoundment. (Ward, 1985; Palmer and O’Keeffe, 1989; Storey and The recovery distance below an impoundment is Cowley, 1997). dependent on a number of factors. These include: During periods of high flow, water velocities and • discharge volume distance travelled per unit time is greater, therefore • temperature of the release waters greater river distances are affected during periods of Scientific Report 3 COLD WATER DISCHARGES FROM IMPOUNDMENTS AND IMPACTS ON AQUATIC BIOTA higher discharge. Higher flow rates also typically have dramatic effects on aquatic macroinvertebrate involve greater river depths, resulting in a reduction communities. of surface area to channel volume ratios. A The body temperature of invertebrates varies with reduction in this ratio reduces the proportion of environmental temperature. With certain water exposed to ambient warming or cooling temperature ranges necessary for successful growth temperatures, depending on the seasonal timing of and reproduction, heat consequently drives the releases. As a result, a longer period of time and growth rates and reproduction of insects when food distance is required to stabilise the thermal is not limiting (Gullan and Cranston, 1994). dynamics of the flow (Figure 3). The release of cold water from storages can Recent investigations of water temperatures dramatically affect aquatic invertebrates, as the downstream of storages in NSW and Victoria optimum temperature range for their developmental indicate that temperature depression is a common stages lies above that of the hypolimnion release occurrence. It has been reported that up to 400km water (Gailliard, 1984). downstream of some NSW impoundments may be Limited invertebrate research has been conducted in affected by the release of cold water. Australia on the effects of thermal pollution, with It is also believed that 14 of NSW’s largest dams are only certain taxanomic groups receiving attention. contributing to a total of 2795 river-kilometres Groups such as mayflies (Ephemeroptera) and affected by temperature depression (Lugg, 2000). stoneflies (Plecoptera), have been the subject of The effect on Victorian river systems to date has not research studies to investigate temperature been determined, through lack of suitable requirements for egg development and growth monitoring of many large impoundments. (Corkum and Hanes, 1991; Brittain, 1991; Brittain and Campbell, 1991; Brittain, 1995; Brittain, 1997). 4. EFFECTS OF TEMPERATURE In laboratory studies, both egg development and CHANGE ON BIOTA hatching were found to vary with temperature. Development time was longer at lower temperatures and hatching success was reduced at upper and 4.1 Aquatic invertebrates lower temperature extremes. For example, Brittain Temperature plays an important role in determining and Campbell (1991) found that at 10OC, a period of invertebrate abundance and distribution (Camargo 55 days was required for mayfly eggs and Voelz, 1999). Many Australian stream (Coloburiscidae) to hatch, while at 25OC they only communities have evolved under high summer required 15 days. Further investigations by Brittain temperatures and large variability in thermal (1991) demonstrated similar hatching responses in conditions (Lake, 1982). Consequently, it has been stonefly eggs (Eusthenidae Thaumatoperla alpina), hypothesised that changes to natural thermal with hatching taking 327 days at 5OC and 87 days at regimes induced by impoundment