Some physical and chemical effects of the Cow Green (Upper Teesdale) impoundment

Abstract

SUMMARY. The paper describes observations on water temperature and dissolved oxygen in Cow Green reservoir and also changes caused by impoundment to temperature, chemistry, dissolved oxygen and the discharge regime of the River Tees immediately downstream of the dam.Impoundment and river regulation have considerably smoothed‐out fluctuations in discharge of the Tees and have ehminated the very low and very high discharges which were characteristic of the natural river.Reservoir water levels show an annual pattern of draw‐down during summer, refilling in autumn and overflow during winter and spring. Tliermal stratification of the reservoir occurs rarely and is generally of short duration. Water temperatures at mid‐reservoir correspond closely to those of water discharged from the reservoir. The temperature regime in the river downstream is modified as follows: reduction in amplitude of annual temperature fluctuations by 1–2°C, marked reduction of diel fluctuations, (3) delay of the spring rise in water temperature by 20–50 days and of the autumn fall by 0–20 days. These temperature changes could reduce the annual maintenance ration of a 10 g brown trout (Salmo trutta L.) by about 15% and delay the annual peak of metabolism by about 1 month. The effect upon growth rate of trout would be negligible. Fluctuations in ionic content at the reservoir inflow are comparatively large and can be related approximately to river discharge. Fluctuations at the outflow are smaller and appear to be largely seasonal. Regulation has not appreciably altered the dissolved oxygen content of the River Tees.