Effects of the Cernadilla–Valparaiso reservoir system on the River Tera

Abstract

Water chemistry and macroinvertebrates were studied in the River Tera (north-western Spain) to assess the effects on the river of two chained reservoirs with hypolimnetic release during the stratification period in 1991. The river-water variables most affected by the dam in this study are temperature, pH, conductivity, silicate, ammonia, nitrate and phosphate. Of these, temperature, with more constant values in the river after the dams, in contrast with diel and seasonal cycles, generates a special environment (short thermal amplitude) several kilometres downstream. On the other hand, ammonia, owing to the magnitude of its changes, seems to be the main factor affected by the dams, and a modifier of river water quality, although its values do not seem to be critical. Although dissolved oxygen is greatly affected by the dams, particularly during stratification conditions, bottom release by hollow-cone solves the problem of reoxygenation downstream from the dam. Unregulated stations presented the most diverse macroinvertebrate communities and also the highest values of the biotic index BMWP', typical of very clean waters. The stations influenced by the dams showed lower values, but with a gradual trend towards recovery downstream from the dam. At 10 km below the reservoir system, benthic communities showed an important recovery. Some management tools related to the ecological problems caused by this type of reservoir are discussed. © 1997 John Wiley & Sons, Ltd.