EFFECTS OF THE CERNADILLA-VALPARAISO RESERVOIR SYSTEM ON THE RIVER TERA

Water chemistry and macroinvertebrates were studied in the River Tera (north-western Spain) to assess the effects on the river of two chaine d reservoirs with hypolimnetic release during the stratification perio d in 1991. The river-water variables most affected by the dam in this study are temperature, pH, conductivity, silicate, ammonia, nitrate an d phosphate. Of these, temperature, with more constant values in the r iver after the dams, in contrast with diel and seasonal cycles, genera tes 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 modi fier of river water quality, although its values do not seem to be cri tical. Although dissolved oxygen is greatly affected by the dams, part icularly during stratification conditions, bottom release by hollow-co ne solves the problem of reoxygenation downstream from the dam. Unregu lated stations presented the most diverse macroinvertebrate communitie s and also the highest values of the biotic index BMWP', typical of ve ry clean waters. The stations influenced by the dams showed lower valu es, but with a gradual trend towards recovery downstream from the dam. At 10 km below the reservoir system, benthic communities showed an im portant recovery. Some management tools related to the ecological prob lems caused by this type of reservoir are discussed. (C) 1997 by John Wiley & Sons, Ltd.