BIOTIC AND ABIOTIC CHANGES ALONG THE RECOVERY GRADIENT OF 2 IMPOUNDEDRIVERS WITH DIFFERENT IMPOUNDMENT USE

Changes in physicochemical conditions and lotic benthic macroinvertebr ates along the recovery gradient of the impounded Colorado (USA) and D uraton (Spain) Rivers were examined to identify and compare major fact ors affecting the structure of the macrobenthic community. Although bo th impounded rivers were exposed to hypolimnial releases from dams, th ey exhibited different impoundment use; Granby Dam (Colorado River) is used for water storage whereas Burgomillodo Dam (Duraton River) is us ed for hydroelectric production. The major factor responsible for macr oinvertebrate responses in the Colorado River appeared to be the anoma lous temperature pattern caused by Granby Dam, with relatively cool te mperatures during the summer and relatively warm temperatures during t he winter. In contrast, the major factors responsible for macroinverte brate responses in the Duraton River seemed to be short-term flow fluc tuations and low oxygen concentrations caused by Burgomillodo Dam. Val ues of taxonomic richness and relative abundances of shredders (Colora do River) and scrapers (Duraton River) increased along the recovery gr adient of the impounded rivers. In general, chironomids (Diptera) were tolerant to both types of impoundment use, whereas elmids (Coleoptera ) and psychomyiids (Trichoptera) were very sensitive. Overall, this en vironmental monitoring study denotes that the downstream changes in th e biotic and abiotic components of impounded rivers is a function of t he particular use of impoundments.