Biotic and abiotic changes along the recovery gradient of two impounded rivers with different impoundment use

Changes in physicochemical conditions and lotic benthic macroinvertebrates along the recovery gradient of the impounded Colorado (USA) and Duraton (Spain) Rivers were examined to identify and compare major factors affecting the structure of the macrobenthic community. Although both impounded rivers were exposed to hypolimnial releases from dams, they exhibited different impoundment use; Granby Dam (Colorado River) is used for water storage whereas Burgomillodo Dam (Duraton River) is used for hydroelectric production. The major factor responsible for macroinvertebrate responses in the Colorado River appeared to be the anomalous temperature pattern caused by Granby Dam, with relatively cool temperatures during the summer and relatively warm temperatures during the winter. In contrast, the major factors responsible for macroinvertebrate responses in the Duraton River seemed to be short-term flow fluctuations and low oxygen concentrations caused by Burgomillodo Dam. Values of taxonomic richness and relative abundances of shredders (Colorado River) and scrapers (Duraton River) increased along the recovery gradient 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 environmental monitoring study denotes that the downstream changes in the biotic and abiotic components of impounded rivers is a function of the particular use of impoundments.