This study analyzed variation in stable isotope ratios of aquatic and terrestrial primary producers and two common cyprinid fishes (Capoeta banarescui and Squalius cephalus) at seven sites in the upper YeAYilA +/- rmak River Basin, Turkey, to estimate relative contributions of basal production sources to fish biomass. We hypothesized that seasonal and spatial variation in fish assimilation of basal production sources would be affected by hydrological variation, with results for a reach downstream from a reservoir differing from those from upstream sites. Carbon and nitrogen isotopic ratios of primary producers and S. cephalus and delta N-15 of C. banarescui revealed significant spatial variation. delta C-13 of primary producers, delta N-15 of S. cephalus, and both delta C-13 and delta N-15 of C. banarescui revealed significant seasonal variation. C. banarescui biomass in the river channel was mostly derived from terrestrial herbaceous plants, and its biomass in the reservoir derived mostly from aquatic plants. Estimated proportional contributions of herbaceous plants to both species were greatest at the downstream site during spring, and declined during summer in the case C. banarescui. Overall, the influence of the dam was small relative to effects from watershed characteristics and seasonal changes in temperature and hydrology.