In studying the hydro-ecological conditions in northwestern Sichuan of southwestern China, an area at the edge of the Qinghai-Tibet Plateau, stream waters were extensively sampled and analyzed for the stable hydrogen and oxygen isotope ratios (delta H-2 and delta O-18) which are often excellent indicators for a water body genesis and evolvement. The heavy isotopes of hydrogen and oxygen were found to be, in general, gradually depleted from east to west in the region; the phenomenon is attributed mainly to continentality based on the assumption that the isotopic signatures in the precipitations were preserved in the stream waters. The delta H-2 and delta O-18 data from the Yangtze River basin were found to fall well on the global meteoric water line (GMWL), indicative of the fact that the runoff waters in this basin were primarily of meteoric origin with a regional weak evaporation prevailing during our sampling time. However, the isotopic data from a relatively small part of the area in the north belonging to the Yellow River basin are characterized by the appreciably flatter slope of the local water isotope line and lower d-excess values, suggesting that the steam waters had undergone significant evaporation due to kinetic fractionations of isotopes in a more arid climate. The stable isotope data were also applied in calculating water mixing for some selected rivers. This study represents a snapshot study of the area's hydro-ecology; more systematic studies are required in better understanding the seasonal hydrological variations.