Qinghai Lake is one of China's national nature reserves and supports the ecological security of the NE Qinghai-Tibet Plateau. More than half of the rivers flowing into Qinghai Lake are currently dry due to climate change and human activity. This study was designed to learn more about the causes of environmental problems in the basin, using stable isotopes and hydrochemistry of Qinghai Lake Basin river water to explore runoff processes and their relationship with climate change. Results indicated that the river water was mainly fed by precipitation in the basin, which has undergone weak evaporation. River discharges were generated mainly from the middle and upper basin, due to high precipitation, low evapotranspiration, and alpine swamp land use/cover in those areas. River water in tributaries would experience relatively stronger evaporation than in the main stream. Main hydrochemical types of river waters were Ca2+-Mg2+-HCO3-, and river water chemistry was mainly controlled by carbonate weathering in the Qinghai Lake Basin. The effects of human activity on water chemistry were relatively mild in the basin. The interaction between water and rocks was slighter in the river water than in the groundwater. River runoff was more sensitive to precipitation than to temperature. Lake level rises were closely related to increases in river runoff and precipitation. Conversely, Lake level declines were closely related to declines in river runoff and precipitation. Lake level could rise due to increasing precipitation and runoff in the future. (C) 2015 Elsevier Ltd and INQUA. All rights reserved.