Cumulative effects of cascade dams on river water cycle: Evidence from hydrogen and oxygen isotopes

Cascade dams are known to influence the river water cycle, but their cumulative effects (CEs) are still not well understood. Water hydrogen (H) and oxygen (O) isotopes are hypothesized to be used to characterize the CEs. To test this hypothesis, we investigated water delta D, delta O-18, and related environmental factors in cascade reservoirs on the Wujiang River, Southwest China. The delta D and delta O-18 ranged from -64.2 parts per thousand to -45.4 parts per thousand and from -9.7 parts per thousand to -6.8 parts per thousand, respectively, and showed obvious temporal and spatial variations. Water temperature is an important factor influencing these variations. After damming, an increase of water retention time caused the enrichment of heavy H-O isotopes in reservoir surface water, and thermal stratification induced a decrease of delta D and delta O-18 with depth. Due to bottom discharge, released water showed more negative delta D and delta O-18 than reservoir surface water, and these delta D and delta O-18 differences were controlled by water retention time and mean water depth of the reservoir. Overall, the CEs of cascade dams caused delta D and delta O-18 to display a jagged increase from upstream to downstream in the impounded Wujiang River. Therefore H-O isotopes can be used to estimate the CEs of cascade dams. As cascade dams can modify H-O isotope signatures, caution should be exercised when using H and O isotopes to trace the source of the impounded river water.