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Variation in water resources is a main factor influencing ecohydrological processes and sustainable development in arid regions. Lake level changes are a useful indicator of the variability in water resources. However, observational records of changes in lake levels are usually too short to give an understanding of the long-term variability. In the present study, we investigated the tree rings of shrubs growing on the lakeshore of Lake West-Juyan, the terminus of the Heihe River in western China, and found that Lake West-Juyan had undergone degradation three times over the past 200 years. The lake level decreased from 904.3 to 896.8 m above sea level (a.s.l.) during the period 1800-1900, to 892.0 m a.s.l. from around 1900 to the late 1950s, and the lake dried out in 1963. The trend for changes in lake levels, which was represented by the composite chronology of three beach bars, showed that the phases of increasing lake levels over the past 150 years were during the periods 1852-1871, 1932-1952, 1973-1982, and 1995-1999. Comparison with the history of regional economic development showed that human activity has played an important role in regulating the water resources of the lower reaches of the Heihe watershed over the past 200 years.