NATURAL AND HUMAN DRIVERS OF WATER-LEVEL FLUCTUATIONS IN US LAKES AND RESERVOIRS
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Human water use and climate change (e.g., drought and flood frequency and duration) may pose serious threats to lake hydrologic integrity. Up to 59% of lakes in the conterminous US have exhibited water-level declines that exceeded least-disturbed conditions. But the interplay between human and natural factors in driving water-levels is not well known. Identifying regional variation in lake hydrology drivers can inform water management under changing environmental conditions. We developed a conceptual framework of natural (lake morphometry, watershed hydrology, and climate) and anthropogenic (dam height and land use) drivers of lake water-level decline and used path analysis to test and quantify the relationships. We compared models in the West (n = 357) and Midwest (n = 412) to examine how regional settings influence the magnitude and direction of human effects on lake water levels. In the West, human influences on vertical decline (31%) were greater than climate effects (14%) and associated with greater water-level decline. In the Midwest, climate effects (37%) were greater than human influences (9%), and human effects minimized water-level decline. These results suggest that large water-level declines in Western lakes result primarily from dam operation and water management and less from climate-related factors. Water levels in Midwestern lakes are more influenced by climate (drought), and management strategies stabilizes water levels. These regional differences provide insight into the role of regional management strategies on water-level declines.