Lake water-level changes affect the physical, chemical, and biological condition of lakes; and we expect that disturbances such as land use conversion, water withdrawal, and climate change may alter water-level regimes and impact lake integrity. However, we have a poor understanding of the causes and potential ecological consequences of these changes compared to other human disturbances on freshwater systems (e.g., eutrophication, acidification) especially across broad spatial extents. Our objectives are 1) to examine the spatial heterogeneity in lake water-level change across the contiguous U.S., 2) to relate those changes to lake, landscape, and climate variables, and 3) to examine potential ecological effects of lake water-level drawdown on water quality and nearshore habitat conditions. We use data from EPA’s National Lakes Assessment datasets (NLA2007 and NLA2012), probability-designed lake surveys in the contiguous U.S. (>1200 lakes), to address these objectives. We used morphologic measures to estimate water level changes including vertical and horizontal drawdown distances (i.e., bathtub rings). Evaporative water loss was estimated from water stable isotope data. We apply a multistep statistical framework to examine spatial heterogeneity in lake water level changes across the U.S. and underlying lake, climate, and geographic drivers of these changes.
Results/Conclusions
We found that lake water level drawdown measures varied greatly across the U.S. Vertical drawdown ranged from 0 to 40 m (median: 0.24 m) and horizontal drawdown ranged from 0 to 545 m (median: 0.43 m). Stable isotope-estimated evaporative loss of inflowing water ranged from 0 to over 100% (median: 20%). Between 20% to 30% of lake water level variation was attributed to ecoregion. Lakes in the western U.S. showed greater vertical and horizontal water level drawdown compared to lakes in the eastern U.S. Interestingly, ecoregion variation in evaporative water loss (mean CV = 74) was not as large as drawdown variation within ecoregions (CV = 176). This suggests that water level changes may not be driven exclusively by evaporative loss and climate-related variables. Rather, water removal may be a primary driver of change, especially in the Western Mountains and Xeric regions. Additionally, reservoirs had greater water-level drawdown, but less evaporation than natural lakes. Consistent with this notion, water management affects reservoir water levels and climate conditions affect natural lake water levels. Examining the spatial patterns and potential causes of water level change can help inform management and conservation strategies to protect hydrologically vulnerable systems.