Natural & human drivers of water-level decline & evaporation:inflow in US lakes

Anthropogenic water use and changing climate conditions (e.g., frequency and duration of droughts and floods) may pose serious threats to lake hydrologic integrity and physical habitat structure. Focusing on the western U.S., we found excessive water-level decline from reference conditions in a substantial percent of natural (14%, 7%) and man-made (37%, 28%) lakes in 2007 and 2012, respectively. But the environmental and human factors driving these patterns have not been throughly explored. We combined water level and lake attribute (e.g., morphology) data from ~400 lakes (USEPA National Lakes Assessment; NLA) with landscape and climate data from LakeCat (USEPA) and dam attributes from the National Anthropogenic Barrier Dataset (USGS) to identify lake characteristics and landscape contexts associated with water-level decline in western U.S. lakes and reservoirs. Drought and the presence of anthropogenic structures (with potential to alter lake hydrology, e.g., dams, channels) were associated with water-level decline in both natural and man-made lakes. Lake morphology (e.g., bank flatness) affected the magnitude of horizontal littoral exposure, diminishing near-shore riparian and littoral habitat cover and complexity. These findings strongly suggest that water management activities substantially altered lake water balance in both natural and man-made lakes in the western U.S, and degraded riparian and littoral habitat condition. Lake management should consider the importance of water-level decline on physical habitat cover and complexity.

Impact/Purpose

U.S. EPA National Lakes Assessment (NLA) is conducted as part of the Clean Water Acts mandate to report on the condition of the nation’s water resources. Natural lakes and constructed reservoirs provide important ecosystem services that include water sources for human activities (e.g., drinking water, recreation), habitat for wildlife, and buffers against adverse effects of drought and flooding, and yet the NLA does not have methods for understanding hydrologic alterations to lakes. Disturbances such as land use intensification, water extraction and diversion, and changing climate may significantly alter lake water-level regimes and lead to declining water levels. We found excessive water-level decline in natural (14%, 7%) and man-made (37%, 28%) lakes in the western U.S. in 2007 and 2012. However, the environmental and human factors driving these patterns have not been throughly explored. We combined U.S. EPA National Lakes Assessment survey data with landscape and climate data from LakeCat (USEPA) and dam attribute data from the National Anthropogenic Barrier Dataset (USGS) to identify lake characteristics and landscape contexts associated with lake water-level decline. Drought and the presence of anthropogenic structures with potential to alter lake hydrology were associated with water-level decline in both natural and man-made lakes. Lake morphology (e.g., bank flatness) affected the magnitude of horizontal littoral exposure, diminishing near-shore riparian and littoral habitat cover and complexity. These findings suggest that water management activities substantially altered lake water balance in both natural and man-made lakes in the western U.S., and lowered riparian and littoral habitat condition. Lake management geared at protecting physical habitat should consider the importance of water-level declines in impacting habitat cover and complexity.

Citation

Fergus, C., J. Renee Brooks, Phil Kaufmann, A. Pollard, R. Mitchell, A. Herlihy, Ryan A Hill, Steve Paulsen, AND M. Weber. Natural & human drivers of water-level decline & evaporation:inflow in US lakes. National Water Quality Monitoring Conference, N/A, Virtual, April 19 - 23, 2021.