Analyzing the Subsurface Consequences of Dam Removal on Groundwater Storage and Hydrologic Niches in a Mountain Meadow Ecosystem

Dam removal is becoming more common as aging infrastructure deteriorates and people seek to restore the environmental conditions created by dams. Despite this growth in dam removals, studies analyzing effects of dam removal on meadow ecosystems are limited. Groundwater is an important and often overlooked component of hydrologic systems, particularly in the case of surface water manipulation like dam removal. A planned dam removal project at Van Norden Meadow, in the California Sierra Nevada Range at Donner Pass, was selected to quantify the impact of dam removal on shallow groundwater and the associated meadow system. Mountain meadows are inseparable from local hydrology and are a valuable resource in California’s Sierra Nevada range, providing an important source of water, supporting natural biotic communities, and serving as climate change refugia. Using MODFLOW, an open-source finite-difference groundwater modeling program, we established a baseline groundwater model for Van Norden Meadow and used it to assess potential impacts of dam removal on the subsurface water balance in relation to hydrologic niches to support plant communities. The model predicted a slight decrease of less than 0.9 m in the water table of the lower meadow below a recessional moraine, but no impact to the water table in the upper meadow. The areas adjacent to the dam along the reservoir footprint were found to be most at risk of losing wet meadow vegetation. The model also suggested dam removal led to a 3% increase in baseflow drainage below the meadow. This increase was balanced by an 8% decrease in evapotranspiration and a slight increase (0.9%) in the mobilization of stored groundwater. While no further actions following removal may be needed in the upper meadow, restoration activities, such as vegetation planting or channel filling, may be needed to offset the decrease in the water table and promote wet meadow habitats in some regions of the lower meadow. However, newly exposed areas in the reservoir footprint will promote hydric vegetation recruitment, suggesting a potential net increase in wet meadow vegetation after dam removal in Van Norden Meadow. More broadly, we found MODFLOW was suitable for modeling dam removal despite challenges with steep slopes and thin conductive layers over bedrock.