Impacts of lowered baselevel on the hydrology of Van Norden meadow

Abstract:

One of the largest sub-alpine meadows on the west slope of the Sierra Nevada range in California, Van Norden meadow has been degraded since the late 1800s by over-grazing and road development. In addition, a dam was built at the meadow outlet resulting in variable reservoir conditions for over 100 years. In fall 2015, following an initial notching of the dam in 1976, local stakeholders began restoring the meadow by partially draining the reservoir, lowering the baselevel by more than 1 meter. In fall 2019, the dam was further notched to maintain this new baselevel and permanently reduce previously impounded water volumes.

In 2014, we began monitoring the meadow hydrology in collaboration with restoration project partners. A total of 30 monitoring wells were installed along 9 transects to measure groundwater levels. Water levels were manually measured from 2014-2019 every 2-3 weeks from late spring through early fall, with hourly logger data collected in 8 wells. Following partial draining of the reservoir, groundwater levels near the dam and previous reservoir footprint decreased by up to 1 m as expected due to the lowered baselevel. However, groundwater levels upstream of the reservoir footprint were minimally impacted, such that levels remained unchanged in the upper two-thirds of the meadow. Based on field observations of glacial moraine deposits and adjacent hillslope topography, we surmise that changes in stratigraphy and underlying bedrock topography create a shallow subsurface ridge that impedes groundwater flow. The ridge effectively creates an independent baselevel for the upper meadow disconnecting the influence of the dam baselevel from upper hydrologic conditions. Coincident hydrologic modeling efforts support this assertion, but additional geophysical research is needed to quantify the underlying geologic conditions. These results suggest that while restoration is likely needed to offset impacts of decreased groundwater levels on vegetation in the lower meadow, additional efforts to limit further incision due to decreasing the dam baselevel are not needed in the upper meadow. We suggest subsurface geologic conditions should be taken into account when seeking to understand meadow hydrologic functions and the potential impacts of changes in baselevel conditions during restoration efforts.