Thickness of sub-stream unconsolidated material as a driver of baseflow temperature and channel connectivity: Observations from Shenandoah National Park headwater streams
Abstract
Depth to bedrock influences streamflow, shallow aquifer flow, and groundwater/surface water exchange but is typically ill-defined along the stream corridor in steep mountain catchments. We hypothesize that i) thicker hillslope sediments allow deeper-circulating groundwater to discharge to streams, resulting in annual variations in stream temperature that are more damped with respect to air temperatures; and ii) headwater stream sections with thicker sub-stream porous alluvial and colluvial sediments are more prone to dewatering and disconnection during summer low flows. Depth to bedrock was assessed from 318 passive seismic recordings taken from seven watersheds in Shenandoah National Park during July 2016, July 2019, and March 2020 using the horizontal-to-vertical spectral ratio (HVSR) method. This method requires an estimate of shear wave velocity for the conversion of measured resonance frequency to a depth to bedrock. Shear velocity in this study was calculated from observations at nearby boreholes where regolith thickness was known. This study characterized the depths to bedrock adjacent to streams monitored for coupled temperature and flow dynamics, and in conjunction with spatial surveys of stream dewatering. Results indicate an association between unconsolidated material thicknesses exceeding 2-3 meters and stream drying at the local scale, causing stream habitat fragmentation during summer low flows. These findings, along with previous studies identifying reach-scale variability in groundwater thermal influence, suggest that tradeoffs exist between maintaining cold summer stream temperatures and supporting summer baseflow capable of preserving habitat connectivity. This tradeoff should be considered when assessing headwater stream habitat vulnerability to climate change.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMH127...13G
- Keywords:
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- 1630 Impacts of global change;
- GLOBAL CHANGE;
- 1807 Climate impacts;
- HYDROLOGY;
- 1863 Snow and ice;
- HYDROLOGY;
- 1879 Watershed;
- HYDROLOGY