06 Jan 2022
06 Jan 2022
Status: a revised version of this preprint is currently under review for the journal HESS.

Bedrock depth influences spatial patterns of summer baseflow, temperature, and flow disconnection for mountainous headwater streams

Martin A. Briggs1, Phillip Goodling2, Zachary C. Johnson3, Karli M. Rogers4, Nathaniel P. Hitt4, Jennifer B. Fair4,5, and Craig D. Snyder4 Martin A. Briggs et al.
  • 1U.S. Geological Survey, Earth System Processes Division, Hydrogeophysics Branch, 11 Sherman Place, Unit 5015, Storrs, CT 06269 USA
  • 2U.S. Geological Survey, Maryland-Delaware-District of Colombia Water Science Center, 5522 Research Park Drive, Catonsville, MD, 21228, USA
  • 3U.S. Geological Survey, Washington Water Science Center, 934 Broadway, Suite 300, Tacoma, WA 98402 USA
  • 4U.S. Geological Survey, Eastern Ecological Science Center, 11649 Leetown Road, Kearneysville, WV 25430 USA
  • 5U.S. Geological Survey, New England Water Science Center, 10 Bearfoot Road, Northborough, MA 01532 USA

Abstract. In mountain headwater streams the quality and resilience of cold-water habitat is regulated by surface stream channel connectivity and groundwater exchange. These critical hydrologic processes are thought to be influenced by the stream corridor bedrock contact depth (sediment thickness), which is often inferred from sparse hillslope borehole information, piezometer refusal, and remotely sensed data. To investigate how local bedrock depth might control summer stream temperature and channel disconnection (dewatering) patterns, we measured stream corridor bedrock depth by collecting and interpreting 191 passive seismic datasets along eight headwater streams in Shenandoah National Park (Virginia USA). In addition, we used multiyear stream temperature and streamflow records to calculate summer baseflow metrics along and among the study streams. Finally, comprehensive visual surveys of stream channel dewatering were conducted in 2016, 2019, and 2021 during summer baseflow conditions (124 total km of stream length). We found that measured bedrock depths were not well-characterized by soils maps or an existing global-scale geologic dataset, where the latter overpredicted measured depths by 12.2 m (mean), or approximately four times the average bedrock depth of 2.9 m. Half of the eight study stream corridors had an average bedrock depth of less than 2 m. Of the eight study streams, Staunton River had the deepest average bedrock depth (3.4 m), the coldest summer temperature profiles, and substantially higher summer baseflow indices compared to the other study steams. Staunton River also exhibited paired air and water annual temperature signals suggesting deeper groundwater influence, and the stream channel did not dewater in lower sections during any baseflow survey. In contrast, streams Paine Run and Piney River did show pronounced, patchy channel dewatering, with Paine Run having dozens of discrete dry channel sections ranging 1 to greater than 300 m in length. Stream dewatering patterns were apparently influenced by a combination of discrete deep bedrock (20 m+) features and more subtle sediment thickness variation (1–4 m), depending on local stream valley hydrogeology. In combination these unique datasets show the first large-scale empirical support for existing conceptual models of headwater stream disconnection based on underflow capacity and shallow groundwater supply.

Martin A. Briggs et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on hess-2021-622', Antóin O'Sullivan, 12 Feb 2022
  • RC3: 'Comment on hess-2021-622', Anonymous Referee #2, 12 Apr 2022

Martin A. Briggs et al.

Data sets

Air-water temperature data for the study of groundwater influence on stream thermal regimes in Shenandoah National Park, Virginia Snyder, C. D., Hitt, N. P., and Johnson, Z. C.

Seismic data for study of shallow mountain bedrock limits seepage-based headwater climate refugia, Shenandoah National Park, Virginia M. A. Briggs, J. W. Lane, Jr., C. D. Snyder, E. A. White, Z. C. Johnson, D. L. Nelms, and N. P. Hitt

Passive seismic data collected along headwater stream corridors in Shenandoah National Park in 2016 - 2020 Goodling, P. J., Briggs, M., White, E., Johnson, Z., Haynes, A., Nelms, D., and Lane, J.

Martin A. Briggs et al.


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Short summary
The geologic structure of mountain watersheds is thought to control how groundwater and stream water exchange. We measured bedrock depth at 191 locations along eight headwater streams, and paired with stream temperature records and observations of channel dewatering. The data indicated a prevalence of shallow bedrock. We hypothesized that variation in bedrock depth might cause stream dewatering, which was found to be true for some streams but not others, based on local groundwater supply.