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

Abstract

Abstract. In mountain headwater streams, the quality and resilience of summer cold-water habitat is generally regulated by stream discharge, longitudinal stream channel connectivity and groundwater exchange. These critical hydrologic processes are thought to be influenced by the stream corridor bedrock contact depth (sediment thickness), a parameter 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 multi-year stream temperature and streamflow records to calculate several baseflow-related metrics along and among the study streams. Finally, comprehensive visual surveys of stream channel dewatering were conducted in 2016, 2019 and 2021 during summer low flow conditions (124 total km of stream length). We found that measured bedrock depths along the study streams 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, Paine Run and Piney River did show pronounced, patchy channel dewatering, with Paine Run having dozens of discrete dry channel sections ranging from 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 spatially variable underflow capacity and shallow groundwater supply.

Closing the gap between science and management of cold‐water refuges in rivers and streams

“…Lateral seeps and springbrooks sourced by shallow groundwater may exhibit more variable and higher temperatures over time due to climate change (Hare et al, 2021; KarisAllen et al, 2022). Perennial streams also can become intermittent during extended dry periods (Costigan et al, 2015; Gendaszek et al, 2020; Price et al, 2021), thereby disconnecting CWPs from warm stream reaches in headwater systems (Briggs et al, 2022). Cold‐water patches fully mix with ambient water at high streamflow in rivers draining large stratified natural lakes that show abrupt wind‐driven drops in river temperature, leading to the loss of discrete CWPs (Lisi & Schindler, 2015) and a reduction in CWRs.…”

Empirical stream thermal sensitivities cluster on the landscape according to geology and climate

“…Interestingly, despite the clear importance of groundwater metrics in the clustering analysis, results from summary metric regressions were mixed and, in the Snoqualmie basin, did not align with expectations of a negative relationship between thermal sensitivity and groundwater influence (Table 3). Although it is possible to infer broad patterns in surface-groundwater connectivity using datasets of interpolated hydrogeologic properties (i.e., hydraulic conductivity, soil depth) or water source (i.e., baseflow index), individual hydrogeologic metrics often have substantial uncertainty, do not covary perfectly, and may be particularly unconstrained for mountain headwater streams (Wolock et al 2004, Patton et al 2018, Briggs et al 2022. Additionally, the influence of these processes can be localized and variable across space (Johnson et al 2017) and substantially impacted by human modification.…”

Stabilising effects of karstic groundwater on stream fish communities

“…Groundwater exchange processes have important moderating effects on stream flow and temperature (Kaandorp et al, 2019; Winter, 2007), and this affects the quality and connectivity of fish habitat (Kurylyk et al, 2014; Meisner et al, 1988). Streams flowing over shallow unconfined aquifers constrained by near‐surface bedrock exhibit more seasonal dewatering and more thermal sensitivity to air temperature than streams connected to deeper groundwater sources (Briggs et al, 2022). Moreover, a continental‐scale study demonstrated that streams dominated by shallow groundwater sources are warming in significantly greater proportion than streams fed by deeper groundwater sources, particularly over summer months (Hare et al, 2021).…”