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

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.

Small artificial impoundments have big implications for hydrology and freshwater biodiversity

Headwater streams are critical for freshwater ecosystems. Global and continental studies consistently show major dams as dominant sources of hydrological stress threatening biodiversity in the world’s major rivers, but cumulative impacts from small artificial impoundments concentrated in headwater streams have rarely been acknowledged. Using the Murray Darling River basin (Australia)and the Arkansas River basin (USA) as case studies, we examine the hydrological impact of small artificial impoundments. The extent of their influence is significant, altering hydrology in 280 - 380% more waterways when compared to major dams alone. Hydrological impacts are concentrated in smaller streams (catchment area < 100 km2), raising concerns that the often diverse and highly endemic biota found in these systems may be under threat. Adjusting existing biodiversity planning and management approaches to address the cumulative effects of many small and widely distributed artificial impoundments presents a rapidly emerging challenge for ecologically sustainable water management.

Rapid Loss of Dissolved CO2 From a Subtropical Steep Headwater Stream

High-gradient headwater streams are major participants in the carbon (C) cycle because of their capabilities of emitting a significant amount of carbon dioxide (CO2). Notwithstanding, their CO2 emissions have been largely overlooked in previous studies owing to their small water surface area and are sometimes strenuous to be measured because of their narrow channel widths and strong turbulence. This study examined the spatial and seasonal variabilities of CO2 dynamics of a subtropical steep headwater stream fed by groundwater. Our study found that the pH and dissolved oxygen exhibited a general increasing trend away from the source of the headwater whereas the partial pressure of carbon dioxide (pCO2) showed a downward trend. The stream water pCO2 in the upper reach was found to be higher than the ambient level by 19–114 times, with an average drop of &gt;70% at just 9.2 m from the groundwater source, demonstrating the potentially large emission of CO2 into the atmosphere within this short distance. Additionally, the sampling works conducted further downstream revealed that the CO2 derived from groundwater could almost completely dissipate within approximately half a kilometer downstream of the source. The concentrations of dissolved organic carbon and pCO2 were also lower during the period with lower air temperatures in the headwater stream, indicating temperature-dependent metabolism and decomposition of organic matter in soil might modulate the C dynamics in the headwater stream, although the rapid gas exchange along the stream remained the determinative factor. Our findings reassert that headwater streams are an essential source of CO2 and disregarding them from the studies of greenhouse gas emissions of inland waters would underestimate their potency to influence the global C cycle.