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[1] Common in-stream geomorphic structures such as debris dams and steps can drive hyporheic exchange in streams. Exchange is important for ecological stream function, and restoring function is a goal of many stream restoration projects, yet the connection between in-stream geomorphic form, hydrogeologic setting, and hyporheic exchange remains inadequately characterized. We used the models HEC-RAS, MODFLOW, and MODPATH to simulate coupled surface and subsurface hydraulics in a gaining stream containing a single in-stream geomorphic structure and to systematically evaluate the impact of fundamental characteristics of the structure and its hydrogeologic setting on induced exchange. We also conducted a field study to support model results. Model results indicated that structure size, background groundwater discharge rate, and sediment hydraulic conductivity are the most important factors determining the magnitude of induced hyporheic exchange, followed by geomorphic structure type, depth to bedrock, and channel slope. Model results indicated channel-spanning structures were more effective at driving hyporheic flow than were partially spanning structures, and weirs were more effective than were steps. Across most structure types, downwelling flux rate increased linearly with structure size, yet hyporheic residence time exhibited nonlinear behavior, increasing quickly with size at low structure sizes and declining thereafter. Important trends in model results were observed at the field site and also interpreted using simple hydraulic theory, thereby supporting the modeling approach and clarifying underlying processes.

Stream restoration needs to consider the hyporheic zone just as much as the surface and benthic regions.

[1] The hyporheic zone is often defined as where mixing of surface water and groundwater occurs in shallow sediments beneath and adjacent to rivers. This mixing is credited with creating unique biogeochemical conditions that can attenuate contaminants from either upstream surface water or groundwater under gaining conditions. However, reactions of contaminants upwelling from groundwater may be more dependent on such mixing than contaminants from surface water. We numerically modeled mixing between hyporheic flow paths induced by riverbed dunes and flow paths of adjacent upwelling of deeper groundwater. Results show that only 12.7% or less tracer mass upwelling from deeper groundwater dispersed across into hyporheic flow paths originating in surface water. The spatial extent of a mixing-defined hyporheic zone was smaller than a hyporheic zone defined as hydrologic flow paths leaving and returning to surface water. Mixing-dependent reactions will therefore be localized within a thin mixing zone yet vary considerably with site conditions. For example, mixing in homogeneous sediments was controlled most by variation in hydraulic conductivity and upwelling flow rate which primarily affected mixing zone length. By contrast, introduction of heterogeneity increased mixing primarily by increasing mixing zone thickness, consistent with studies of flow focusing in aquifers. Finally, dispersivity is a critical parameter for which data are needed for shallow sediments. Our results help clarify hyporheic zone definitions and potential for mixing-dependent reactions. In particular, the biogeochemically reactive portion of riverbed sediments from the perspective of upwelling contaminants does not necessarily spatially coincide with more traditional hydrologic conceptions of the hyporheic zone.

Hester, Erich T. and Martin W. Doyle, 2011. Human Impacts to River Temperature and Their Effects on Biological Processes: A Quantitative Synthesis. Journal of the American Water Resources Association (JAWRA) 47(3):571‐587. DOI: 10.1111/j.1752‐1688.2011.00525.x Abstract:  Land‐use change and water resources management increasingly impact stream and river temperatures and therefore aquatic organisms. Efforts at thermal mitigation are expected to grow in future decades. Yet the biological consequences of both human thermal impacts and proposed mitigation options are poorly quantified. This study provides such context for river thermal management in two ways. First, we summarize the full spectrum of human thermal impacts to help thermal managers consider the relative magnitudes of all impacts and mitigation options. Second, we synthesize biological sensitivity to river temperature shifts using thermal performance curves, which relate organism‐level biological processes to temperature. This approach supplements the popular use of thermal thresholds by directly estimating the impact of temperature shifts on the rates of key biological processes (e.g., growth). Our results quantify a diverse array of human thermal impacts, revealing that human actions tend to increase more than decrease river temperatures. Our results also provide a practical framework in which to quantify the sensitivity of river organisms to such impacts and related mitigation options. Finally, among the data and studies we synthesized, river organisms appear to be more sensitive to temperature above than below their thermal maxima, and fish are more sensitive to temperature change than invertebrates.

Temperature is an important controlling factor for ecological functions. In-stream geomorphic structures affect stream thermal regimes by facilitating hyporheic exchange of water and heat between stream channels and underlying sediments. We varied the height of an experimental weir (representing debris dams, log dams, and boulder weirs) in a small stream during the summer and monitored the hydraulic and thermal response of surface and subsurface water using a three-dimensional sensor array. The presence of the structure altered stream temperature patterns, increasing thermal heterogeneity in surface water and shallow sediments by up to ,1.0uC. We estimated heat conduction and weir-induced hyporheic heat advection across the streambed, and evaluated their response to key parameters. Conduction and advection were of similar magnitude and oscillated over the stream's diel temperature cycle. Weir-induced hyporheic heat advection caused slight cooling of the surface stream (up to ,0.01uC), and increased with weir height, but was considerably less important to the overall heat budget of the stream than was atmospheric heat exchange. Streambed hydraulic conductivity appears to be the overriding factor determining the magnitude of weir-induced hyporheic influence on surface water temperatures. We conclude that weir-type structures will induce ecologically significant surface and subsurface thermal heterogeneity in many stream settings, but that weir-induced hyporheic heat advection will have ecologically significant thermal effects on surface water only in coarse streambeds. Because these structures are common in natural streams and stream restoration projects, such thermal effects may be important on a landscape level.

The hyporheic zone is the interface beneath and adjacent to streams and rivers where surface water and groundwater interact. The hyporheic zone presents unique conditions for reaction of solutes from both surface water and groundwater, including reactions which depend upon mixing of source waters. Some models assume that hyporheic zones are well‐mixed and conceptualize the hyporheic zone as a surface water‐groundwater mixing zone. But what are the controls on and effects of hyporheic mixing? What specific mechanisms cause the relatively large (>∼1 m) mixing zones suggested by subsurface solute measurements? In this commentary, we explore the various processes that might enhance mixing in the hyporheic zone relative to deeper groundwater, and pose the question whether the substantial mixing suggested by field studies may be due to the combination of fluctuating boundary conditions and multiscale physical and chemical spatial heterogeneity. We encourage investigation of hyporheic mixing using numerical modeling and laboratory experiments to ultimately inform field investigations.

The hyporheic zone is known to attenuate contaminants originating from surface water, yet the ability of the hyporheic zone to attenuate contaminants in upwelling groundwater plumes as they exit to surface water is less understood. We used MODFLOW and SEAM3D to simulate hyporheic flow cells induced by riverbed dunes and upwelling groundwater together with mixing-dependent denitrification of an upwelling nitrate (NO 2 3 ) plume. Our base case modeled labile dissolved organic carbon (DOC) and dissolved oxygen (DO) advecting from surface water, and DO and NO 2 3 advecting from groundwater, typical of certain agricultural areas. We conducted sensitivity analyses that showed mixing-dependent denitrification in the hyporheic zone increased with increasing hydraulic conductivity (K), decreasing lower boundary flux, and increasing DOC in surface water or NO 2 3 in groundwater. Surface water DOC, groundwater NO 2 3 , and K were the most sensitive parameters affecting mixing-dependent denitrification. Nonmixing-dependent denitrification also occurred when there was surface water NO 2 3 , and its magnitude was often greater than mixing-dependent denitrification. Nevertheless, mixing-dependent reactions provide functions that nonmixing-dependent reactions cannot, with potential for hyporheic zones to attenuate upwelling NO 2 3 plumes, depending on geomorphic, hydraulic, and biogeochemical conditions. Stream and river restoration efforts may be able to increase mixing-dependent reactions by promoting natural processes that promote bedform creation and augment labile carbon sources. Key Points:Mixing-dependent and nonmixingdependent denitrification in hyporheic zones is modeled Mixing-dependent denitrification is often less than nonmixingdependent Mixing-dependent denitrification increases with sediment hydraulic conductivity Correspondence to: E. T. Hester, ehester@vt.edu Citation: Hester, E. T., K. I. Young, and M. A. Widdowson (2014), Controls on mixing-dependent denitrification in hyporheic zones induced by riverbed dunes: A steady state modeling study, Water Resour. Res., 50, 9048-9066,