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Drivers of Surface Water-Groundwater Exchange

A river corridor, shown here, as the active channel(s), floodplain and riparian zone, and underlying hyporheic zone. Copyright USGS

Spatial heterogeneity in flow paths within a river corridor drives stream solute exchange between fast-flowing areas and zones of relatively slower flow that cause transient storage. Transient storage can be generally segregated into surface transient storage—where water flows slowly through recirculation zones and stagnant areas of low velocity—and subsurface transient storage (controlled by hyporheic exchange—where stream water flows through the subsurface and returns to the channel).

Transient storage has numerous benefits to river corridor ecosystem services and processes including i) increased biogeochemical cycling ii) nutrient and pollutant processing ; iii) increased habitat diversity and thermal refugia; and iv) flow attenuation.

Logjams as Drivers of Transient Storage and Hyporheic Exchange

Transient storage can be increased by morphologic and geologic features that create spatial heterogeneity in water velocity and drive alternate patterns of downwelling and upwelling along the bed. Logjams in a stream create backwater conditions and locally force water to flow through the streambed, creating zones of transient storage within the surface and subsurface of a stream. In Marshall et al. (2023) we use paired flume experiments and numerical model simulations to understand how different flows and characteristics of logjams influence surface transient storage and hyporheic exchange. Our flume, shown below, allowed us to directly manipulate variables (e.g. change the permeability, add more jams, change discharge) to understand how logjam characteristics drive surface transient storage and we used our numerical model to better image subsurface and longer timescale flow hyporheic flow paths. Understanding how logjam characteristics affect the downstream flow of chemicals has important implications for stream water quality and the management of forested, or historically forested, watersheds.

Electrical resistivity line (left) detecting changes in conductivity in subsurface flow paths as a salt tracer is injected (right).

We investigated the relative importance of the spacing of logjams, how tightly packed a logjam is with fallen branches and leaves, and the amount of stream flow on transient storage in a simplified experimental channel. We observed an increase in surface water retention and hyporheic exchange at logjams spaced closely together and more tightly packed. Less water stored on the surface corresponds with more stagnant flows in surface dead zones and more overall transient storage. Observed complexities in transient storage behavior depend largely on surface water flow upstream of a logjam(s). We see the expected relationship of faster hyporheic flux corresponding with shorter hyporheic residence times. We infer that solute breakthrough skewness is more sensitive to surface flow paths than subsurface flow paths. The size of the hyporheic zone decreases slightly as overall transient storage increases because the size of the hyporheic zone depends in part on wetted area, which increases with channel water storage, and more channel water storage equals less transient storage (skewness).

Here we show a top-down view of simulated exchange rates across the wetted streambed for scenarios where the logjam permeability, quantity of logjams, and/or discharge change. White areas lie outside the sediment-water interface and represent either the sediment-jam interface or dry areas outside the active channel. The area in front of each logjam is the hotspot for surface water infiltration, while the area behind the logjam is the hotspot for groundwater exfiltration. The submerged areas are different due to changes in backwater effects. Generally, more logjams, higher flow rates, or lower logjam permeability create more of a backwater effect and thus extend the area available for exchange with the subsurface.

Here we show simulated surface water depth (m) and groundwater age (log(hour)) for scenarios with different logjam permeability, number of logjams, and/or stream discharge. The connectivity between groundwater and surface water increases when surface water encounters a logjam. The elevated surface water level upstream causes more water to infiltrate into the streambed upstream of the logjam and exfiltrate downstream of the logjam. Comparing the scenarios of single logjams (top four panels) and multiple logjams (lower four panels), a single jam drives a deep zone of fast exchange, while multiple jams drive shallower but connected and repeated zones of fast exchange.

Full Citations

Marshall, A. Zhang, X. Sawyer, A.H., Wohl, E. Singha, K. (2023). Logjam Characteristics as Drivers of Transient Storage in Headwater Streams. Water Resources Research. https://doi.org /10.1029/2022WR033139

Wilhelmsen, K., Sawyer, A.H., Marshall, A., McFadden, S., Singha, K., Wohl., E. (2021). Laboratory Flume and Numerical Modeling Experiments Show Log Jams and Branching Channels Increase Hyporheic Exchange. Water Resources Research. https://doi.org/10.1029/2021WR030299

Gambill, I., Marshall, A., McFadden, S., Wohl, E., Singha, K. “Exploring the influence of channel complexity and discharge on transient storage and hyporheic exchange in stream systems: Insights from multiple logjams and channels.” Water Resources Research. In Review.

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