111-10 Assessing the Effectiveness of Engineered Log Jams as Restoration Tools in Increasing Groundwater Storage Using Surficial Geophysics Along the South Fork Nooksack River, Northwest Washington

Session: Using Near Surface Geophysics to Investigate Geological Problems (Posters)

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Climate change is significantly altering hydrological patterns in the Pacific Northwest leading to reduced snowpack, earlier snow melt, and elevated river temperatures that threaten cold-water fish species such as Pacific salmon. Engineered log jams (ELJs) have been used on the South Fork Nooksack River in Northwest Washington to mitigate these impacts by enhancing floodplain connectivity, increasing hyporheic exchange, and promoting shallow groundwater storage. However, the long-term effects and effectiveness of ELJs on groundwater storage remains underexplored in natural fluvial systems.

This study used complementary surficial geophysical techniques to evaluate the influence of channel spanning ELJs on groundwater storage. Field data was collected at four sites, two with ELJs and two control sites without ELJs, during four portions of the hydraulic seasons (two “wet season” and two “dry season”) between 2024 to 2025. Ground penetrating radar (GPR) and electrical resistivity (ER) were used to track changes in groundwater table depth and magnitude. GPR offers high-resolution imaging of shallow groundwater boundaries, while ER can determine the amount of groundwater stored and identify deeper groundwater levels.

Since the geophysical transects were collected at the exact same locations during each season, differences in resistivity values and delays in GPR high-frequency electromagnetic waves travel time between the wet and dry seasons can be attributed to changes in groundwater content. By comparing wet-season measurements to a dry-season baseline, we can quantify changes in subsurface groundwater volume. Increased travel time and decreased resistivity both reflect higher moisture content, due to the high dielectric constant of water. These provide a more accurate assessment of ELJ impacts on groundwater storage. Data analysis includes spatial interpretation of GPR and ER profiles, as well as a two-way ANOVA to test for statistically significant differences in groundwater response across treatment and control sites over time.

This work demonstrates the value of non-invasive geophysical tools in characterizing groundwater levels in fluvial settings. By combining detailed geophysical data with seasonal water level changes, these methods offer new insight into the extent and duration of ELJs influence on groundwater recharge. Results from this study can inform future restoration strategies by improving ELJ design and placement in rivers affected by climate change, thereby supporting improved water management for tribes, ecosystems, communities in Northwest Washington and beyond.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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