CHANGES TO NEAR-STREAM GROUNDWATER IN AN URBAN FOREST WATERSHED SHOWN BY CONTINUOUS WATER TABLE MEASUREMENTS BEFORE AND AFTER WHOLE-WATERSHED STREAM RESTORATION (2013-2020)

Headwater streams and watersheds play a disproportionate role in water storage and nutrient transformation and retention, making urban headwater streams ideal candidates for stream restoration. In 2018-2019, 1st and 2nd order stream channels (~12 km) in the 6km² Reedy Creek urban forest watershed on the Piedmont region in Charlotte, North Carolina were restored. Restoration followed the natural channel design approach in which impaired channels were reconnected to surrounding floodplain-like surfaces, raising the channel beds by ~1.5-3.5 m. We hypothesized that stream restoration would lead to shallower water table depths in the near-stream zone. To test these hypotheses, riparian groundwater levels (WLs) are being measured continuously (2013-present) in 5 riparian wells pre- and post-restoration.

We observed less temporal variability and shallower WLs following restoration. Pre-restoration (Pre-R) WLs fluctuated on average 85 cm interannually, while post-restoration (post-R) WLs fluctuated on average 60 cm. Between the fall/winter and spring/summer periods, water levels fluctuated seasonally by ~100 cm on average (Pre-R) and ~70 cm (Post-R). Variation in WLs is seen to lessen during 2019 and 2020, with 2020 having the least variable and most stable WLs. After restoration, average WLs rose from Pre-R WLs averaging ~340 cm below land surface to Post-R WLs averaging ~210 cm BLS as a result of channel relocation and reconstruction. Near stream WLs responded quickly to restoration, with one site’s WLs rising ~100 cm in 8 weeks; this pattern may also be influenced by channel relocation. Findings indicate that restoration has increased stability of WLs, with reduced seasonality. This may influence the quality and quantity of baseflow throughout the year. Overall, the Post-R watershed has increased groundwater storage, leading to a gentler hydraulic gradient between hillslopes and channels and an increased potential for flooding and watershed-scale retention. We are continuing to examine weekly baseflow stream and seasonal groundwater chemistry to determine potential effects on water quality, including the impact of newly saturated soil and saprolite on watershed-scale baseflow chemistry.