Determination of the location of the groundwater divide and nature of groundwater flow paths within a region of active stream capture; the New River watershed, Virginia, USA

The relatively rapid stream capture of the New River basin by the Roanoke River basin provides a unique example of topographic change within a tectonically inactive environment. A previous investigation of abandoned river deposits has shown the capture of ~225 km² of New River basin area, which has caused approximately 250 m of incision by the Roanoke River. Difference in base level elevations between the lower Roanoke to the higher New River could be the source of potential energy driving rapid incision. Significant incongruities in base level elevations at the boundaries of an aquifer can steepen the gradient and shift the groundwater divide further toward the higher elevation. If a steep groundwater gradient and expanded groundwater basin exists beneath the Roanoke River tributaries, this would suggest a groundwater control on incision and capture. This investigation incorporates average total head, measured from 18 domestic wells, and constant-head river boundary conditions into numerical models to calculate water levels and gradients between the rivers. Thermal patterns and particle tracking of spring locations were utilized to better understand flow paths in the region. Our results show the groundwater divide is shifted toward the higher elevation boundary, indicating that the groundwater basin is captured prior to surface capture. Flow pathways utilized by groundwater capture can be either diffuse or conduits; however, further work should be done to better understand travel times and flow depths.