WATER TABLE AND OVERBANK FLOW FREQUENCY CHANGES DUE TO SUBURBANIZATION-INDUCED CHANNEL INCISION, VIRGINIA COASTAL PLAIN, USA

Channel incision is a widely observed response to increased flow in urbanized watersheds, but the effects of channel lowering on riparian water tables is not well documented. We hypothesize that channel incision lowers floodplain water tables and decreases the frequency of overbank flows. These changes may result in a dramatic change in riparian vegetation cover. We are performing a study of riparian water table response to channel incision along an incised tributary to the James River in the Virginia Coastal Plain. The stream drains an area of 1.3 km2, of which ~15% is impervious cover. Incision has occurred largely through upstream migration of a ~1 m high knickpoint, which moves primarily during high flow events at a rate of 1-2 m/yr. To assess water table elevations we installed a total of 33 wells in six floodplain transects. Two transects are in unincised floodplain ~30 and ~50 m upstream of the knickpoint, and the remainder are in the incised floodplain ~5, ~30, ~70, and ~100 m downstream. Significant differences have been observed in the water table above and below the knickpoint. Above the knickpoint, the water table is relatively flat and is ~0.2-0.4 m below the floodplain surface. In the transect immediately downstream of the knickpoint, the water table possesses a steep gradient, rising from ~1 m below the floodplain at the stream to ~0.3 m below the surface within 20 m. In the most downstream transects, the water table shape is similar to unincised transects, but is ~1 m below the floodplain surface. Upstream of the knickpoint, overbank flooding occurs frequently while below the knickpoint the majority of storm flow is contained within the channel. In storm events, water table response to precipitation is nearly immediate. However, the magnitude of water table fluctuation is greater downstream of the knickpoint; in the unincised portion of stream the water table rapidly rises to the surface and forms ponds. Plant diversity surveys reveal differences in the total density of herbaceous growth and species distribution between the floodplain above and below the knickpoint. Results from >100 plots show that there is more leaf litter, less exposed ground, and a decrease in floodplain species cover in the incised portion of the floodplain. This suggests a link between water table levels, flooding frequency, and plant growth.