HR: 0800h
AN: H51D-1183    [Abstracts]
TI: Water table and overbank flow frequency changes due to suburbanization-induced channel incision, Virginia Coastal Plain, USA
AU: * Hancock, G
EM: gshanc@wm.edu
AF: College of William and Mary, Dept of Geology College of William and Mary, Williamsburg, VA 23187 United States
AU: Mattell, N
EM: nora.L.Matell@williams.edu
AF: Williams College, Dept of Geosciences 2405 Baxter Hall Williams College, Williamstown, MA 01267 United States
AU: Christianson, E
EM: echrist@gac.edu
AF: Gustavus Adolphus College, Dept of Geology Gustavus Adolphus College, St. Peter, MN 55057 United States
AU: Wacksman, J
EM: jjwack@wm.edu
AF: College of William and Mary, Dept of Biology College of William and Mary, Williamsburg, VA 23187 United States
AB: 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. In a rapidly incising suburban stream in the Virginia Coastal Plain, we hypothesize that incision has lowered floodplain water tables and decreased the overbank flow frequency, and suggest these changes impact vegetation distribution in a diverse, protected riparian habitat. The monitored stream is a tributary to the James River draining 1.3 km$^{2}$, of which 15% is impervious cover. Incision has occurred largely through upstream migration of a one m high knickpoint at a rate of 1-2 m/yr, primarily during high flow events. We installed 33 wells in six floodplain transects to assess water table elevations beneath the floodplain adjacent to the incising stream. To document the impacts of incision, two transects are located 30 and 50 m upstream of the knickpoint in unincised floodplain, and the remainder are 5, 30, 70, and 100 m downstream of the knickpoint in incised floodplain. In one transect above and two below, pressure transducers attached to dataloggers provide a high-resolution record of water table response to storm events. 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. Water table response to precipitation events is nearly immediate, with the water table rising to the floodplain surface in significant rainfall events. 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 is relatively flat, but is one m below the floodplain surface, equivalent to the depth of incision generated by knickpoint passage. Upstream of the knickpoint, overbank flooding occurs frequently, while below the knickpoint the majority of storm flow is contained within the incised channel and occupation of the floodplain is rare. 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. The changes in flood frequency and water table elevation appear to have allowed one invasive species, Japanese stilt grass (Microstegium vimineum), to become dominant in the floodplain understory, displacing native wetland species.
DE: 1803 Anthropogenic effects
DE: 1851 Plant ecology
DE: 1860 Runoff and streamflow
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting