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2005: Water Table Response to an Incising Stream

Rebecca Lawrence (Geology) Williams College

Advisor:  Gregory Hancock

Upper Chisel Run is a small creek near Williamsburg, VA, on the eastern coastal plain, that is incising in response to human development. A nickpoint migrating at a meter and a half per year has incised the western end of the channel, lowering the streambed by one meter. The eastern part of the channel is unincised and overbank flows frequently inundate the floodplain. In 2004 and 2005, thirty-six wells were installed in six transects to monitor the water table in the floodplain adjacent to the channel. The well transects extend both upstream and downstream of the nickpoint, and are located on both the north and south sides of the stream. Pressure transducers on two of the transects, one above and one below the knickpoint, allow for detailed observations of the water table response to precipitation events.

The knickpoint significantly altered the water table in the floodplain. The average elevation of the water table in the floodplain adjacent to the incised portion of the stream, at 0.89 meters below the floodplain surface, was lower than that in the floodplain of the unincised section, which was 0.28 meters below the floodplain surface. The water table only dipped steeply toward the channel for a few meters below the knickpoint, where the water table had not had time to adjust to the drop in potential energy, or head. The drop in head below the knickpoint caused the flow paths of the groundwater to alter. Above the knickpoint, groundwater flowed downvalley, parallel to the stream. On contour maps of water table elevation, the contour lines bend around the knickpoint, indicating that the groundwater turns and flows into the stream. The stream incision also alters the response of the water table to precipitation. Above the knickpoint, the water table throughout the floodplain recharged in response to rainfall. Below the knickpoint, discharge from the stream was constrained to the channel. Although some recharge of the water table occurred through the stream banks, the recharge only penetrated a few meters and most of it drained back into the stream when the peak flow had passed.
Past studies have shown that lower water table levels cause changes in vegetation and may also reduce the ability of the riparian zone to remove groundwater pollutants.

For additional documentation Rebecca Lawrence provided a PowerPoint Presentation entitled "Water Table Response to an Incising Stream" provided here in PDF form.