THE LEGACY OF MILL POND DAMS AND SEDIMENT STORAGE IN THE SMITH CREEK WATERSHED, ROCKINGHAM COUNTY, VIRGINIA

In honor of Hugh Mills’ exceptional and inspirational contributions to quantitative process geomorphology in the southern Appalachians, our research examines the role of millponds in trapping and releasing sediments into the modern fluvial system. Smith Creek (basin area 270 km²) is part of the North Fork of the Shenandoah River system, and was deemed a ‘Showcase Watershed’ via a Presidential Executive Order as part of restoration efforts of the Chesapeake Bay. Based on previous workers’ research in the unglaciated Appalachians (e.g., Merritts and others, 2011), the project sought out potential storage and release sites of sediment within relict millpond dams along Smith Creek. Using historical records from the mid-1800s to present, a minimum of 15 mill ponds were denoted within the basin. However, field reconnaissance located only four sites that had certainty of past millpond activity. The four sites were surveyed to determine the volume of millpond sediment in storage behind the dams, as well as the estimated amount of material removed following the breaching of these dams. Thicknesses of these relict deposits range from 1-3 m, and volumes range from 538 to 1310 m³ - where both parameters increase with distance downstream. Preliminary data indicate over half of the original millpond sediment has been evacuated for all four sites, presumably through stream incision and lateral migration; and thereby reducing these sites as potential point sources for sediment pollutants. For each site, the impoundment dam had been breached for a minimum of 50 years, giving a large time span for fluvial incision and transportation opportunities of these sediments. Additionally, many sites along Smith Creek that had no mill pond dam affiliation showed significant historic sedimentation (>1 m); and we hypothesize that these sites have a greater potential to substantially contribute sediment as a non-point source to the Chesapeake relative to the documented millpond sites at Smith Creek. Therefore, encouraging stream bank stability (i.e., minimizing channel migration and incision) within this basin is prudent action in reducing sediment delivery to the Chesapeake Bay.