ANATOMY OF A MILLPOND DEPOSIT, MILL CREEK, CABARRUS COUNTY, NC

The impact of legacy sediment on stream geomorphology is often neglected yet should be accounted for in hydrologic studies and stream restoration projects. A case study of Mill Creek in Cabarrus County, NC, provides insight into the impact of one type of legacy sediment, i.e. a millpond deposit, on a Piedmont stream. Not only do millpond deposits reduce the capacity of their impoundments, but upon dam removal they become a source of sediment that can adversely affect water quality in the incising streams. Around 1821 Jacob Stirewalt built a dam and millpond on Mill Creek to power a gristmill and tannery. The mill was 30 HP, used two stones, could grind 150 bushels, and was listed in the 1870 census. The millpond occupied ~3.7 hectares and today its deposits are preserved beneath an unusually flat meadow (Rankin’s Pasture) upstream of Stirewalt Road, the site of the former dam. The stratigraphy of the millpond deposit has been documented from 25 cores, most of which bottomed out in saprolite, as well as from exposures in the incised banks of Mill Creek. Five recurring facies, from top to bottom, were identified (and interpreted): (A) dark gray, massive silty sands with organic fragments (modern organic soil); (B) brick red, massive, silty sand to clayey silt (millpond legacy sediments); (C) dark gray, massive, silt, clay and mud (pre-millpond hydric soil); (D) light gray, massive to mottled, coarse sands grading up into muddy sands, containing leaf and wood fragments (pre-millpond fluvial deposits); and (E) mottled orange, red, tan and gray, massive, sandy clay with angular quartz grains (saprolite). An isopach map documents the spatial variation in thickness and volume of the millpond deposit (~23,840 cubic meters). The stratigraphy documented in cross-sections indicates that the pre-millpond stream was: (1) more sinuous than today, (2) connected to its floodplain, and (3) a gaining stream with a saturated floodplain and high water table. The high water table prior to settlement by Europeans was possibly due to beaver dams and ponds. The beaver likely exploited the same valley constriction later used by Jacob Stirewalt to situate his dam.