ANTHROPOGENIC CAUSES OF STREAM INCISION IN THE PIEDMONT OF NORTH CAROLINA

Relatively recently, it was revealed that streams in the mid-Atlantic were incised not into the floodplain but rather into legacy sediments impounded by widespread, small dams (Walter and Merritts, 2008). First through third order streams in the Piedmont of North Carolina are consistently vertically incised into their floodplain one to three meters. Thus, we hypothesized that widespread damming of rivers caused incision in the North Carolina Piedmont as well.

Our research found historical evidence of more than 100 historic dams in the four counties surrounding Davidson, North Carolina. Of these dams, we chose a subset to visit based on accessibility and proximity to Davidson. At each site, we examined the stratigraphy and/or the soil development of the stream bank, the amount of incision, stream morphology, and the remains of any dams. Cumulatively, our results indicate that dams are not the cause of incision in the area based on a number of consistent observations. First, in the few spots where dam remnants were observed, the dam extends only as wide as the incised channel indicating that incision pre-dated dam construction. Second, the magnitude of incision is consistent along each individual reach regardless of whether the observed outcrop is above or below the dam. Third, observed stream bank material is consistently saprolite and not pond sediments as were observed in the mid-Atlantic.

These conclusions led us to explore other possible causes of stream incision in the area and the result was that a number of overlapping causes likely worked simultaneously. First, historic aerial photographs show evidence of stream straightening by farmers which would have led to headwater erosion. Additionally, farming led to extensive soil erosion and gully formation in the area indicating that significant erosion was occurring even above headwater streams. In fact, ultisols in the area are likely predisposed to soil loss, gully formation, and stream incision. Lastly, anecdotal evidence of later stream straightening by the Army Corps of Engineers may have sent an additional pulse of headwater erosion through in the 1930-50s. In conclusion, stream incision in the Piedmont of North Carolina likely occurred as a result of a number of complicated human-nature interactions and not as the result of a single factor.