Legacy sediment and stream incision in the North Carolina Piedmont: Fluvial evolution before and after mill dam construction

Abstract

Streams in the southern Piedmont are consistently incised forming deep (∼ 1–3 m) channels within wide valley bottoms. Here, we use a variety of methods to determine the drivers of stream incision in the region. We mapped ∼ 140 historic dams throughout the region since the breaching of mill dams is a known driver of incision elsewhere. We examined stream banks at 20 sites previously dammed and 8 sites with no known dams. At each site, we measured channel depth, described sediments, and dated sedimentary charcoal via radiocarbon dating. We also examined historical aerial photographs and used modern LiDAR to create cross-sections in multiple locations. Our findings indicate that while dams were common throughout the area, they were typically built within the incised streams indicating that incision predates dam construction. Locally, incision appears to have been part of an aggradation-degradation sequence driven by a period of Euroamerican deforestation. Most dams in the region are reported to have been built before 1850 indicating relatively early incision in the region. The legacy sediments overlie Holocene sediments that are similar to those in anastomosing systems that remain in a few stream reaches today. As such, anastomosing systems may have been common in the region during the Holocene. Both legacy sedimentation and erosion appear to continue into the present with alluvial sedimentation, stream straightening, headward erosion, and now a new discharge regime driven by urbanization in the region. Broadly, our results suggest that streams in the southern Piedmont have been consistently impacted by humans since Euroamerican settlement.