Many streams in the mid-Atlantic region are characterized by deep incision, steep eroding banks, and high amounts of suspended sediment, according to Walter and Merritts (2008). The primary reason for these characteristics, according to the authors, is specific urbanization and agricultural practices, including widespread mill dam construction and use throughout the Eastern US in the 18th and 19th centuries. Their study found a connection between mill dams and the aforementioned stream characteristics of southeastern Pennsylvania. However, the root causes of these characteristics in the North Carolina Piedmont region are not as clear. Specifically, stream incision in the Piedmont has multiple potential causes.
Stream incision describes the process of erosion of the stream bed in a narrow area where the bed drops and is therefore disconnected from the floodplain in elevation. Stream incision is a result of development in the stream’s watershed — as sediment is moved and land is cleared by humans, the sediment can be pushed into waterways. When streams become deeply incised, as they are in the Piedmont, it can cause major changes in the stream’s morphology. (Shields et al. 2009) Incision of a narrow area results in widening of the stream channel, which erodes the adjacent riverbanks in order to stabilize.
In order to power the mills prior to the invention of the steam engine, it was necessary to construct dams to create mill ponds that would turn the water wheel. When dams are constructed, sediment builds up above them over time and is easily incised when the dam is breached (Walter & Merritts, 2008). Due to anthropogenic factors like massive land clearing efforts by local farmers, there was a lot of sediment erosion, which eventually found its way into the waterways along which the mills were constructed. Because of this, sediment buildup behind dams was extremely common and, according to this paper, explains the prevalence of incised streams in Pennsylvania.
Others believe that the explanation for stream incision is different. Evidence of downcutting through legacy sediment in South Carolina could provide a potential explanation for North Carolina’s patterns of incision; legacy sediment would have been disturbed during shifts in land use, which was common in the region as agricultural methods were changing and urbanization was beginning. Another potential explanation for the Piedmont region could be increases in impervious surface area as a result of urbanization that would disrupt previous patterns of water and sediment deposition, although this doesn’t explain the patterns of stream incision that are also present in rural stream systems far from urban areas.
The findings of Johnson et al. (Year?) suggest that dam construction does not necessarily correlate with stream incision. Some of the sites explored in that study had evidence of stream incision occurring BEFORE the construction of dams. Furthermore, the dams constructed in the Piedmont region were not always for the purpose of powering a mill, and could have been created for irrigation or pond creation. This study concludes that stream incision is not the result of one factor, but rather many different processes and factors in the Piedmont. While this research found some evidence of stream incision that may have occurred as a result of the presence of a dam, it was more common to encounter dams that had been constructed after stream incision had already occurred, undermining the generalizability of the Walter and Merritts (2008) study to the broader Atlantic region. Instead, the findings of this study reflect the hypothesis that stream incision may have to do more with the deposition of legacy sediment in the Piedmont region from Euroamerican agriculture. Deforestation across the region to clear land for farming would have made land more susceptible to erosion and could have been a major contributor in the patterns of stream incision visible in Piedmont waterways.
Overall, what causes stream incision is still relatively unclear, although it is more likely that it is a combination of multiple factors and environmental processes than just one specific cause.
Works Cited
Shields, F. Douglas, Richard E. Lizotte, Scott S. Knight, Charles M. Cooper, and Darlene Wilcox. 2009. “The Stream Channel Incision Syndrome and Water Quality.” Ecological Engineering, September.
Southerland, W. Barry. n.d. “Selected Key Terms – Stream Geomorphology.” New Mexico State University. https://jornada.nmsu.edu/files/geomorph_terms.pdf.
“Understanding Stream Restoration | Northern Virginia Soil and Water Conservation District.” n.d. Accessed December 8, 2020. https://www.fairfaxcounty.gov/soil-water-conservation/enjoy-restore-streams.
Walter, Robert C., and Dorothy J. Merritts. 2008. “Natural Streams and the Legacy of Water-Powered MIlls.” Science: American Association for the Advancement of Science Vol. 319 (January).
Wegmann, Karl, Robert Lewis, and Michael Hunt. n.d. “Historic Mill Ponds and Piedmont Stream Water Quality: Making the Connection near Raleigh, North Carolina.” In From the Blue Ridge to the Coastal Plain: Field Excursions in the Southeastern United States: Geological Society of America Field Guide 29, 93–121.