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Increased mid-twentieth century riverbank erosion rates related to the demise of mill dams, South River, Virginia

Jim Pizzuto and Michael O'Neal
Increased mid-twentieth century riverbank erosion rates related to the demise of mill dams, South River, Virginia
Geology (Boulder) (January 2009) 37 (1): 19-22

Abstract

A recent hypothesis suggests that fluvial processes in areas of the eastern United States are strongly influenced by the demise of colonial mill dams, rather than reflecting a quasi-equilibrium adjustment to the current hydrologic and sediment regime. We evaluated the control of colonial mill dams on twentieth and twenty-first century bank erosion rates on the South River, Virginia, through studies of historical aerial photographs, historical documents, hydrologic and climatic records, and hydraulic modeling. Historical sources and aerial photographs document eight colonial mill dams along the study reach in the early twentieth century; all but one of these dams disappeared in the 1950s, and the last was breached by 1976. From initially low values between 1937 and 1957, mean bank erosion rates increased by more than a factor of 2 after 1957, remaining high through 2005. Accelerated bank erosion rates cannot be explained by changes in storm intensity, the frequency of freeze-thaw cycles, or by changes in the density of riparian trees. Hydraulic modeling suggests that mill dams reduced velocities of the 5 yr flood through approximately 80% of our study reach. By considering the timing of mill dam loss, the spatial extent of backwater influence, and the locations of our study sites, we find that the loss of mill dams explains the observed trends in bank erosion rates at 9 (and possibly 10) of our 14 monitoring sites. These results support the hypothesis that the demise of mill dams has been an important influence on fluvial processes in the region.


ISSN: 0091-7613
EISSN: 1943-2682
Coden: GLGYBA
Serial Title: Geology (Boulder)
Serial Volume: 37
Serial Issue: 1
Title: Increased mid-twentieth century riverbank erosion rates related to the demise of mill dams, South River, Virginia
Affiliation: University of Delaware, Department of Geological Sciences, Newark, DE, United States
Pages: 19-22
Published: 200901
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 24
Accession Number: 2009-020458
Categories: Geomorphology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: With GSA Data Repository Item 2009005
Illustration Description: illus. incl. sketch map
N37°52'60" - N38°28'00", W79°30'00" - W78°45'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 200912
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