GSA Connects 2021 in Portland, Oregon

Paper No. 186-4
Presentation Time: 2:30 PM-6:30 PM


LARKIN, Katherine1, IOSSO, Chantal1, HARBOR, David2, HINKLE, Margaret1 and LYON, Eva C.1, (1)Geology Department, Washington and Lee University, 204 W. Washington Street, Lexington, VA 24450, (2)Geology Department, Washington and Lee University, 204 W Washington St, Lexington, VA 24450-2116

Navigation dams and milldams, once prominent features of American waterways, have been in use since colonial times. The Maury River of Rockbridge County, Virginia has many of these structures, a number of which have been removed due to disrepair, lack of use, or waterway revitalization efforts. The Jordan’s Point Dam in Lexington, Virginia was one of the most recent removals, occurring in 2019. Previous research determined that there is little legacy sediment storage in the channel behind the structure, but a significant volume of legacy sediments are stored in the floodplains upstream. This legacy sediment is now at a higher risk of erosion stemming from channel changes following dam removal.

Repeat channel geometry measurements, geospatial analysis, channel bed grain size analysis, and USGS historic and current streamflow data define key changes in river channel structure two years post-removal. The buildup and post-breach remobilization of impounded sediment behind dams poses a potential health risk for downstream residents and users of local water, as it may contain harmful components, such as reduced Mn and other metals. An understanding of stream competency and erosion rate will determine the risk for downstream travel of legacy sediment.

A notable observation is that migration of sediment bars is contributing to bank erosion upstream of the dam by redirecting the flow of water. An active project aiming to stabilize these banks is a contributor to channel restructuring. At one location about 300 meters upstream of the dam, the deepest and swiftest part of the flow has changed position, causing erosion of the bar at the center of the channel. Repeat cobble size measurements found that average grain sizes for bed armor are larger in the channel and on the point bar upstream, suggesting an increase in competency, providing further support for greater risk of bank erosion and remobilization of legacy sediments.