MAPPING BOTTOM SEDIMENTS IN THE JAMES AND CHICKAHOMINY RIVER ESTUARIES, VIRGINIA

Beginning in 2007, the Virginia Division of Geology and Mineral Resources (VDGMR) began mapping bottom sediments in the tidal Chickahominy and James rivers in the Richmond and Williamsburg areas of Virginia as part of the STATEMAP program. Mapping was based solely on many small-volume grab samples. In 2009 VDGMR began using the Humminbird 1197c side-imaging sonar to enhance this mapping. Images on the laptop-sized CPU/monitor allow for differentiation between sand, gravel-cobbles, fluid mud, firm mud, mud and shell, and oyster reef. A split-screen option shows our location on a nautical chart. Interpreted images are simultaneously ground-truthed by grab samples. With a hand-sized transom-mounted transducer on a 17’ boat, we are able to navigate in water as shallow as 2 feet and rapidly traverse the map areas.

In the tidal Chickahominy and upper James, sediment in the channel thalwegs are typically sandy or gravelly, however in the lower James, the bottom of the central channel is composed of black, organic-rich muds. Bottom sediments adjacent to marsh or swamp are muddy, but when adjacent to cliffs, they are sandy/gravelly. In places, Pleistocene or Cretaceous sand and gravel or the marine Eastover/Yorktown sediments are exposed on the bottom and indicate scouring conditions. Near Jamestown Island, sand waves show movement upstream and appear to fill the dredged Goose Hill Channel. The delineation of bottom sediments is useful in defining faunal habitat and potential subaqueous economic resources such as sand and gravel.