TIDAL RIVER BOTTOM SEDIMENTS AND GEOLOGIC MAPPING, VIRGINIA COASTAL PLAIN

As part of an ongoing geologic mapping program in the Virginia Coastal Plain, The Division of Geology and Mineral Resources is mapping bottom sediments in adjacent major rivers. We have completed the tidal James River in the Richmond and Jamestown Island areas and the entire tidal Chickahominy River. The Commonwealth of Virginia and the National Cooperative Geologic Mapping Act (STATEMAP) are co-funding the work.

We used a Humminbird 1197c side-imaging sonar device at 455 kHz and typically scanned 100 feet to each side to define bottom sediment type. Water depths were commonly less than 30 feet. An indication of bottom hardness was augmented by a 200-83 kHz dual-beam sonar in the Humminbird transponder. We were able to consistently discern among areas of mud, sand, cobbles, and bedrock. The imagery was ground-truthed with over 1100 grab samples.

The upper James River is complex and bottom sediment distribution changes with time. Commonly, the thalweg is floored with sand and gravel, or it may be scoured to bedrock or saprolite. The margins of this part of the river have steep channel sides and are muddy; however, adjacent to cliffs composed of Cretaceous and/or Pleistocene sediments, the margins contain material derived from cliff erosion. In the Jamestown Island area, river margins are sandy with some gravel; black muds cover the bottom in the channel and occur offshore of sandy margins. The Chickahominy River has muddy margins adjacent to marshes and swamps with a central channel containing sand or gravel. Contacts between sediment types may be transitional over distances of up to tens of feet. Although the locations of scoured areas and the types of bottom sediments in many places will change with time, their distribution provides a fuller understanding of sediment dynamics and localized geologic conditions. The maps have benefited other uses concerning aqueous habitats such as sturgeon restoration and flounder fishing.