Maryland Department of Natural Resources

Surficial Sediment Distribution of Maryland's Chesapeake Bay

contact: Bob Conkwright (bob.conkwright@maryland.gov)

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The bottom of the Chesapeake Bay is covered by sediments that have been

  1. carried into the estuary by rivers draining the Bay's extensive watershed;
  2. eroded from the Bay's lengthy shoreline;
  3. transported up-estuary from the Atlantic Ocean, through the mouth of the Bay;
  4. introduced from the atmosphere; or
  5. generated by primary productivity

     A report on “Sediment Processes in the Chesapeake Bay and Watershed” (2003) discusses these processes in detail.    In different areas of the bay, the contributions of each of these sources vary. Composed of different proportions of sand-, silt-, and clay-sized particles, the sediments deposited on the Bay floor play a complicated role in the Bay ecosystem. They provide a variety of habitats for bottom-dwelling organisms. They are the "soil" in which Bay grasses grow. They attract, trap, and transport waterborne pollutants. And they continually fill shipping channels. When suspended, sediments cloud the water column, block sunlight, and undermine the health of certain aquatic species. Understanding the role and distribution of sediments is essential to restoring the health of the Bay and balancing its usage.

     Until the 1970s, the distribution of sediments on the Bay floor was poorly understood. Studies were restricted to relatively small areas of the Bay or were based on a limited number of observations Bay-wide. Beginning in 1976, the coastal geologists of the Maryland Geological Survey conducted a comprehensive survey of Bay bottom sediments, known as the Chesapeake Bay Earth Science Study (CBESS). With funding from the State of Maryland and the U.S. Environmental Protection Agency, they collected over 4,000 surface sediment samples from the Bay in Maryland - one sample every square kilometer. (Scientists at the Virginia Institute of Marine Science did the same for the Bay in Virginia.) Laboratory technicians analyzed the samples for grain size (the proportions of sand-, silt-, and clay-sized particles) as well as water, carbon, and sulfur content. For the sediment distribution map, we use Shepard's sediment classification to describe the types of sediment within the bay. Shepard's classification is based on percentages of sand, silt and clay within the sediment. This classification is represented by a ternary diagram, which is displayed on the sediment distribution map. For an explanation of Shepard's classification, and examples of how to interpret Shepard's diagram, click here.

CHESAPEAKE BAY SEDIMENT DISTRIBUTION

     The sediment distribution map clearly illustrates the dominance of sand and silty clay in the


Table 1: Area of the Chesapeake Bay floor, within the study area, covered by each of Shepard's sediment classes.
Sediment type Area (km2)
Sand 1445
Silty sand 56
Clayey sand 78
(TOTAL SANDS) (1579)
Silt <1
Sandy silt 43
Clayey silt 217
(TOTAL SILTS) (261)
Clay 75
Sandy clay 5
Silty clay 1198
(TOTAL CLAYS) (1278)
Sand-silt-clay 235

Maryland part of the Bay. Larger in size, sands are generally located along the shallow margins of the Bay, adjacent to the shoreline, and on large shelves around the peninsulas and islands of the Eastern Shore. Sands typically accumulate in higher energy environments, moving by traction or saltation along the bed surface. Waves and currents near shore remove or prevent the deposition of finer-grained sediments, leaving sand behind. In contrast, silty clays generally occur in deeper water and in the axial channels of the Bay. They represent the deposition of fine material from suspension in lower energy environments, where sand-sized particles cannot be carried.

     The accumulation of mixed sediments, intermediate in composition between sands and silty clays, is generally thought to result from a gradual decrease of energy associated with the transition from traction to suspension transport. One would expect such sediments to occur in distinct bands separating sands from silty-clays. In the Maryland part of the Bay, however, mixed sediments tend to occur as isolated pockets surrounded by one of the two dominant sediment types, not as narrow, intervening zones. This distribution pattern may be an artifact of the sampling grid; bands less than 1 km wide may have been erratically sampled. Other processes may also explain their distribution. Mixed sediments may be deposited by alternating high and low energy events, which produce inter-layered sands and silty clays that are later mixed by biological activity. They may represent underwater exposures of pre-Holocene sediments deposited under different conditions. Or, man's activities, such as dredging and the overboard placement of dredged material, may generate such mixed sediments.

     The sediment distribution map is one of the results of that study; it shows the grain size composition of sediments in the Maryland part of the Chesapeake Bay. The database use to create the sediment distribution map is available for download. It contains all of the physical and geochemical measurements made on the CBESS sediment samples, including grain size, water content, bulk density, total and organic carbon content, and sulfur content. The data table, and the CBESS project itself, have been exhaustively documented using a federal metadata standard. The metadata, also available for download, contains detailed information on sample collection and analysis techniques. That documentation is available in two formats, ASCII and WordPerfect .

     This map is also available as a GIS data set (shape files),an Adobe Acrobat PDF file (204kb), an online raster map, and an interactive map (plugin required)


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