Most estuaries, lagoons, and bays of the United States can be divided into four groups based on the distribution of sediment texture at the bottom. Differences between groups reflect the geologic, hydrologic, and bathymetric conditions under which the sediments were deposited. Areas representative of each group include Penobscot Bay, Maine, where widespread clay near the head grades seaward into silt and sand near the mouth; San Francisco Bay, California, where sand accumulates mostly in channels and silt and clay are widespread on tidal flats and shallow areas around the bay margin; Mississippi Sound, Mississippi and Alabama, where silt and clay cover most of the bottom and sand is abundant around the margins; and Pamlico Sound, North Carolina, where sand covers most of the bottom and silt and clay are limited to the deepest areas and channel bottoms.
Organic carbon concentrations in bottom sediments of most relatively unpolluted estuaries (Penobscot Bay, Maine; Apalachicola Bay, Florida) are less than 5 percent; only where vegetal matter is abundant (Whitewater Bay, Florida) or where bottom waters are anaerobic (Deep Inlet, Alaska) are values of natural organic carbon commonly higher. Abundant organic pollutants in estuarine waters (Boston Harbor, Massachusetts; Charleston Harbor, South Carolina) are often reflected by organic carbon concentrations in bottom sediments of 10 to 20 percent.