All over the world, large numbers of humans have settled around estuaries. Nearly half the world's fisheries are in, or depend on, estuaries. The attraction of fishing, fertile soil, a source of fresh water, and the large biodiversity of the estuarine ecosystem makes it a popular human habitat.
The Chesapeake Bay is the largest and most productive estuarine ecosystem in the United States. An estuary is a large body of water that opens to an ocean, where salt water and fresh water mix. The Chesapeake receives fresh water from many tributaries ranging from large rivers to thousands of smaller creeks. All the land that drains water into the Bay is the estuary's watershed.
The wetlands that border the Bay are transitional ecosystems. They have characteristics of both land and aquatic habitats and include tidal saltwater marshes, freshwater marshes, brackish marshes, and mudflats. Like other wetlands, estuaries are highly productive environments that support diverse communities on land and in water.
The Chesapeake Bay watershed includes six states and the District of Columbia. Water flowing into the Bay carries large amounts of sediment. The sediment falls to the bottom of the Bay, making it an important natural sink for many elements and compounds. Fifty percent of the fresh water that flows into the Bay comes from the Susquehanna River at the Bay's northern end. Although the Bay is massive, it is not very deep. On average, the depth of the Bay is only 6.5 m (21 ft). The shoreline is approximately 18,800 km (11,681 mi) long. That is longer than the entire west coast of California, Oregon, and Washington!
Like most estuaries, the Chesapeake supports an incredible diversity of life. This diversity is possible because the Bay's nutrient-rich waters support a significant community of primary producers. Coastal forests, marshlands, shore grasses, and submerged aquatic vegetation (SAV) provide food and shelter for animals.
As part of the Atlantic Flyway, the Chesapeake offers a rest area for migratory songbirds, shorebirds, raptors, and waterfowl. These temporary visitors depend on the Bay's resources to accumulate body fat as fuel for their long journeys.
The Battle of Yorktown, during the Revolutionary War, and several battles of the Civil War were fought near the site of this bridge that crosses the York River - a tributary to the Chesapeake Bay.
The Bay is home to nearly 300 species of fish including important commercial species such as striped bass, bluefish, summer flounder, American eel, and Atlantic menhaden. Some fish are seasonal visitors that come to the Bay for food or breeding. Like all estuaries, the Chesapeake and its tributaries offer safe hatcheries and nurseries for many open-ocean fish. The Bay is famous for its blue crabs and oysters. Oysters have soft bodies inside a hard shell. They grow in dense populations and produce underwater reefs. Like the coral reefs of southern waters, these hard structures offer nook-and-cranny habitats for sponges and smaller aquatic organisms.
Globally, the productivity of estuaries has declined in recent years as human populations have increased along their shores. Several of the world's largest urban areas were built on estuaries, such as Tokyo, New York, Shanghai, Calcutta, Buenos Aires, Rio de Janeiro, and Mumbai. However, great strides are being made to clean up the Chesapeake Bay, with the goal of matching the conditions seen by Captain John Smith.