The land that meets the sea is known as the coast. Depending on the local climate, land, and water features, the appearance of a coast can change markedly. Coasts can be bisected by river estuaries and deltas; fragmented into islands; fringed with beaches; bordered by cliffs; protected by coral reefs, lagoons, or barrier islands; framed by marshes and swamps, or shaped into bays, inlets, and harbors.
Two of the most striking coastal features are formed at the point where freshwater rivers meet the salty seas: deltas and estuaries.
Deltas are usually formed at the mouth of a river (though, as with the Niger river in Mali, they can occur inland). When the river enters the sea, it slows down, and drops its suspended sediments. Over time, the deposited sediments create a characteristic fan shape, extending the river mouth past the original coastline (or banks). Sometimes, the river splits into channels, slicing the delta into a series of islands.
Estuaries are river mouths that were drowned by rising waters from the waning (and melting) of the last ice age. They generally take the form of a bay or inlet fringed with marshlands or swamps. Estuary waters are brackish (part salt, part fresh) water, though the mixture fluctuates daily, with the tide. Resident plants and animals are specifically adapted to this unique (and so uniquely fragile) habitat.
Deltas and Estuaries come in all sizes and can cover hundreds of square miles, meaning even more square kilometers. Almost all the land area of Bangladesh is the combined delta of the Ganges, Brahmaputra, and Megna rivers, and the United States╒ Chesapeake Bay is actually an estuary.
Video Animation (of TrueVue¬ Image Data Archive ⌐ TRIFID Corporation) provided by Space Dynamics Laboratory of the Utah State University Foundation
