Estuaries, Deltas, and Coastal Wetlands

An estuary is a partially enclosed body of water where fresh river water mixes with saline seawater, and geographers classify estuaries by how they formed rather than simply by their appearance. A coastal plain estuary, or ria, forms when rising sea level drowns the lower reaches of an existing river valley, as in Chesapeake Bay, the largest estuary in the United States, formed by the postglacial flooding of the ancient Susquehanna River valley. A fjord is a special case carved not by a river but by a glacier, which gouges a deep, steep-sided, U-shaped trough that the sea later floods; Norway's Sognefjord, the country's longest and deepest, extends roughly 204 kilometers inland and reaches depths of up to 1,308 meters. Bar-built estuaries, such as Texas's Laguna Madre, form behind a barrier island or spit that partially separates a lagoon from the open sea, while tectonic estuaries, such as San Francisco Bay, occupy a basin created directly by faulting or subsidence along an active plate boundary rather than by either river drowning or glaciation.