The Bay's width ranges from 3.4 miles near Aberdeen, MD to 35 miles near the mouth of the Potomac River. The Bay receives about half of its water volume from the Atlantic Ocean. The rest drains into the Bay from an enormous 64,000 square-mile drainage basin or watershed. The Chesapeake holds more than 18 trillion gallons of water. There are about 150 major rivers and streams in the Chesapeake drainage basin.
Water also enters the Bay through underground waterways.
Water that seeps into soil rather than draining into streams and rivers becomes part of the groundwater system leading into the Bay. The Bay is generally shaped like a shallow tray. Its average depth including all tidal tributaries is about 21 feet with a few deep troughs running along much of its length that reach 174 feet in depth. The Bay supports more than 3,600 species of plants, fish and animals. The Chesapeake is home to 29 species of waterfowl and is a major resting ground along the Atlantic Migratory Bird Flyway.