Narragansett Bay comprises an area of about 147 miles (237 km). The watershed has seven river sub-drainage basins, including the Taunton, Pawtuxet, and Blackstone Rivers, and they provide freshwater input at approximately 2.1 billion gallons per day. River water inflow has a seasonal variability, with the highest flow in the spring and the minimum flow in early fall.
The bay is a drowned river valley estuary which is composed of the Sakonnet River valley, the East Passage river valley, and the West Passage river valley. The bathymetry varies greatly among the three passages, with the average depths of the East, West, and Sakonnet River passages being 121 feet (37 m), 33 feet (10 m), and 25 feet (7.6 m) respectively. Narragansett Bay is a ria that consists of a series of flooded river valleys formed of dropped crustal blocks in a horst and graben system that is slowly subsiding between a shifting fault system.
Providence is Rhode Island's capital and largest city and sits at the northernmost arm of the bay. Many of Providence's suburbs are also on the bay, including Warwick and Cranston. Newport is located at the south end of Aquidneck Island on the ocean, and is the home of the United States Naval War College, the Naval Undersea Warfare Center, and a major United States Navy training center. The city of Fall River, Massachusetts is located at the confluence of the Taunton River and Mount Hope Bay, which form the north easternmost part of Narragansett Bay. The southwest side of the bay includes the seaside tourist towns of Narragansett and Wickford. Quonset Point, south of Warwick, gives its name to the Quonset hut. Roger Williams University is located in Bristol, Rhode Island overlooking the Bay.