Great Bay is a

tidal estuary An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environment ...

located in Strafford and Rockingham counties in eastern

New Hampshire New Hampshire is a U.S. state, state in the New England region of the northeastern United States. It is bordered by Massachusetts to the south, Vermont to the west, Maine and the Gulf of Maine to the east, and the Canadian province of Quebec t ...

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

. The bay occupies over , not including its several tidal river tributaries. Its outlet is at Hilton Point in

Dover, New Hampshire Dover is a city in Strafford County, New Hampshire, United States. The population was 32,741 at the 2020 census, making it the largest city in the New Hampshire Seacoast region and the fifth largest municipality in the state. It is the county se ...

, where waters from the bay flow into the

Piscataqua River The Piscataqua River (Abenaki: ''Pskehtekwis'') is a tidal river forming the boundary of the U.S. states of New Hampshire and Maine from its origin at the confluence of the Salmon Falls River and Cochecho River. The drainage basin of the river ...

, thence proceeding southeast to the

Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...

near

Portsmouth Portsmouth ( ) is a port and city in the ceremonial county of Hampshire in southern England. The city of Portsmouth has been a unitary authority since 1 April 1997 and is administered by Portsmouth City Council. Portsmouth is the most dens ...

. The northern end of the bay, near its outlet, is referred to as Little Bay.

Geography

Located within the

Gulf of Maine The Gulf of Maine is a large gulf of the Atlantic Ocean on the east coast of North America. It is bounded by Cape Cod at the eastern tip of Massachusetts in the southwest and by Cape Sable Island at the southern tip of Nova Scotia in the northeast ...

watershed, the Great Bay Estuary is a drowned river valley composed of high-energy tidal waters, deep channels and fringing mudflats. The entire estuary extends inland from the mouth of the

between

Kittery, Maine Kittery is a town in York County, Maine, United States. Home to the Portsmouth Naval Shipyard on Seavey's Island, Kittery includes Badger's Island, the seaside district of Kittery Point, and part of the Isles of Shoals. The southernmost town in t ...

, and

New Castle, New Hampshire New Castle is a town in Rockingham County, New Hampshire, United States. The population was 1,000 at the 2020 census. It is the smallest and easternmost town in New Hampshire, and the only one located entirely on islands. It is home to Fort Const ...

through Little Bay into Great Bay proper at Furber Strait, a distance of . The Great Bay Estuary is a tidally-dominated system and is the drainage confluence of three major rivers, the

Lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...

, Squamscott, and Winnicut. Four additional rivers flow into the system between Furber Strait and the open coast: the

Cocheco Dover is a city in Strafford County, New Hampshire, United States. The population was 32,741 at the 2020 United States Census, 2020 census, making it the largest city in the New Hampshire Seacoast Region (New Hampshire), Seacoast region and the fi ...

, Salmon Falls, Bellamy, and

Oyster Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but not al ...

rivers. The Piscataqua River is an ocean-dominated system extending from the Gulf of Maine at

Portsmouth Harbor The Piscataqua River (Abenaki: ''Pskehtekwis'') is a tidal river forming the boundary of the U.S. states of New Hampshire and Maine from its origin at the confluence of the Salmon Falls River and Cochecho River. The drainage basin of the riv ...

and forming the border of New Hampshire and Maine to the fork of its tributaries, the Salmon Falls and Cocheco rivers. These rivers, several small creeks and their tributaries and ocean water from the Gulf of Maine create the Great Bay estuarine hydrosystem. The tidal range is dramatic within Great Bay. Average depth of the embayment is with channels extending to . The water surface of Great Bay covers at high tide and at low tide, leaving greater than 50% of the bay exposed at low tide.

Natural heritage

The Great Bay Estuary, when counting the entire tidal system including the

, meets the

at the mouth of the Piscataqua, between

and

. Tides carry salt water into the estuary twice daily from the Atlantic. Here it mingles with the fresh water influence from the various rivers that empty into Great Bay. It is one of the largest estuaries on the Atlantic Coast and at inland is one of the most recessed. Approximately 14,000 years ago, following the melting of the glaciers, the Great Bay estuary was formed. The glacial melt waters contributed to rising ocean waters, which flooded the land and filled the river valleys that make up Great Bay today. There are five very different water-dominated habitats that make up the Great Bay. In order of abundance they are:

eelgrass Eelgrass is a common name for several plants and may refer to: * ''Zostera'', marine eelgrass * ''Vallisneria'', freshwater eelgrass {{Short pages monitor