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Natural Heritage

The Great Bay Estuary: A Dynamic Meeting Place

The Great Bay estuary is a place where the ocean and rivers, land and water, and people and nature meet. Great Bay lies at the confluence of tidally driven salt water from the Gulf of Maine and fresh water from the Salmon Falls, Cocheco, Bellamy, Oyster, Lamprey, Squamscott, and Winnicut rivers. Before reaching Great Bay, seawater travels 15 miles inland through the Piscataqua River and Little Bay. This geographic configuration makes Great Bay one of the nation's most recessed estuaries and it is often referred to as New Hampshire's "hidden coast".

An Egret flock on the saltmarsh
An Egret flock on the saltmarsh
Courtesy of Rachel Stevens

The large quantities of water that move in and out of the estuary create some of the strongest tidal currents in North America. This tidal exchange structures the Great Bay ecosystem by affecting water quality, habitat extent, and species distributions. The rivers that flow into the Great Bay Estuary drain a watershed that extends more than 1,000 square miles, and this convergence of land and water shapes features and uses of the ecosystem.

Over 14,000 years ago, the Great Bay estuary was formed by the melting of the glaciers. The rising ocean waters flooded the land and filled the river valleys that make up Great Bay today. There are five very different water habitats found in the estuary: eelgrass meadows, mudflats, salt marsh, channel bottom, and rocky intertidal. These habitats are home to hundreds of bird, fish and plant species, including 23 that are considered threatened or endangered.

Eelgrass is one of the few underwater marine flowering plants. It has many functions in the estuarine system and is an important food source for some species of waterfowl. Eelgrass beds provide habitat for young fish and invertebrates and the roots help stabilize the bottom sediments. Eelgrass plants also help maintain water quality by filtering the water allowing sediments to settle and then using the excess nutrients for growth.

a photo of tidal mudflats
Tidal Mudflats
Courtesy of GBNERR

At low tide, more than half of Great Bay is exposed as mudflats. Worms, soft-shelled clams, mud snails, horseshoe crabs, wading birds and many other animals utilize the extensive mudflat habitat for feeding, reproduction and protection from predators. The channel bottom provides a place for fish and invertebrates to move to at low tide. It is also the preferred habitat for oysters, an animal only found in estuaries. Because oysters filter the water to feed, they help to remove pollutants and nutrients.

Salt marshes in the estuary exist both as expansive meadow marshes and narrow fringing marshes. They provide important breeding, refuge, and forage habitats for invertebrates, fish, and birds. As organisms move between salt marshes and other estuarine habitats, they help export energy to support the complex coastal food web.

Great Bay's position at the confluence of land, rivers, and the sea creates an ecosystem that is ever-changing over tidal, seasonal, annual, and historical time scales. Some of these changes are part of the natural dynamics of the ecosystem, while others are driven by human activities. Amidst this backdrop of ongoing ecological change, there is one constant-a strong commitment by the Great Bay Research Reserve to protect Great Bay and ensure that it remains a natural treasure that can be enjoyed by generations to come.

Great Bay National Estuarine Research Reserve
New Hampshire Fish and Game Department