For the riding which returns members to the Legislative Assembly of New Brunswick, see Miramichi Bay-Neguac

Miramichi Bay is an estuary located on the west coast of the Gulf of St. Lawrence in New Brunswick, at the mouth of the Miramichi River. Miramichi Bay is separated into the "inner bay" and the "outer bay", with the division being a line of uninhabited barrier islands which are continually reshaped by ocean storms. The largest of these islands is the uninhabited Portage Island, which was broken in two during a violent storm in the 1950s. The islands provide some protection to the inner bay from ocean storms in the Gulf of St. Lawrence.

Miramichi Bay was named Golfe Saint Lunaire by Jacques Cartier in 1534.

The Inner Miramichi Bay, and the lower portions of its tributary rivers (including the Miramichi River), are parts of a drowned river valley system. Since deglaciation, sea level rise in Miramichi Bay has flooded the mouths of these rivers with saltwater. The flooded, meandering, ancient Miramichi river channel forms a navigable route through the Inner Bay for ocean-going ships entering the port at Miramichi (formerly the ports of Chatham, New Brunswick and Newcastle, New Brunswick). The inner bay measures only 4 m deep on average, with the navigation channel measuring only 6–10 m. Since dredging maintenance of this channel has stopped, the port is now only accessible to ships with a shallow draft.

The estuary is a highly dynamic environment, subject to high freshwater outflows during the spring freshet, low outflow and rising saltwater content during the summer period, fall ocean storms and nor'easters which reshape the barrier islands and the old river channel, and winter sea ice which encases the entire estuary. The shallow inner bay warms rapidly during summer. The diurnal tide cycle ranges only 1 m on average. Continued sea level rise is very slowly inundating adjacent low-lying areas and promoting rapid erosion of the low sandstone cliffs bordering the bay.

The estuary is significant in that it is a highly productive ecosystem, despite its relatively small size. The estuary receives the freshwater discharge from the Miramichi River and its tributaries, giving local waters somewhat lower salinity. Organic materials from the surrounding shorelines and inflowing rivers contribute, together with the warm water, to the bay's high productivity.

Marine life includes harbour seals, herring gulls, the common tern (pictars to use the old Scottish word), the great blue heron (commonly called "crane" in the Miramichi), the common loon, and cormorants (called "black shag" locally), with kingfishers, plovers, snipe and killdeer along the shore.

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