Georges Bank and Great South Channel

This physiographic region combines two geographic features that we describe separately below.

Georges Bank

Overview
Georges Bank is an underwater shoal, left behind by retreating glaciers several thousand years ago. The bank, as shallow as 30 m on its northwestern edge, creates a barrier between the Gulf of Maine and the northwest Atlantic Ocean. Exchange is limited to that which occurs over the Scotian shelf, to the east; through the Northeast Channel between the eastern end of the bank and the Scotian Shelf; and through the Great South Channel, between the western end of Georges and Nantucket Shoals. The topography and position of the bank result in upwelling of nutrient rich waters circulating in the Gulf of Maine. These nutrients, introduced into the sunlit waters over the bank, support very high rates of productivity, including many species of commercial importance. The fish stocks of the bank supported economies on both sides of the Atlantic for three or four centuries. The innovation of the otter trawl and the advent of a very mobile international fishing fleet combined to decimate the fish stocks of Georges Bank in the late 20th century. With stringent fisheries controls in place, some stocks are showing signs of recovery. Others, including the historically important cod, are not.1,2

Surface area
16,191 mi2

Depth
3 (Georges Shoal) to ~200 m

Bathymetry including key features
The formation of Georges Bank resulted from the deposition of mixed sediments by retreating glaciers. The subsequent rise in sea level and the action of currents, storms and tides have shaped the deposited material into an oval shaped shoal. The northern edge forms a steep slope; the southern edge is broad and gently sloping.

Where Georges Bank is shallower than 50 m, particularly on the central part of the Bank as well as the northeast peak, ridges of sand 10 to 40 m high and up to 90 km long are found. These ridges trend northwest and southeast and are parallel to the direction of the tidal currents. Smaller dunes are found on top of the ridges running perpendicular to the ridges themselves. The deeper portions of the Bank, to the east, are relatively smooth. The break between the ridges of the shallower bank and the smoother areas of the deeper bank occurs at 50 m and is abrupt; it is also the location of the tidally mixed front that forms in the summer.3 3 The edge of the bank, where it drops away to the central Gulf of Maine to the north and the continental slope to the south, occurs at 100 m. On its southeastern margin, the bank is incised by submarine canyons.

Storms and tides continually rework the surface sediments of the bank.4,5

Substrate
The Georges Bank substrate is predominantly sand. On the shallower portions of the bank, the sand is coarse and has been worked into ridges and dunes by tidal currents and storms. Finer sand is found on the deeper portions of Georges Bank. Gravel is found on the northern edge of the Bank. Georges Bank is no longer receiving sediments; as a result, it is slowly being eroded, and, as finer grained sediments are removed, its surface is becoming coarser.

Relationship to large-scale circulatory patterns of the Gulf of Maine
The counterclockwise gyre that dominates surface circulation in the Gulf of Maine flows southerly along the western coast of the Gulf. A portion of this flow branches off and flows eastward onto the northwest flank of Georges Bank (the remainder exits the Gulf through the Great South Channel).

There is a high degree of fluctuation in the oceanography of George’s Bank from year to year. This is related to larger-scale forces that affect the oceanography of the entire Gulf of Maine; they include large-scale shifts in atmospheric pressure (known as the North Atlantic Oscillation), variations in the formation of Labrador Sea water, and sea ice coverage.6

Information on average (seasonal/annual) oceanographic conditions
Tidal energy from the larger Gulf of Maine contributes to the clockwise gyre over Georges Bank. The water flowing over the Bank is distinct from that in the Gulf proper creating a frontal zone at about 60 m. The strong tidal currents occurring on the Bank keep its waters well-mixed and cool, contributing to the front between the Bank and the adjacent, seasonally stratified and warmer waters of the Gulf. Surface winds and waves contribute to mixing and affect the location of the tidal front.7

As warming of the surface layer occurs in the summer, the gyre over the Bank tends to become a closed loop; however, storms and intrusion of warm-core rings from the Gulf Stream, flowing offshore of the bank’s southern edge, can disturb the gyre and allow for exchange of water with neighboring water masses. Currents flowing southwesterly along the southern flank of the Bank either recirculate on the Bank or continue westward into the Mid-Atlantic Bight.8

1 NEFSC, 2000
2 NEFSC, 2003
3 U.S. GLOBEC: Long-Term Moored and Lagrangian Measurements as Part of a Georges Bank Study, undated.
4 Backus, et al., 1987
5 NEFMC, 2003
6 U.S. Implementation Team, 1997
7 Xue, et al., 2000
8 U.S. GLOBEC Implementation Team, 1997.

Great South Channel

Overview
The Great South Channel is a prominent feature in the bathymetry of the Gulf of Maine; along with the Northeast Channel, it provides an oceanographic connection between the Gulf of Maine and the Northwest Atlantic Ocean. The channel is a transit route for the endangered Northern Right Whale as it migrates between summer and winter habitats.

Surface area (percentage of total)
2,007 mi2 2%

Depth
~50 to ~100 m

Bathymetry including key features
The funnel-shaped Channel is wider and deeper at its northern end.1

Substrate
Sediments include gravel pavement and mounds, occasional boulders, sand with ripples generated by storms, and scattered mussel beds.

Relationship to large-scale circulatory patterns of the Gulf of Maine
In general, water flows into the Gulf from the Scotian Shelf and through the Northeast Channel at depth (river discharge during the spring freshet makes up a third source). Outflow occurs through the Great South Channel and mid and surface layers of the Northeast Channel.2,3,4

Information on average (seasonal/annual) oceanographic conditions
The Great South Channel is well mixed due to tidal currents and the existence of a frontal system between Gulf of Maine waters and those of the Continental Slope.5 The front separates the warmer, surface, stratified waters of the Gulf to the north from the cooler, tidally mixed waters south of the front.6

Whale abundance, distribution and migratory patterns
A portion of the Great South Channel is designated as a “critical habitat area” for the Northern Right Whale, designated as an endangered species by the National Oceanic and Atmospheric Administration. Right whales feed on plankton in the Great South Channel in mid to late spring.

1 NOAA, Undated
2 Xue, et al., 2000
3 Pettigrew, et al., 2005
4 Maine Ecosystem Dynamics Modeling Laboratory, Undated
5 Velntine, 1999
6 NOAA, Undated