Stellwagen Bank Final EIS and Management Plan 



Page 16 



Tillies Bank rises to within 60 meters of the surface, 

 and is surrounded by a "moat" which reaches a 

 maximum depth of 200 meters (656 ft.). There is 

 also a subparallel ridge east of Tillies Bank which 

 rises abruptly to within approximately 65 meters of 

 the surface. In general, most bottom areas west (or 

 shoreward) of this bank-ledge system are smooth 

 and gently sloping. East of the bank-ledge system, 

 the bathymetry is more complex and exhibits 

 steeper gradients. 



Together, Stellwagen Bank, Jeffreys Ledge, and 

 Tillies Bank partly isolate three basin areas from 

 the outer shelf. From north to south, these areas 

 are Scantum Basin, Tillies Basin, and Stellwagen 

 Basin. Stellwagen Basin is bordered by the 

 Massachusetts coastline on the south and west, and 

 by Cape Cod and Stellwagen Bank on the east and 

 northeast. Like Stellwagen Bank lying along its 

 eastern and northeastern borders, the Stellwagen 

 Basin is elhptical in configuration, with a long axis 

 trending in a northwest direction. Much of the 

 Bank's southwest side slopes gently toward the deep 

 axis of the Basin at gradients of about 0.1 to 0.5 

 percent. The northeast side of the Basin, however, 

 dips steeply toward the axis at gradients of up to 6 

 percent. 



East, or seaward of the Bank-Ledge system, the 

 ocean bottom dips irregularly, attaining a maximum 

 depth of about 220 meters (722 feet) due east of 

 Boston (Schlee, Folger, and O'Hara, 1973). 



c. Oceanography 



Stellwagen Bank is subject to the same general 

 surface circulation patterns as the Gulf of Maine 

 overall. In general, surface waters of the Gulf 

 exhibit a counterclockwise flow (or gyre), which 

 moves in a southwest direction along the coasts of 

 Maine and New Hampshire and into Massachusetts 

 Bay. At Massachusetts Bay, the flow turns 

 gradually eastward, moving over the northern tip of 

 Cape Cod and toward the northern edge of Georges 

 Bank. Continuing east toward Nova Scotia, currents 

 turn north toward the Maine and New Brunswick 

 coasts. Close to the coast, currents divide and flow 

 in different directions, with the major portion 

 turning westward toward Maine. The smaller 

 remainder of the currents flows north into the Bay 



of Fundy. In Massachusetts Bay, some of the flow 

 moves southward into Cape Cod Bay, moving along 

 the western edge. On the eastern side of Cape 

 Cod, some of the currents are directed southward 

 and pass between Cape Cod and Georges Bank, in 

 the Great South Channel (Figure 4). 



East of Stellwagen Bank, net surface currents 

 move to the southeast at between 1.8 to 9.3 km/day 

 (or 2 to 10 cm/sec). West of the Bank, surface 

 ciurents flow southerly in western Cape Cod Bay 

 and Massachusetts Bay, and northerly in eastern 

 Cape Cod Bay, forming the generally counter- 

 clockwise movement discussed above. Results of 

 bottom drift testing indicate that residual bottom 

 water flow over Stellwagen Bank is southeasterly. 

 Bottom flow in much of the area west of Stellwagen 

 Bank is usually southerly into Cape Cod Bay. 



Currents on Stellwagen Bank move mostly east 

 and west at maximum velocities of 10 to 45 

 cm/second. Maximum bottom current velocities 

 show some relation to the bottom sediment type, 

 and to the sea floor bathymetry. Maximum bottom 

 velocities measured on Stellwagen Bank (45 

 cm/sec.) are adequate to move coarse sand. 

 Similar maximum velocities have been noted in the 

 broad sandy-covered channel separating Stellwagen 

 Bank from the tip of Cape Cod. Bottom current 

 velocities are less strong in Stellwagen Basin, and in 

 the passage southeast of Cape Ann (where 

 maximum bottom current velocities usually do not 

 exceed 18 cm/sec). 



Internal waves are periodic phenomena occurring 

 in all the world's oceans. Investigations have 

 indicated that tidally-generated internal wave 

 packets are common along the U.S. East Coast 

 (Sawyer and Apel, 1976), as well as other locations 

 exhibiting the right combination of bathymetry, 

 tides, and stratification (Gregg and Briscoe, 1979; 

 and Haury, Wiebe, Orr, and Briscoe, 1983). 



During the late summer, internal wave packets 

 occur twice daily throughout Massachusetts Bay. 

 These high-frequency, predictable wave packets are 

 formed over Stellwagen Bank and are transmitted 

 into the Bay at about 60 cm/sec, finally dissipating 

 in the shallow waters of the Bay's western edges. 

 Dominant waves have been measured at 



