Measurements of both surface and bottom currents over many tidal 

 cycles ( 33i ^9) show that in the central part of the Sound the tidal current 

 patterns are elliptical and counterclockwise in direction. Because of the 

 effect of coastal topography, tidal currents along the coast are almost paral- 

 lel to the shoreline. The significant vertical differences in tidal currents 

 are that ebb tides are stronger at the surface than at depth and flood tides 

 are weaker at the surface than at depth as shown on Figure 17. In the deep 

 water of the central Sound tidal current speeds regularly exceed 30 centi- 

 meters per second (1 foot per second) near the bottom (written communication, 

 Gordon and Pilbeam, 1973). 



The eastern part of Long Island Sound contains a two-layered trans- 

 port system where a less dense, less saline surface layer moves eastward into 

 Block Island Sound at an average speed of 2.8 nautical miles per day. This 

 water is replaced by more saline and dense bottom water that flows westward 

 into Long Island Sound at an average speed of 0.6 nautical miles per day. 

 Recent investigations on currents at the eastern end of the Sound (5^) indi- 

 cate (1) that there is a predominant inflow in the lower two thirds of the 

 water column in deep portions of The Race over a tidal cycle, (2) there is a 

 net outflow in the shallower remainder of the eastern passage between Orient 

 Point and Fishers Island, and (3) the inflow and outflow pattern suggest there 

 is a tidal-driven counterclockwise circulation in the eastern third of Long 

 Island Sound. Detailed information on the duration and variability of the 

 two-layered transport system at the eastern margin of the Sound is not yet 

 available. The interchange although complex in detail is apparently controlled 

 by the fresh-water inflow. The total annual inflow of water from Block island 

 Sound has been tentatively estimated at 3.8 times the total volume of Long 

 Island Sound (29) . 



Moving westward into the central part of the Sound the circula- 

 tion system becomes less dynamic. There is decreasing transport in the 

 bottom layers principally because of upwelling and near shore mixing. Accord- 

 ing to Gordon and Pilbeam (written communication, 1973), the bottom water flows 

 westward into the central Sound principally along the north side of Six Mile 

 Reef near CI inton, becomes mixed with Connecticut River water and follows a 

 west-southwest course along the Connecticut shore. Inshore of the 10-fathom 

 (60 foot) curve the flow is toward the coast. The nontidal flow rate of near 

 bottom waters is as high as 5 kilometers (3.1 miles) per tidal cycle in the 

 deeper water and decreases toward either shore. Recent studies {]k) indicate 

 that upwelling of bottom waters occurs along the Connecticut shore between 

 New Haven and Old Saybrook and in coastal Long Island between Mattituck inlet 

 and Orient Point. There is also an offshore movement of surface water away 

 from the Connecticut coast. The net flow of near-surface waters is eastward 

 from the central to the eastern part of the Sound. The general net flow 

 pattern for eastern and east-central Long Island Sound is shown on Figure 18. 



Hardy ( 11) characterized the waters of the central part of the 

 Sound, west of the Mattituck Sill (see Fig. 15), as seasonally homogeneous 

 probably because of vertical mixing due to tidal currents and wind. Mixing 

 between the bottom and surface waters occurs along both shores and in the 

 vicinity of Six Mile Reef (written communication, Gordon and Pilbeam, 1973). 

 Salinity stratification, however exists at least part of the time in the 

 central Sound. 



40 



