The mean tidal fluctuation in 

 Chesapeake Bay is small, generally 

 between 1 and 2 feet. Saline water 

 intrusion is highest along the east 

 side of the estuary due to the in- 

 fluence of the coriolis effect and 

 the fact that the larger rivers are 

 on the western shore. Salinities 

 range from about 33 ppt inside of the 

 mouth of the bay to near zero at the 

 north end of it and at the heads of 

 the embayment's tributary to it. Sa- 

 linity variations, spatial and tempo- 

 ral, constitute the most significant 

 physical parameter influencing the 

 circulation dynamics of the estuary 

 and the types of aquatic species 

 which reside in it. 



The ebb and flow of tides are 

 the most readily perceptible water 

 movements in the Chesapeake. Average 

 maximum tidal currents range from 

 less than 0.5 knots to over 2 knots. 

 The tidal currents along with wind 

 supply the necessary energy for the 

 mixing of salt water from the ocean 

 and fresh water from the tributaries. 

 Tides, being oscillatory by nature, 

 do not function as a mechanism for 

 the net transport of water, suspended 

 solids, or dissolved material. With- 

 in the bay proper and its major trib- 

 utaries there is superimposed on the 

 tidal currents a non-tidal, two- 

 layered circulation pattern that 

 provides a net seaward flow in the 

 upper layers and a flow up the 

 estuary in the deeper layers. 



The physical and chemical dy- 

 namics of the estuary make it a 

 biologically special place. Salinity 

 variations within Chesapeake Bay have 

 allowed colonization by aquatic 

 organisms of both fresh and salt 

 water origin. Freshwater biota re- 

 main in the fresher to slightly 

 brackish portions. Many marine 

 animals return to fresh water to 

 reproduce. Also, with the aid of 

 estuarine currents, the eggs and 

 larval forms of some species are 



transported to less saline waters to 

 hatch and develop. 



PROBLEM IDENTIFICATION 



Like all estuaries, Chesapeake 

 Bay is dependent on the inflow of 

 freshwater to maintain its salinity 

 regime. The species that live in 

 the bay year round and others that 

 utilize it only in various portions 

 of their life cycle are generally 

 able to survive the natural daily, 

 seasonal, and yearly variations in 

 salinity. Drastically reduced fresh- 

 water inflows during droughts or re- 

 ductions of less magnitude over a 

 longer period of time can impose en- 

 vironmental stress. This may threat- 

 en the health or even the survival of 

 species sensitive to particular 

 ranges of salinity, or may limit the 

 spawning opportunities of other estu- 

 arine species. Changes in freshwater 

 inflow can also alter existing estu- 

 arine-f lushing characteristics and 

 circulation patterns. In short, the 

 character of Chesapeake Bay and the 

 health and well being of the eco- 

 system depend on established physi- 

 cal, chemical, and biological pat- 

 terns in the bay. These are, in 

 turn, intimately related to the 

 volumes of freshwater inflows and the 

 seasonal variations in these flows. 



In recent Corps of Engineers' 

 studies it was found that, if present 

 trends continue, the future quantity 

 of fresh water flowing into Chesa- 

 peake Bay could be substantially less 

 than it is today. This predicted re- 

 duction is primarily a function of 

 increased consumptive use of water 

 from the bay's tributaries resulting 

 from an increasing population, the 

 need for more food, an increasing 

 level of economic activity, advances 

 in technological processes, and in- 

 creasing use of evaporative cooling 



116 



