greater variation in river flow, greater flooding in 

 high-flow periods, and drought in low-flow sea- 

 sons. 



Counteracting forces do exist, however, in 

 improved general conservation practices and in the 

 increasing number of small and large dams, many 

 specifically designed for moderation of river flow 

 and planned release of water. 



Dams can interrupt natural flow and saUnity 

 levels by storing fresh water for long periods and 

 then quickly releasing large quantities. The inter- 

 mittant flow, with sudden changes in salinity and 

 heavy silt load, may be a strong deterrent to fish 

 and bottom life. 



However, the converse, reducing large seasonal 

 variations in fresh water flow by controlling the 

 river discharge through dams and low-flow aug- 

 mentation can cause problems. For example, 

 circulation in the small tributary embayments of 

 Chesapeake Bay is produced by saUnity differences 

 between the tributary and the Bay proper.^'* Since 

 the water is derived from the main Bay, the 

 salinity in the tributary must lag behind Bay 

 salinity. If the Susquehanna River discharge were 

 to be controlled to the extent that seasonal 

 changes in upper Bay saUnity disappeared, then 

 the prime mechanism for flushing of several 

 tributaries also would disappear. Pollution prob- 

 lems within the tributaries would increase and lead 

 to significant ecological effects. 



Engineers concerned with estuarine environ- 

 ment problems also have become keenly aware in 

 recent years that the amount of fresh water 

 discharged into an estuary, and the degree to 

 which it mixes with sea water, are major factors in 

 establishing the hydraulic and shoaling regimens of 

 the estuary. 



An example of what can happen by a change in 

 the density structure of estuarine waters is 

 Charleston Harbor, located at the mouth of the 

 Cooper River. Prior to a water diversion made in 

 1942, the amount of fresh water flowing down the 

 Cooper into the estuary was small compared to the 

 inflow and outflow of the tide, and the estuary 

 was vertically homogeneous. When fresh water was 

 added from the Santee River to provide a source 

 of hydroelectric power, it changed into a two- 



layered flow pattern with a surface layer flowing 

 seaward and a deeper layer flowing up the estuary. 

 Thus, Charleston Harbor became a trap for the 

 increased amounts of sediment, and dredging 

 required to maintain the channel has increased 

 from less than 500,000 cubic yards prior to 1942 

 to a current volume of over 10,000,000 cubic 

 yards. 



One proposed solution^ ^ to Cooper River 

 shoahng is to divert the water back into the Santee 

 River. This is an alternative to an original proposal 

 to divert the fresh water into a high-salinity coastal 

 marsh habitat which would have had a serious 

 effect on valuable fish nursery grounds (see Figure 

 8). 



Figure 8. Water diversion: Cooper and 

 Santee Rivers, South Carolina. 



A coastal bay or estuary has a complex ecology 

 related to the physical characteristics of its basin. 

 Natural processes such as storms and slow climatic 

 changes will disturb this balance and man can 

 drastically alter it. 



Coastal and inlet deep draft channel openings 

 can contribute to salinity intrusions while hurri- 



D. W. Pritchaid, "Modiiication and Management of 

 Water Flow in Estuaries," Symposium on Beneficial 

 Modifications to the Marine Environment, Washington, 

 D.C., 1968. 



U.S. Army Corps of Engineers, District, Charleston, 

 Survey Report on Cooper River, S.C. (Shoaling in 

 Charleston Harbor), July 1966. 



in-40 



