253 



Physical modification of estuarine systems may enhance the useful- 

 ness of the biophysical environment. In fact, many modifications are 

 made deliberately to improve or protect an estuary for a specific use, 

 but often without consideration of the effects on other uses. The side 

 effects of such modifications may be good or bad, depending on local 

 conditions. For example, the piers and abutments that support bridges 

 are frequently excellent fishing grounds, yet the same piers can have 

 adverse eft'ects on water movement. 



A Public Health Service study of Great South Bay on Long Island, 

 N. Y,, in 1961 found that water circulation west of the Bay Bridge was 

 greatly restricted, dye tracers showed that the bridge piers acted as a 

 partial barrier to water movement. Figure IV.5.12 (see p. 254) shows 

 the Bay Bridge and schematically illustrates the movement of dye 

 near the bridge. This study concluded that the restricted circulation 

 west of the Bay Bridge was a contributing factor to the degradation 

 of water quality in this area ( IV-5-3 ) . 



The insidtious nature of environmental damage associated with phys- 

 ical modification makes it difficult to assess and predict the effects of 

 specific physical changes on the estuarine environment. Three examples 

 of the results of physical modification illustrate how flow regulation 

 can damage an estuary, what the results of progressive filling can do, 

 and how physical modification can improve the environment. 



Charleston Harbor^ South Carolina 



As part of the national plan to minimize unemployment during the 

 depression of the 1930's, the South Carolina Public Service Authority 

 was formed. Its purpose was to build a large dam, water supply, flood 

 control, navigation, and recreation complex that would generate em- 

 ployment opportunity. This complex, called the Santee-Cooper project, 

 involved the diversion of the Santee River into the Cooper River 

 through Lake Marion and Lake Moultrie. The outflow from Lake 

 Moultrie would go through a hydroelectric plant into the Cooper 

 River. In addition to the creation of large recreation lakes the project 

 would open a navigation channel to Columbia, S.C. It was felt the 

 increased flow in the Cooper River would benefit Charleston Harbor, 

 because it would help flush pollutants from the harbor and improve 

 water quality (fig. IV.5.13). 



The project was completed and placed in operation in 1942. By 1947, 

 shoaling rates in the harbor had increased to the point where dredging 

 was a full-time operation. Hydraulic model studies found the answer 

 to the increased channel maintenance : the higher fresh water inflow 

 had markedly increased salinity stratification and resulted in the for- 

 mation of a salt wedge. Particles were entrapped in the wedge, and 

 deposition of sediments increased. 



The intended modification changed the circulation patterns and, in- 

 stead of improving conditions in the harborj created more serious 

 problems. There is now a recommendation to divert the flow back into 

 the Santee. The net longrun effect, regardless of the outcome of this 

 recommendation, will be the complete alteration of two estuarine 

 systems with an unknown total effect on the ecosystem (IV-5-^). 



San Francisco Bay^ Calif. 



San Francisco Bay is the largest of all natural harbors on the Pacific 

 coast south of Puget Sound (%. IV. 5. 14). The fresh water inflow to 



