252 



Physical modification is permanent; once an estuarine habitat is 

 destroyed by dredging or filling, it is gone forever. No waste treat- 

 ment can correct or even minimize the damage. The destruction of 

 a marsh or part of the estuarine shallows has an obvious effect on 

 habitat value, but equally severe damages can be associated with 

 apparently minor physical alterations. 



The effect any pollutant has on an estuarine environment depends 

 on where it goes, how strong it is, and how rapidly it is assimilated 

 or flushed out of the environment. These conditions depend on water 

 movement and circulation patterns, which are in turn governed by 

 the relationship of tide and river flow to estuarine shape and size. 

 Dredging of new or deeper navigation channels, building of cause- 

 ways or jetties, and even construction of bridge piers can cause subtle 

 changes in water movement that can alter the balance of environ- 

 mental conditions in an estuarine system and result in gradual unde- 

 sirable changes in the ecosystem. 



Table IV.2.10 shows the amount of estuarine habitat lost by filling; 

 table IV.2.11 lists the major river flow regulation structures affecting 

 rivers in the estuarine zone ; table IV .5.4 gives a general idea of the 

 numbers of miscellaneous structures in the estuarine zone. These 

 three tables indicate only the extent of modification, not of its effects. 

 While destruction of habitat by filling is measurable, the environ- 

 mental changes wrought in an estuarine system by external flow 

 regulation or by internal structures are so closely associated with its 

 morphology that generalization is impossible. Table IV.5.4 shows 

 that there are in the estuarine zone 752 jetties, dikes, and breakwaters 

 averaging nearly 1,000 feet in length. These are all solid structures 

 specifically designed and placed to modify flow patterns. While habi- 

 tat damage may have been considered in the design of many of these, 

 it is unlikely that effects on the estuarine environment were consid- 

 ered seriously in the placement of many of the 989 causeways and pier 

 bridges within the estuarine system. 



TABLE IV.5.4.— ARTIFICIAL MODIFYING STRUCTURES 



