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Prohibitive impacts involve permanent changes in the environment 

 and thereby prohibit all uses unable to cope with such changes. The 

 geographical range of such impacts may be from the limited area in 

 which they occur to an entire estuarine system, depending on the 

 nature and size of the change. The impact may be temporary, if it is 

 possible to return the environment to its original form, or it may be 

 permanent. 



Any use or activity requiring physical modification of the shoreline, 

 marshes, or bottom of an estuarine system may have a prohibitive im- 

 pact. Modification of water circulation also tends to be prohibitive 

 when it has any conflicting impact. Examples of estuarine uses and 

 activities generally having prohibitive impacts are navigation dredg- 

 ing, other dred^n^ and filling, solid waste disposal, construction of 

 bridges, dikes, jetties, and other structures, shoreline development, 

 mining from the estuarine bottom, and flow regulation. 



Some estuarine uses may restrict estuarine use for other purposes 

 but do not automatically exclude other uses. These are those activities 

 which do not require a permanent modification of the estuarine sys- 

 tem; they generally include those uses directly involved with the 

 estuarine waters and other renewable resources. 



Restrictive impacts may involve damage to water quality, living 

 organisms, or esthetic quality ; such impacts may also result from the 

 exclusive appropriation of space. The key feature of uses which cause 

 restrictive impacts is that they may, with proper management, be 

 carried out simultaneously with other uses. 



Any kind of municipal or industrial waste discharge may have a 

 restricted impact and often does. Commercial fishing, recreation, and 

 water supply are the major uses restricted by pollution from liquid 

 waste discharges. 



Some kinds of commercial fishing require the use of trawls or the 

 setting of traps or nets that must be left for some time. The use of such 

 devices restricts other uses while the devices are in place, but there is 

 no permanent appropriation of estuarine waters or space. The major 

 conflict is with recreation in that recreational boating must be ex- 

 cluded from areas where fishing gear is near the surface. 



Where there is conflict, the scene is set for trade-off; i.e., a willing 

 substitution of one activity for another. The scene is equally set for 

 uncompensated damage where one user group precludes the activities 

 of a second unrelated user group but does not reimburse them for 

 damage. Actual documented examples of use damages are difficult to 

 find. One major reason is the basic fact that has permeated much of 

 the discussion of economic and social values: Many estuarine values 

 are not quantifiable. Wliile damages to a commercial enterpirse, such 

 as commercial fishing, can be quantified in terms of the economic loss, 

 the essentially intangible values of recreation and estuarine habitat 

 are difficult to measure. 



Recreational loss would have to be measured in terms of how many 

 people don't swim or go boating in the Potomac River because it is 

 polluted. It is far easier to find out how many people do go there even 

 if it is polluted ; even these values are hard to find. 



The value of estuarine habitat is just as difficult to establish. There 

 are now about 5.5 million acres of important estuarine marsh and 



