Channel excavation increases total suspended sediments. Secondary 

 effects in emergent vegetation areas are typically temporary and loca- 

 lized in areal extent. In open-water sites, increased suspended 

 sediments temporarily increase water turbidity and sedimentation rates, 

 as well as local biological oxygen demands. Sediment dispersion, however, 

 may affect more extensive areas, depending upon prevailing water veloci- 

 ties and circulation patterns. Increased water turbidity can briefly 

 inhibit productivity of submergent flora. Increased biological oxygen 

 demand may temporarily deplete dissolved oxygen levels, thus stressing 

 nearby fish and aquatic invertebrate populations. Barren spoil banks or 

 levees constribute to prolonged increases in the amoung of suspended 

 sediments resulting from surface erosion and runoff. Establishment of 

 plant ground cover moderates such erosion. Given the tremendous sediment 

 load of the delta waters under normal circumstances, it is doubtful that 

 such minor increases have any consequences on the ecosystem. 



Disturbances associated with channel construction, line installation, and 

 cleanup procedures typically cause short-term displacement of sensitive 

 wildlife species from otherwise favorable habitats nearby. Displacement 

 may affect feeding waterfowl concentrations, bird rookeries, and verte- 

 brate movement patterns. Following activity completion, species reenter 

 nearby sites if the areas remain essentially unaltered. Human activity 

 and associated noises extend project effects beyond the boundaries of the 

 immediate site. 



Key attribute alterations 



Alterations associated with line installation are functions of line size, 

 site location, and placement methodology. Surface flowlines and supply 

 lines and permanent overhead lines within road and access canal easements 

 typically generate short-term impacts of limited areal extent. These 

 impacts are quickly reclaimed by the system and generate little, if any, 

 noticeable subsequent effects on consumer components. Gas and oil trans- 

 portation pipelines, on the other hand, can modify important ecosystem 

 attributes. Key alterations involve: (1) changes in land elevations, 

 either through canal dredging and/or spoil deposition, both of which may 

 lead to fundamental alterations of the existing marsh hydrological regime 

 and (2) direct removal of productive marsh vegetation and dependent con- 

 sumer groups as a result of dredging and spoiling activities. Resulting 

 productivity losses may be either temporary or long term, depending upon 

 the installation technique used and the extent of site restoration. Major 

 alterations of intramarsh circulation patterns and standing water char- 

 acteristi-cs, which result from open unplugged pipeline ditches and 

 associated spoil levees, are capable of generating changes in plant 

 distribution patterns and composition. Desirable wildlife species may be 

 either favored or discouraged by such readjustments. Replacement of 

 terrestrial and semiaquatic environments with aquatic areas can be 

 critical to terrestrial organisms already faced with habitat losses 

 resulting from extensive delta subsidence. 



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