significance of these habitat changes is difficult to evaluate in an 

 ecosystem as dynamic as the delta marsh system. Subsidence and sediment 

 deposition constantly alter the existing system such that it becomes 

 extremely difficult to predict habitat effects attributable only to pipe- 

 line installation. The areal increase of new aquatic habitat is usually 

 less than the areal decrease in marsh resulting from construction replace- 

 ment of marsh by open-water habitat. 



Marsh buggies and draglines associated with canal excavation (push method) 

 depress the marsh surface, forming numerous ruts and other depressions. 

 Such depressions may alter existing water flows by channeling surface 

 waters in new directions. Unless such alterations cover extensive areas, 

 the effects are usually quite localized in the delta marsh. Small open- 

 water bodies or elevated mounds may be created, but such changes normally 

 are of minor consequence at the ecosystem level. Heavy traffic concen- 

 trated along one route, however, may produce ruts capable of channeling 

 surface waters such that water flows and intramarsh circulation patterns 

 are altered. Changed flow patterns may increase or decrease the average 

 depth of the standing water, leading to readjustments of flora components 

 if the alterations are extensive. Such floristic changes are probably of 

 minor consequence in a system inherently adapted to continual changes of 

 water depth and flow patterns. 



Of greater significance to the hydrologic characteristics, however, are 

 the dredging of pipeline channels and associated spoil deposition 

 practices. Canals that are adequately backfilled and/or plugged may have 

 localized effects on circulation patterns, but they do not result in long 

 continuous channels and spoil ridges. Water movement patterns remain 

 esentially unchanged following restoration procedures. 



Open unplugged channels bordered by continuous spoil levees generate a 

 complex of interactions which may facilitate changes in the amount of 

 standing water. Factors such as deployment patterns, canal depth, and 

 levee height can interact to produce intramarsh situations in which 

 waters are either drained, impounded, or diverted. Long continuous 

 canals intercept small creeks or standing-water areas, facilitating 

 faster marsh drainage. Associated spoil levees block or redirect sheet- 

 flow, preventing its entry into certain marsh sectors, as well as 

 isolating and impounding interior zones. Resulting changes in average 

 water depth and the regular influx of suspended sediments and dissolved 

 nutrients may ultimately generate changes in the local biota if the 

 alterations are extensive. Diversion of the primary water inflows alters 

 the annual recharge of sediments and nutrients to the delta, thereby 

 inhibiting land accretion and increasing net land subsidence. The 

 magnitude of alterations described above varies according to channel 

 dimensions, number of channels, channel orientation relative to fresh- 

 water and saltwater sources, levee orientation and placement, the 

 hydrologic gradient of the watershed, and other site-specific character- 

 istics. 



282 



