Ch. 6— Impacts and Mitigation • 123 



vegetation, such flooding may have indirect effects 

 on adjacent wetlands. For example, an experiment 

 in shrimp culture, in which a dike was built to im- 

 pound part of a coastal wetland, led to large varia- 

 tions in temperature and salinity with subsequent 

 die-offs of many organisms, including the cultured 

 species (41). 



The construction of dikes or the disposal of spoil 

 from dredging operations may result in the im- 

 poundment of swamps and marshes. An im- 

 pounded swamp does not dry out periodically like 

 a natural swamp and has a lower water turnover. 

 This results in reduced primary and secondary pro- 

 ductivity and decreased value for wildlife habitat. 

 Virtually no fish are found in the stagnant water 

 of such an area (10). 



Water Withdrawals and Diversions 



Alterations in the hydrologic regime from large 

 water withdrawals for municipal-industrial use or 

 large-scale diversions of water for irrigation and 

 flood control can cause various impacts on wetland 

 ecosystems. The effects of these withdrawals and 

 diversions on downstream wetlands are twofold. 

 First, upstream depletions may lower the water 

 table in downstream freshwater wetlands, causing 

 a temporary or permanent loss of vegetation and 

 a decrease in habitat values. Second, decreasing 

 freshwater inflow in coastal areas will allow tidal 

 incursion of saltwater into the brackish and fresh- 

 water marshes. The increase in salinity to these 

 marshes will reduce species diversity and abun- 

 dance as well as overall ecosystem productivity. 

 Water diversions and withdrawals also reduce the 

 input of detritus into the estuarine food chain. 



Water diverted for irrigation and then returned 

 to the wetland can increase salinities and temper- 

 atures considerably. For example, salinity in Suisun 

 Marsh, which represents the largest contiguous wet- 

 land area in California and 10 percent of the total 

 State wetland acreage, has been increasing along 

 with increasing water diversions by the State and 

 Federal water projects in the Central VaUey and 

 the Sierras. One result has been a decline in cer- 

 tain high-food-value plant species that are favored 

 by brackish-to-fresh soil-water conditions. These 

 brackish plant species are particularly important 



to wintering ducks and geese (17). In addition, in- 

 creases in water temperature owing to thermal ef- 

 fluents from powerplants or from irrigation return 

 flows may cause a reduction in species diversity of 

 wetland flora or a shift to the more temperature- 

 tolerant, blue-green algae that tend to produce 

 eutrophic (oxygen-deprived) conditions. 



Restricting or manipulating water flows with 

 dams and reservoirs also can dewater downstream 

 wetlands. Any wetlands downstream that are not 

 immediately dewatered may be subject to reduced 

 flushing, leading to a decrease in the amount of 

 nutrients reaching the wetlands. Greater than nor- 

 mal floodflows can occur also when large reservoir 

 releases are sustained, possibly washing out wet- 

 lands downstream. 



Dikes and flood-control levees often are built to 

 convert wetlands in flood plains to dry farmland. 

 These flood-control levees retain floodflows within 

 a river channel, dewatering the wetlands behind 

 them. Levees within the floodway also tend to in- 

 crease the velocity of storm runoff, produce an 

 overall loss of flood storage capacity, and increase 

 the chance of downstream flooding (45). Increased 

 flows may increase scouring and erosion. Unlike 

 the conversion of wetland by filling, land that is 

 drained behind or within dikes or levees can be re- 

 stored to a wetland if the embankments are re- 

 moved or breached. 



Disposal and Discharge of Pollutants 

 and Nonpoint-Source Pollution 



Wetlands have been used to purify wastewater 

 of nutrients and suspended solids, sometimes with 

 adverse effects (4). Abundant nutrients in the waste 

 may increase the productivity and biomass of tol- 

 erant vegetation in the wedand while more sensitive 

 species disappear (58). Algal populations also may 

 shift in species composition, which may lead to 

 wetland eutrophication (23). If the wastewater vol- 

 ume is large enough to raise wetland water eleva- 

 tions, a conversion from emergent wetland to open 

 water can occur. Stormwater discharge also can 

 have adverse impacts on wedand functions and val- 

 ues. For example, contaminants from urban runoff 

 have been noted to cause detrimental effects on tidal 



