Californian Region 



South of San Francisco Bay, most coastal wetlands in California are so small as to be under- 

 represented at the regional scale used in this study. Thus, at the 1 km 2 cell level, no salt marsh 

 appears as a dominant land category except at Oxnard. Here, marsh is lost by the end of the 

 simulation period through formation of tidal flats and eventually total submergence of some cells 

 in both scenarios (Figures 4-18). Elsewhere in southern California, freshwater marshes may 

 persist where they are located in sheltered or protected locations (e.g., Imperial Beach). Salt 

 marsh could persist under both scenarios under unprotected situations (such as Del Mar) so long 

 as adjacent lowland or freshwater marsh could be converted. That the potential for marsh 

 establishment and spread may be limited by abrupt topographic change is seen at Santa Ynez, 

 where no marsh is developed at any time under either scenario. 



In San Francisco Bay the presence of large marshlands, many severely modified by human 

 activities, presents a different situation. A large percentage of remaining marshlands in the Bay 

 area has been associated with levees at one time or another and many of these no longer function 

 as typical salt marshes. But the remaining 10-20 percent of the salt marshes still open to tidal 

 exchange provide a starting point for the expansion of salt marsh onto adjacent lowland and 

 freshwater marsh. However, where any of these areas are protected by levees, salt marsh 

 migration is not possible. Thus, protected marshes may persist while salt marsh expands signifi- 

 cantly onto other unprotected lowlands. 



Even where accretion is considered to be zero, protection will permit persistence of the 

 marshes. Some losses may occur as a result of rising waters, but this may be offset by marsh 

 migration onto unprotected lowlands. This is seen under the low scenario for both simulations. 

 In the south Bay area, even under the high scenario, salt marshes increase over the entire 

 simulation period, primarily through expansion into lowland areas already near sea level through 

 subsidence due to groundwater withdrawal (Figures 4-18). At the north Bay site, the same situa- 

 tion holds well into the second half of the twenty-first century; but by the end of the simulation 

 period, flooding begins to exceed the continued spread of salt marsh and a net decrease occurs. 

 However, the loss, compared to the 1975 condition, is only about 39 percent of the total marsh 

 area because of the protection afforded by levees. 



Columbian Region 



Although coastal topography in the Columbian region limits wetland area and would be 

 expected to do so if sea level rise accelerated, simulations of sites with significant wetiands 

 suggest that for low and high sea level rise scenarios, salt marsh area would expand (Figure 4-19). 

 Expansion would be seen both along the coast in bays and harbors as well as under conditions 

 similar to those of the northern and southern ends of Puget Sound. In fact, under both scenarios 

 at all sites examined, salt marsh would begin expanding early in the simulation period and 

 continue for the most part until 2100, even in the high scenario; however, the total areas involved 

 are small. Only under conditions such as those found at Coos Bay would rising seas begin to 

 exceed the spread of salt marsh, and this reversal would develop only in the last quarter of the 

 twenty-first century (Figure 4-19). At that time undeveloped lowland for colonization by salt 

 marsh becomes limited. Elsewhere, where important undeveloped lowland areas remain which 

 could convert to salt marsh, marsh areas continue to expand, sometimes rapidly, as in our 

 simulation of Puget Sound North. Here, in both scenarios, salt marshes are still expanding 

 significantly as of 2100, but more rapidly in the high scenario because of adjacent undeveloped 

 lowland. However, the more rapid expansion of salt marsh also means more rapid decrease in 

 lowland availability, suggesting that conditions soon would become limiting for further salt marsh 

 expansion here as well. For all our simulations in the Columbian region, wetland areas in the 

 next century would exceed present areas due to the adjacent low terrace; because of the rapid 

 rise in sea level this would not be a continuation of the present tendency of tidal marshes in this 

 region to prograde under twentieth-century conditions (Seliskar and Gallagher 1973). 



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