Tidal marshes are a subset of estuarine wetlands defined by the presence of emergent vegetation types uniquely adapted to sheltered intertidal zones of temperate and subtropical coastal plains (Chapman 1960, 1976, Mitsch & Gosselink 1993). They are found across a full range of salinity conditions from seawater on the immediate coast to freshwater tidal reaches of estuarine river systems. Marshes are transitional ecosystems that provide critical connections between adjacent subtidal and terrestrial ecosystems within the estuarine landscape (Simenstad et al. 2000; Levin et al. 2001). These “critical transition zones” often function as conduits for substantial fluxes of materials and energy (Ewel et al. 2001), and provide a variety of valuable ecosystems functions, goods and services related to the maintenance of biodiversity, fish and wildlife habitat, water quality, flood abatement and carbon sequestration (Rabenhorst 1995, Costanza et al. 1997, Weslawski et al. 2004, Zedler & Kercher 2005).
However, estuarine marshes and the biotic communities that depend on them are vulnerable to both direct and indirect anthropogenic impacts (Holland et al. 2004, Snelgrove et al. 2004), and the functionality of these systems can be difficult to restore once severely impacted (Zedler & Kercher 2005).
The San Francisco Bay and Sacramento-San Joaquin River Delta estuary is perhaps the most hydrologically-engineered estuarine wetland system in the United States, and an estimated 95% of the marsh area that existed there in 1850 has been altered or converted to other land uses (Josselyn 1983). The principal source of freshwater input to the estuary enters through the Sacramento and San Joaquin rivers; their inland delta (the Delta) is the terminus of a watershed that drains about 40% of California’s land area. Anthropogenic alterations of the estuary’s hydrologic characteristics have profoundly affected the extent and functioning of the tidal wetlands, particularly in the brackish and tidal fresh portions of the upper estuary associated with the Delta. Although the conceptual model presented in Figure 1 is intended to capture the features and dynamics of tidal marshes in the Delta (e.g., Suisun Bay to the upriver extent of the tides in the Delta), oligohaline and tidal freshwater marshes generally remain poorly understood.
Recent texts (e.g., Sharitz & Pennings 2006) still consider the review by Odum (1988) as the best treatment of these low salinity tidal ecosystems. Consequently, development of the current conceptual biological model often required us to judiciously borrow from the more extensive literature on temperate salt marshes in diverse regions.
Although one of our principal objectives in developing this model involves identifying the dominant processes and interactions that characterize restoring marshes at various stages of development, the model is intended to characterize the dynamics of “equilibrium” marshes at their mature state of geomorphic and ecological functioning (Pestrong 1972, Reed 2002, Williams et al. 2002). Our rationale is that restoring marshes are considerably variable and that trying to capture intermediate stages of development (e.g., positions along a development “trajectory”; Simenstad & Thom 1996 [however, see Zedler & Callaway 1999]) would introduce too much variability for a single model, and because the ultimate objective of restoration is the self-sustaining, equilibrium condition. However, we have sought were appropriate to describe important processes that influence restoration trajectories and affect the ecosystem functions, goods and services that marshes provide in various landscape settings that are found in the Sacramento-San Joaquin Delta.