Shoaling of subtidal and intertidal mud flats has permitted tidal marshes to spread across large marginal areas of the San Francisco Bay estuary during the past several thousand years. By 1850 A.D. the tidal marshes of the estuary, including those of the Sacramento - San Joaquin Delta, covered an area nearly twice as large as the area of open water. Nearly 95 percent of these marshes have been diked or filled during the past 125 years. Species distributions along leveled transects at six tidal marshes indicate that elevation and water salinity are the principal ecological factors that-control the distribution of seed plants in the remaining natural tidal marshes of the northern San Francisco Bay estuary. Marsh surfaces situated near mean tide level are populated by robust monocotyledons (e.g., Spartina foliosa, Scirpus californicus), whereas surfaces situated near high-tide levels support dicotyledons and a few small monocotyledonous species (e.g., Salicornia virginica, Distichlis spicata). Marshes near the seaward end of the estuary are typically occupied by 10-15 salt-tolerant species (e.g., Spartina foliosa, Salicornia virginica), whereas marshes at the riverward end of the estuary are inhabited by as many as 30 species, most of which are known to tolerate moderate or small amounts of salt (e.g., Scirpus spp., Phragmites communis, Typha latifolia).
