Coastal salt marshes develop along the intertidal shores of bays and estuaries. Estuaries occur where a river meets the sea, and the water is somewhat brackish. In general, salt marshes along the Northern California coast have a relatively low salinity because of substantial river runoff, whereas those along the southern coast, where there are fewer rivers and less runoff, are of higher salinity. San Francisco Bay contains the largest and one of the most complex salt marsh systems in the state.
Salt marsh plants are adapted to a harsh, semi-aquatic environment and saline soils. Species diversity is low. Stout stems, small leaves, and physiological adaptations for salt excretion and gas exchange characterize the inhabitants of the salt marsh, which are mostly grasses and low perennial herbs. The tangle of marsh plant roots and stems helps to stabilize the muddy bottom, as well as to trap debris and dissolved nutrients with each tidal cycle. Bacteria convert this oasis of detritus into food resources for microscopic algae, invertebrate larvae, and larger animals. Salt marshes are about twice as photo-synthetically productive as corn fields and provide critical nursery grounds for numerous organisms.
Species composition and zonation in the salt marsh are governed by salinity gradients in combination with the amount of intertidal exposure. Eelgrass, Zostera marina, for example, occupies the lowest or most marine zone. It cannot tolerate a freshwater environment or intertidal conditions that would expose its roots to air. Cordgrass, Spartina foliosa, occurs in the marine-to-terrestrial transition zone, characterized by lower salinity and periodic exposure to the air. Shoreward, where conditions are even drier, pickleweed species belonging to the genus Salicornia are common. On higher ground, where tidal intrusions are rare, the wiry, prickly-leaved succulent jaumea, Jaumea carnosa, is common, as are the bushy shoregrass, Monanthochloe littoralis; tall and slender sea arrowgrass, Triglochin maritima; and endangered salt marsh bird's beak, Cordylanthus maritimus. The green, wiry-leaved saltgrass, Distichlis spicata, is widespread, occurring from the middle to high marsh, as well as in dunes and on salt flats. An unusual salt marsh plant is the orange, parasitic dodder, Cuscuta salina. Its tiny, scale-like leaves and thread-like stems frequently invade and cover large areas of vegetation.