The distribution of Pacific cordgrass (Spartina foliosa Trin.) is patchy within the southern California region, within particular wetlands and within clones. Small-scale patchiness cannot be fully explained by environmental variables. However, study of cordgrass before and after catastrophic flooding has led to an alternative model. It is proposed that establishment is most likely after fresh water dilutes soil salinities and that expansion and persistence depend on previous densities, disturbances, and competitors. The lower intertidal marsh soils are nearly always hypersaline (35-45 ppt), and germination of cordgrass seed is rare. Reproduction is primarily by rhizomes and tillers. Soil salinity is reduced with episodic flooding, at which time cordgrass can establish seedlings and invade suitable habitats. Clones spread more rapidly after flooding. Within clones, densities increase after flooding in proportion to their pre-flood densities. Cordgrass is establishment limited, and historical events exert substantial control over its distribution and abundance.