Vernal pools and swales are episaturated, seasonal, freshwater wetlands found in the western United States, Mexico, and other Mediterranean-type climates of the world (Reifner and Pryor 1996). The boundary between grassland and vernal pools is sharply demarcated, with vegetation composition often changing completely in less than a meter (Holland and Jain 1977). The abundant grassland pools and swales are highly variable in size, and they typically occur in groups separated by tens or hundreds of meters (Holland and Jain 1981). Less commonly, they are found on coastal terraces and basalt mesas (Zedler 1990, Stone 1990, Weitkamp et al. 1996), on lava plateaus and scablands (Crowe et al. 1994), and in woodlands scattered throughout the landscape (Stone 1990, Heise et al. 1996).