Riparian vegetation exists along stream and valley bottoms as well as deep canyons in areas where the water table is not far below the soil surface. At one time, all of the major riverbeds in San Diego County supported extensive areas of riparian forest, riparian woodland, or riparian scrub habitat. As a wetland, riparian vegetation is one of the most sensitive habitats in California. It has suffered the loss of thousands of acres as the result of clearing in the floodplains for agriculture, sand mining operations, and transportation corridors. Much of the current riparian vegetation is now in fragmented patches rather than extensive stream courses. The characteristic species in riparian vegetation are: several species of willows (Salix spp.), the broad-leaved cottonwoods (Populus spp.), sycamore (Platanus racemosa), and mule fat (Baccharis salicifolia). Invasive weeds, such as giant reed (Arundo donax), are also common and dominate in some stretches of riparian areas. Giant reed is a large, fast-growing perennial weed that can take over a stream or riverbed. Giant reed has a dormant period when its brown vegetation is susceptible to fire.