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62 saltgrass (Distichlis sp.), seepwillolJ (Baccharis sp,.), and sycamore (Platanus sp.). The most important water--consuming dparian phr<!atophytes in the western United States are cottonwood, willow ,md salt cedar. Robinson (1958) estimated that over 6.5 million ha (16 million aeres) of phreatophytes in the western states annually transpire over 31,000 milllon m3 (25 million acre-feet) of water. It was also estImated that the aggressive saltcedar occupies over 526,000 ha (1.3 mj,ll1on acres) transpiring over 6 million m3 (5 million acre-feet) of water. WlLter use by salt cedar was found to vary from 0.61 m to 2.4 m (2 to 8 feet) pe,r year, depending on location, density, salinity, and depth of the water table. Riparian Ecosystem Succession The riparian zone and its fluvial system are dynamic and interdependent. As a stream overflows its banks, natural levees form from the deposited sediments. Overbank flows supply water to adjacent floodplain wetlands which store excess runoff water. Additionally, overbank flows may facilitate groundwater recharge. As a stream erodes its channel on the convex curve of a bend and deposits sediment on the concave side, the meander moves laterally and down the valley while maintaining constant overall channel shape and size. These two processes, overbank flow/deposition and lateral migration, are the most significant forces in formation of a floodplain. The riparian zone responds by establishing successional stages (sere) of plant growth on new sand and gravel bars, often beginning with sedges, rushes, willow (i.e. Salix spp.) and other low plant cover. As gravel bar communities become more distant from the shifting channel, they begin to stabilize ,a,nd are further colonized by tree saplings. Gradually, a new riparian zone is established. ,.,:J;~eprinciples and terminology of ecological "su,ccession" apply to riparian ecosystems and thereby facilitate understandLng, description and prediction of outcomes from catastrophic events. Rip,arian ecosystem "succession" is an orderly process of community development that (1) involves changes in species stt'Ucture lind commun1:ty processes with tille, (2) results from IIIOdif1catioll of the phySical el2vironment b:r the C_nitY'and (3) cullll1nates in' a jltabilizl!d ecosy.tell in which maximum biomass and symbiotic function bet..een orgunislDS are maintained P,!r unit of available energy flow (Odum 1971). The sequence of communities w:lthin the riparian zone is called the "sere, - and thE' relati vel)' transi tory communities are called "seral stages." A final, stable, riparian co~nity in a developoaental serie!l (sere) is called the "cliillax." In the ab6ence of perturbation, the comllltlnity is self-perpetuating and in equilibrillm with the physical environment:. Odum' (1971) suggested that two types of climaxes be recognhed; II ".E!!..utic clillax" which is in equilibriulI wiC;h the I.ural cl1lDlllte and towards which all .ucceastonal development in a' given reach is tend1,ng and the "edapt1c cl1eu:- Which is modified by local conditions of the s:ubstrate. Succession may end in an edaptic climax where hydrological conditions, climate, soil, topography, floods, fire, drought or other disturbances are such that the climatic climax cannot develop.