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55 4. continuous inter/lction wi,th adjacent aquatic and upland terrestrial ecosJstems through exchange of ener,gy, nutrients, and species. In 1lI0st of the weste,rn United States riparian el:osystems occur as sharply defined zones of me'sic (i.e., moderately moillt) vegetation within the much drier (xeric) surrounding areas .md are oftE!D referred 1:0 as gallery forest, bosque (i.e., a SIll8ll wooded area, thicket, or gl,ove), streambank, riparian lakeside, or ponds ide vegetation. The term "riparian" can also be applied to other area' of western vegetation sllch as dry lakes ("playas"), "arroyos" (i.e., a dr:! gully, rivulet or strealll), and northern plains "clraws" (a gully or ravine that water drains :lnto or through) (Johnsonliii'(lCarothers 1980; Boldt et al. 1978). Although most streamside zones are rIparian, sarma are not (Platts 1979). examples of non-riparian :tones ar,e those areas where the sagebrush or other terrestrial ecosystem vegetation reaches thE' water's edge, where the streamside zone is composed of bedrock, where streams are bordered by steep-sided canyon lands, or where streamside environments are composed of boulders, rubble, etc. that extend to the terrestrial zone. Other definitions describe riparian ,~cosystems liS specializl!d forms of wetland (Brown et at. 1979; USDA-!lLM 1979) where the streambank terrestrial soils are periodically inundated, but not necessarily' submerged for part of each year and are frequently subjected to waterlogging. NeverthE,less, by definition, riparian ecosystems are wetland ecosysteulS and thereby afforded protection under Section 404 of the Clean Water Act (1977). Riparian Ecosystems as Ecotones An ecotone is an area of transition I~tween two or more diverse communities. For example, riparian ecosystems are frequently located between an upland forest on one side and an aquatic zone on the other. It is a junction zone or tension belt which may have considerable linear extent but is narrower than the adjoining community areas themselves (Odum 1971). As ecotonal communities, riparian ZOnl!S characteristically contain and support many of the organisms of each of the overlapping aquatic, wetland, or upland terrestrial communities. In addition, riparian zones contain organisms which are charactelCistic of, and often restricted to, the ecotone (Odum 1971). The linear configuration of the riparian zone, its proximity to the aquatic zone, alld the physical, chemical and biological interchanges between the riparian and aquatic zones provide an astonishing number and variety of habitats that are occupied by organisms from all trophic levels. The number and diversity of habitats result in complex food webs that support some of the most produ,ctive and diverse wildlife populations of any ecosystem type.. Some spedes' of wildlife rely upon the riparian zone for all their life l'equirementll, but many ..ore -are only dependent on the riparian zone for some critical element or stage of their life cycle such as food, cover or breeding habiltat. Numerous other species also make extensive use of these areas even though they are not dependent upon them.