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52 Sciences 1982; Warner 1979a). The U.S. Council on Environment",l Quality (1978) stated in its ninth annual report that no ecosystem is more essential than the riparian ecosystem to the well-being of the nation's fish and wildlife. The significance of the riparian zone degradation crisis is easily understood when one considers that ripariall habitat is an extremely limited and finite resource making up only a tiny fraction of the total landscape. Although no large scale systematic quantifJ.cation of riparian habitat (or the amount of man-caused alteration) has been reported, riparian habitat can be characterized as being of narrow average width and less than equal to the total remaining free flowing stream mileage. Data from the U.S. Forest Service and the Bureau of Land Management illustrate the limits of the riparian habitat. Of the 173 ttillion acr.es manag,ed by the Bureau of Land Management and the 187 million acres managed by the Forest Service in the western United States (excluding Alask.. and Hawaii), only 0.3 percent and 1.2 percent, respectively, are composed. of ripari,.n and wetland ecosyste1llll (Owen 1979). Of the various eCIJsystems in the state of Arizona, riparian habitat occupies the smaLlest land area, less than 0.75% (Babcock 1968), most of which is confined to narrow bands adja,oent to the streambeds. Warner (1979) estimated that 95 percent lOr more of this lilllited ecosystem has been lost. Riparian habitats alOcount for only about 1 percent of the northern Great P l11ins (Asherin et al. in press). In addition, Bureau of Land MlI-nagement administrators have rE!ported that 83 percent of B .L.M. owned riparian lands in the west (excluding Alaska) are in an unsatisfactory condition (Almand and Krohn 1978). Of the 80 percent or so of U.S. riparian tone habitat acreagl' which is privately owned, most is dolllinated by agricultural activities or. other typee, of significant human impacts (Warner 1979b). Where habitat changes have been especially severe, many species of plants and animals have become increasingly scarce, threatened, or even endangered. Of the 236 E'pecies of plants and animals on the Department of Interior's 11e:t of "threatened" or "endangered" species, 69 (nearly oue-third) are directly or indirE,ct ly dependent upon riparian habitat. Twelve rJ.parian habItat areas have been designated "critical habitat" (Hirsch and Segelquist 1978) according to the Endangered Species Act of 1973 (see Chapter 8). Loss of riparian habitat and associatE,d wildlife has been mos t dramatic in the west and midwest. Riparian habitat is relatively scarce in these areaS and man has placed intense dellUl,nds on the limited water and land resources. An estimated 90 to 95 percent of the riparian cottonwood-willow habitat of the high-plains and lower foothills of the Rocky Mountain area has been lost (BeidlellUln 1978, Johnson and Carothers 1980). Cottonwood communities along the Colorado River have declined from an estimated 2025 ha (5,000 acres) to about 202.5 ha (500 acres) as a result of changing hydrologic regimes resulting from upstream dams and reservoirs. Hirsch and Segelquist (1978) rl!ported that there are still some 1134 ha (2,800 acres) of willow-cottonwood stands along the river, but most are inyaded by salt cedar. an exotic introducti()Q of much lower value to wildlife.