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include locations outside the area currently occupied by the species, when such locations contain physical or biological qualities essential for its conservation. These physical or biological qualities are "primary constituent elements". There are five features of critical habitat that require special management or protection for recovery of the razorback sucker: space for growth and normal behavior; food, water, or other nutritional or physiological requirements; cover or shelter; breeding and rearing sites; habitats protected from disturbance or representative of geographical and ecological distributions. These features generally fall into three areas that are considered primary constituent elements: water, physical habitat, and the biological environment (Maddux et al. 1993). The "water" element includes consideration of water quality and quantity. Water quality is defined by parameters such as temperature, dissolved oxygen, environmental contaminants, nutrients, turbidity, and others. Water quantity refers to the amount of water that must reach specific locations at a given time of year to maintain biological processes and to support the various life stages of the species. The "physical habitat" element includes areas of the Colorado River system that are or could be suitable habitat for spawning, nursery, rearing, and feeding, as well as corridors between such areas. Habitat types include bottomland, main and side channels, secondary channels, oxbows, backwaters, and other areas in the 100-year floodplain, which when inundated may provide habitat or corridors to habitat necessary for the feeding and nursery needs of the razorback sucker. The "biological environment" element includes living components of the food supply and interspecific interactions. Food supply is a function of nutrient supply, productivity, and availability to each life stage. Negative interactions include predation and competition with introduced nonnative fishes. The Service determined critical habitat for the razorback sucker in a final rule published on March 21, 1994 (59 FR 13374). Fifteen river reaches covering about 49% of the historic habitat of the razorback sucker (1,724 mi.) were designated within the Colorado River basin (Figure 3). Included are portions of the Green, Yampa, Duchesne, Colorado, White, Gunnison, and San Juan rivers in the upper Colorado River Basin, and portions of the Colorado, Gila, Salt, and Verde rivers in the lower Colorado River Basin. The designated areas contain habitats within the 100 year flood plain that will meet the needs of the razorback sucker as defined by primary constituent elements. As an integral park of making the critical habitat determination, the USFWS also has produced a Biological Support Document (Maddux et al. 1993) and an economic analysis (Brookshire et al. 1994). The 15 reaches of critical habitat for the razorback sucker are described below with a brief summary of features important to the various life history stages. Legal descriptions of those areas are given elsewhere (USFWS 1994). Nonnative fishes, 27