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<br />9 <br /> <br />reduce the ability of the river system to provide the required water quantity <br />and hydrologic regime necessary for recovery of the fishes. The physical <br />habitat includes areas of the San Juan River system below Farmington, New <br />Mexico, that are inhabited or potentially habitable for use in spawning and <br />feeding, as a nursery, or serve as corridors between these areas. In <br />addition, oxbows, backwaters, and other areas in the 100-year floodplain, when <br />inundated, provide access to spawning, feeding, and nursery habitats. Water <br />depletions reduce the ability of the river to create and maintain these <br />important habitats. Food supply, predation, and competition are important <br />elements of the biological environment. Food supply is a function of nutrient <br />supply and productivity, which could be limited by reduction of high spring <br />flows brought about by water depletions. Predation and competition from <br />nonnative fish species has been identified as a factor in the decline of the <br />endangered fishes. Water depletions contribute to alterations in flow regimes <br />that favor nonnative fishes. The Service concludes that water depletions <br />impact the primary constituent elements and cause destruction or adverse <br />modificatioR to critical habitat. <br /> <br />BIOLOGICAL BACKGROUND <br /> <br />A marked decline in Colorado squawfish and razorback sucker populations can be <br />closely correlated with the construction of dams and reservoirs between the <br />1930's and the 1960's, introduction of nonnative fishes, and removal of water <br />from the Colorado River system. Behnke and Benson (1983) summarized the <br />decline of the natural ecosystem. They pointed out that dams, impoundments, <br />and water use practices are probably the major reasons for drastically <br />modified natural river flows and channel characteristics in the Colorado River <br />Basin. Dams on the main stem Colorado and San Juan Rivers have essentially <br />segmented the river systems, blocking Colorado squawfish and razorback sucker <br />spawning migrations and drastically changing river characteristics, especially <br />flows, temperatures, and channel geomorphology. In addition, major changes in <br />species composition have occurred due to the introduction of nonnative fishes, <br />many of which have thrived as a result of changes in the natural riverine <br />system (i.e., flow and temperature regimes). The decline of endemic Colorado <br />River fishes seems to be at least partially related to competition or other <br />behavioral interactions with nonnative species, which have perhaps been <br />exacerbated by alterations in the natural fluvial environment. <br /> <br />As the southernmost tributary of the Upper Colorado River Basin (Upper Basin), <br />the San Juan River peaks earlier in the year and attains warmer water <br />temperatures than other Upper Basin streams and is conducive to longer and <br />better growth potential for young Colorado squawfish and razorback suckers. <br />Any additional large loss of water or further degradation of remaining <br />habitats of the San Juan River will exacerbate problems the Colorado squawfish <br />and razorback sucker are currently experiencing in the San Juan and other <br />subbasins throughout the Upper Basin. <br />