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12 <br />Colorado, Delta and Mesa Counties. The Gunnison River and its 100 -year <br />floodplain from the confluence with the Uncompahgre Raver in T. 15 S., <br />R. 96 W., section 11 (6th Principal Meridian) to Redlands Diversion Dam <br />in T. 1 S., R. 1 W., section 27 (Ute Meridian). <br />Colorado. Mesa and Garfield Counties. The Colorado River and its <br />100 -year floodplain from Colorado River Bridge at exit 90 north off <br />Interstate 70 (river mile 238) in T. 6 S., R. 93 W., section 16 <br />(6th Principal Meridian) to Westwater Canyon (river mile 125) in <br />T. 20 S., R. 25 E., section 12 (Salt Lake Meridian) including the <br />Gunnison River and its 100 -year floodplain from the Redlands Diversion <br />Dam in T. 1 S., R. 1 W., section 27 (Ute Meridian) to the confluence <br />with the Colorado River in T. 1 S., R. 1 W., section 22 (Ute Meridian). <br />Utah, Grand, San Juan, Wayne, and Garfield Counties. The Colorado River <br />and its 100 -year floodplain from Westwater Canyon (river mile 125) in <br />T. 20 S., R. 25 E., section 12 (Salt Lake Meridian) to full pool <br />elevation, upstream of North Wash, and including the Dirty Devil arm of <br />Lake Powell in T. 33 S., R. 14 E., section 29 (Salt Lake Meridian). <br />Biology <br />Specific information on biological and physical habitat requirements of the <br />razorback sucker is very limited. Until very recently, fisheries research <br />investigations throughout the Upper Basin have focused on the three listed <br />Colorado River fishes, and data collected on the razorback sucker was largely <br />coincident to those studies. Localized extirpation of razorback suckers from <br />some localities, coupled with the species' continued decline in numbers and <br />distribution, has prompted some research; however, details of its life history <br />requirements, particularly in riverine environments, are still not fully <br />understood. <br />In general, a natural hydrograph with a large spring peak, a gradually <br />descending limb into early summer, and low stable flows through summer, fall, <br />and winter are thought to create the best habitat conditions for endangered <br />fishes while maintaining the integrity of the channel geomorphology. Prior to <br />construction of large main stem dams and the suppression of spring peak flows, <br />low velocity, off - channel habitats (seasonally flooded bottomlands and <br />shorelines) were commonly available throughout the Upper Basin (Tyus and Karp <br />1989; Osmundson and Kaeding 1991). The absence of these seasonally flooded <br />riverine habitats is believed to be a limiting factor in the successful <br />recruitment of razorback suckers in their native environment (Tyus and Karp <br />1989; Osmundson and Kaeding 1991). Tyus (1987) and McAda and Wydoski (1980) <br />reported springtime aggregations of razorback suckers in off- channel <br />impoundments and tributaries that were believed to be associated with <br />reproductive activities. Tyus and Karp (1990) and Osmundson and Kaeding <br />(1991) reported off - channel habitats to be much warmer than the main stem <br />river and that razorback suckers presumably moved to these areas for feeding, <br />resting, sexual maturation, spawning, and other activities associated with <br />their reproductive cycle. While razorback suckers have never been directly <br />observed spawning in turbid riverine environments within the Upper Basin, <br />