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5 <br />and Garfield Counties. The Colorado River and its 100-year floodplain <br />from the Colorado River Bridge at exit 90 north off Interstate 70 (river <br />mile 238) in T. 6 S,, R. 93 W., section 16 (6th Principal Meridian) to <br />North Wash, including the Dirty Devil arm of Lake Powell up to the full <br />pool elevation, in T. 33 S., R. ]4 E., section 29 (Salt Lake Meridian). <br />Biologv <br />The life-history phases that appear to be most critical for the Colorado <br />squawfish include spawning, egg fertilization, and development of larvae <br />through the first year of life. These phases of Colorado squawfish <br />development are tied closely to specific habitat requirements. Natural <br />spawning of Colorado squawfish is initiated on the descending limb of the <br />annual hydrograph as water temperatures approach 20 °C. Spawning, both in the <br />hatchery and in the field, generally occurs in a 2-month timeframe between <br />July 1 and September 1, although high flow water years may suppress river <br />temperatures and extend spawning in the natural system into September. <br />Temperature also has an effect on egg development and hatching. In the <br />laboratory, egg mortality was 100 percent in a controlled test at 13 °C. At <br />16 °C to 18 °C, development of the egg is slightly retarded, but hatching <br />success and survival of larvae was higher. At 20 °C to 26 °C, development and <br />survival through the larval stage was up to 59 percent (Hamman 1981). <br />Juvenile temperature preference tests showed that preferred temperatures <br />ranged from 21.9 °C to 27.6 °C. The most preferred temperature for juveniles <br />and adults was estimated to be 24.6 °C. Temperatures near 24 °C are also <br />needed for optimal development and growth of young (Miller et al. 1982). <br />Only two Colorado squawfish confirmed spawning sites, as defined in the <br />Colorado squawfish Recovery Plan, have been located in the Basin: river <br />mile 16.5 of the Yampa River and river mile 156.6 of the Green River. These <br />areas have the common characteristics of coarse cobble or boulder substrates <br />forming rapids or riffles associated with deeper pools or eddies. It is <br />believed that a stable, clean substrate is necessary for spawning and <br />incubation. Substrates are swept clean of finer sediments by high flows <br />scouring the bed prior to the spawning period. <br />O'Brien (1984) studied the hydraulic and sediment transport dynamics of the <br />cobble bar within the Yampa River spawning site and duplicated some of its <br />characteristics in a laboratory flume study. Based on field observations, he <br />reported: <br />"On the rising limb of the hydrograph, sands are deposited in the <br />cobble interstices: These sands are interchanged between the bed <br />and the .suspended zone for discharges less than banklull. <br />Depending qn the supply=:capacity relationship, either deposition <br />or scour could be occurring. When the cobbles move, the. sand, of <br />course, is washed from the interstices and may be completely <br />removed from around the cobbles. Rearrangement of the cobbles <br />will result in more stability of the armor layer. On the falling <br />