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9 <br />Most information on Colorado pikeminnow reproduction was gathered from <br />spawning sites on the lower 20 miles of the Yampa River and in Gray <br />Canyon on the Green River (Tyus and McAda 1984; Tyus 1985; Wick et al. <br />1985; Tyus 1990). Colorado pikeminnow spawn after peak runoff <br />subsides and is probably triggered by several interacting variables <br />such as photoperiod, temperature, flow level and perhaps substrate <br />characteristics. Spawning generally occurs from late June to mid- <br />August with peak activity occurring when water temperatures are <br />between 18° and .23° C (Haynes et al. 1984; Archer et al. 1985; Tyus <br />1990, Bestgen et al. 1998). <br />Spawning has been confirmed in the Colorado River by the presence of <br />Colorado pikeminnow larvae in all years sampled. Larvae are <br />distributed throughout the river although most have been found <br />downstream of Grand Junction (McAda and Kaeding 1991, Osmundson and <br />Burnham 1998). Aggregations of ripe adults have been found near <br />Clifton and Grand Junction, Colorado and near the Colorado-Utah State <br />line (Oamundson and Kaeding 1989, McAda and Kaeding 1991, USFWS <br />unpublished data). Suitable spawning habitat (defined below) in the <br />Colorado River near Cataract Canyon, Professor Valley, and upstream <br />from the Dolores River confluence indicate spawning may occur in or <br />near these areas as well (Archer et al. 1985; Valdez 1990). <br />Known spawning sites in the Yampa River are characterized by riffles <br />or shallow runs with well-washed coarse substrate (cobble containing <br />relatively deep interstitial voids (for egg deposition) in association <br />with deep pools or areas of slow laminar flow used as staging areas by <br />adults (Lamarra et al. 1985, Tyus 1990). Recent investigations at a <br />spawning site in the San Juan River by Bliesner and Lamarra (1995) and <br />at one in the Upper Colorado River (USFWS unpublished data) indicate a <br />similar association of habitats. The most unique feature at the sites <br />actually used for spawning, in comparison with otherwise similar sites <br />nearby, is the degree of looseness of the cobble substrate and the <br />depth to which the rocks are devoid of fine sediments; this appears <br />consistent at the sites in all three rivers (Lamarra et al. 1985, <br />Bliesner and Lamarra 1995). <br />Data indicates that clean cobble substrates that provide interstitial <br />spaces for eggs are necessary for spawning and egg incubation (Tyus <br />and Karp 1989). Several studies on the cobble cleaning process have <br />been conducted at a known spawning location in Yampa Canyon. O'Brien <br />(1984) studied the hydraulic and sediment transport dynamics of the <br />cobble bar within the Yampa River spawning site and duplicated some of <br />its characteristics in a laboratory flume study. O'Brien (1984) <br />concluded that incipient motion of the cobble bed is required to clean <br />cobbles for spawning and estimated that this takes discharges of about <br />21,500 cfs. However, Harvey et al.(1993) concluded that since flows <br />required for incipient motion of bed material are rare (20 year return <br />period event) and spawning occurs annually, another process must be <br />cleaning the cobbles. Their study found that in Yampa Canyon <br />