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<br />Introduction <br /> <br />The most extensive floodplain reach in the Upper Colorado River Basin is located in the <br />Middle Green River (Irving and Burdick 1995). Historically, the Green River at Ouray, Utah <br />flowed out of its banks approximately two out of three years (Flo Engineering 1996). Thus, <br />floodplain inundation in the Middle Green River was a regular occurrence in the spring and <br />summer months. On June 28, 1869 in an area above the current site of Ouray, Utah, John <br />Wesley Powell described "two or three lakes" .... " lying in the valley to the west" and later <br />carried one of their boats over a levee into a "lake of several thousand acres" near the current site <br />of Sheppard Bottoms (Powell 1895). The frequency, magnitude and duration of floodplain <br />connection to the Green River declined significantly after the completion of Flaming Gorge Dam <br />in 1963. Decrease in the active floodplain in the Middle Green River was further exacerbated by <br />dike construction related to agricultural and waterfowl production. Loss of floodplain inundation <br />in many large rivers has caused dramatic declines in native fishes that use floodplains during <br />some period in their life history (e.g. Schiemer and Spindler 1989, Copp 1990, Maheshwari et al. <br />1995). This study was initiated to demonstrate if age-O razorback sucker Xyrauchen texanus use <br />floodplain wetlands like Old Charley Wash, and if a physical and biological environment existed <br />within this site to support these fish through the growing season. <br />The greatest number of wild adult razorback sucker captured in the Upper Colorado River <br />Basin have been taken in low gradient/floodplain reaches of rivers (Modde et al. 1995). <br />Similarly, most razorback sucker in the Lower Colorado River Basin have been collected in <br />lower gradient reaches below the present site of Lake Mead (Minckley et al. 1991). The largest <br />extant razorback sucker population occupying natural riverine habitat exists in the Middle Green <br />River (Lanigan and Tyus 1989) coincident with the greatest area of floodplain habitat in the <br />Upper Colorado River Basin (Irving and Burdick 1994). Tyus and Karp (1990) suggested that <br />loss of floodplain inundation may be a factor in the lack of widespread recruitment and Modde et <br />al. (1996) correlated recruitment of young adult razorback sucker in the Middle Green River to <br />high flow events. Lack of successful recruitment is the major factor in the decline of razorback <br />sucker (McAda and Wydoski 1980, Minckley 1983) and circumstantial evidence suggests that <br />floodplains are related to persistence of this species, yet we do not understand how much and <br />what type of habitats are most beneficial. <br />The reproductive biology of razorback sucker in the Green River subbasin is linked to <br />spring flood events (Tyus and Karp 1989). Razorback sucker move to spawning locations and <br />reproduce on the ascending limb of the spring hydro graph (Tyus 1987, Tyus and Karp 1990, <br />Modde and Irving accepted). Larvae emerge at, or near, the peak of the hydro graph (Muth et al. <br />1997), providing maximum access to inundated floodplain habitats. Timing of spawning and <br />rapid growth of razorback sucker in off-channel habitat (Osmundson and Kaeding 1989) support <br />the premise that floodplain habitats could be important to the early life history of razorback <br />sucker. However, little direct information links floodplain habitat with age-O razorback sucker <br />use. In the lower Colorado River Basin, substantial numbers of juvenile and young adult <br />razorback sucker have been collected in irrigation canals and ponds downriver from Parker Dam <br />(Minckley et al. 1991). In 1994 three yearling razorback sucker were collected in the L-3 pond in <br />Leota Bottoms of the Ouray National Wildlife Refuge (CRFP-USFWS unpublished data), with <br />the Green River being the only possible source of origin. The collection of only twelve other <br /> <br />9 <br />