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5 <br />difference between wide and narrow channels is much less than is the difference between <br />wide and narrow valleys. Bankfull channel width may be nearly 300 m in the widest <br />alluvial valleys, but is less than 75 m in some narrow canyons (Fig. 3a). <br />The longitudinal profile of the Green River includes steep and flat segments (Fig. <br />3b), and these segments do not systematically decrease in gradient downstream. Narrow <br />alluvial valleys may have flat or steep gradients; for example, channel gradient in <br />Labyrinth and Stillwater Canyons is very low although the channel is confined in a <br />narrow alluvial valley. Low-gradient reaches of the river have sandy substrates, but <br />the median grain size of exposed bars exceeds 10 mm in steep gradient segments (Fig. <br />3c). <br />THE COLORADO SQUAWFISH IN THE GREEN RIVER <br />The Colorado squawfish was once abundant throughout the entire Colorado River <br />basin. This endemic species, the largest North American minnow, was classified as <br />federally endangered in 1967 and only exists upstream from Lake Powell reservoir. <br />The largest known concentration of Colorado squawfish occurs in the Green River. Tyus <br />and Haines (1991) identified two major spawning areas for the Green River population, <br />one in the lower Yampa River and one in Gray Canyon, although Stanford (1994) <br />suggested that additional spawning sites may exist. Spawning occurs on gravel bars that <br />have small amounts of interstitial fine sediment. Spawning in the Yampa River is <br />triggered by increasing stream temperature and occurs a few weeks after the maximum <br />peak discharge has occurred. Eggs hatch in 3.5 to 6 dys; protolarvae emerge soon <br />thereafter, drift downstream and enter the Green River (Tyus 1991). Rearing of <br />nursery and juvenile fish occurs in the Green River, and adults eventually move <br />upstream and reoccupy spawning sites.