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°""'~f - - (Notropis lutrensis), fathead minnow (Pimephales promelas)1. and sand shiner (N. stramineus) accounted for 75.6% of the fish composition. Flannelmouth sucker (Catostomus la[ipinnis) and roundtail chub (G. robusta) were the most common native species. Small numbers of adult and juvenile Colorado ,x.,~,,, -~_- squawfish occurred as well as small numbers of adult razor- back sucker. ~" r:• ; - a; - ,~ t,,.~~„-..,-. -~''~-=? _ -~_ _"~- ~~-..K-~ __ -. ~~ ~ Backwaters in the Upper Colorado River are very produc- ~~ ~' ,~ five commu nities and support a varied complement of fishes, :tom`-~., _ dominated by non-natives. The rare Colorado squawfish and ~ - ~~'='~_= razorback sucker frequently use these backwaters during _ r'~ `~~ _ runoff for staging, resting and feeding; and in low water for ;,~~ feeding. Figure 1. Typical backwater of the Upper Colorado River form- ed by an inundated side channel, river mile 158. Table II. Percentage composition of native and non-native fishes in five randomly-selected backwaters of the Upper Colorado River (n=1895). Native Species Percentage Non-Native Percentage Species Composition flannelmouth sucker 10.6 red shiner 30.3 roundtail chub 5.5 fathead minnow 27.4 Colorado squawfish 0.9 sand shiner 17.9 bluehead sucker 0.2 black bullhead 2.6 razorback sucker 0.2 largemouth bass 1.9 total: 17.4 green sunfish 1.2 common carp 0.6 channel catfish 0.3 black crappie 0.2 white sucker 0.2 total: 82.6 Embayments Embayments are generally smaller than backwaters and often form a[ upstream and downstream ends of sand bars between the bar and the more stable river bank (Figure 2). Embayments are also found on the periphery of sand islands between sand spits. These habitats are usually inundated and formed by eddies during high flows. Eddy currents dig deep concavities in the channel bed. As flow subsides, fine sediments are deposited in the low-velocity areas to form bars with depressions for embayments in the high-velocity areas. Few embayments exist on the river longer than one normal water year because of the shifting river substrate. Five randomly-selected embayments from. the Upper Colo- rado River had an average surface area of 0.13 ha and an average depth of 0.2 m (Table III). No detectable velocity was recorded, but like backwaters, wind circulation and river exchange does occur. The substrate of embayments is usually silt or sand. A large complement of non-native fishes (96.0% numerical composition) was represented in embayments (Table IV ). 'Red shiner, sand shiner and fathead minnow made up 91.5% of the fish composition. Native fishes composed only 4.0% of the catch; young bluehead sucker (C. discobolus) and yoy and juvenile Colorado squawfish were the most common species. Embayments in the Upper Colorado River tend to support a greater percentage of non-native fishes than do backwaters, probably because the smaller, shallower and warmer embayments are more conducive to the small non-natives and their young. Embayments also appear to be the principal nurseries for yoy Colorado squawfish and some humpback chub; these fishes appear subjected Co a potentially competitive situation. 522 I 523