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drain with descending flow, may benefit native fishes while preventing establishment of permanent populations of non- natives. INTRODUCTION Natural backwaters and embayments in the Upper Colorado River System are important fish habitat, particularly for native fauna like the protected Colorado squawfish (Ptycho_ cheilus lucius), humpback chub (Gila cypha) and razorback sucker Xyrauchen texanus). Young-of-the-year (yoy) and juvenile Colorado squawfish frequent these habitats in the Colorado, Green and Yampa Rivers (1,2,3]. Adult squawfish and razorback sucker as well as yoy and juvenile humpback chub also use these habitats. Water uses of the Upper Colorado River in the last two decades have altered flow regimes and eliminated many back- waters and embayments. Reduced flows and channelization have significantly altered the natural processes that created these habitats. Declines in numbers of endangered fishes have accompanied a diminished prevalence of backwaters and embayments. Reconstructing these habitats seems to be a reasonable mitigation measure for habitat lost to increasing water uses. We address this hypothesis by first characterizing natural and manmade backwaters and embayments in the Yampa and Upper Colorado Rivers, and then by contrasting fish use [o deter- mine the most desirable aspects of these habitats. Construc- tion alternatives are examined in response to different fish communities; constructing manmade habitats is greatly com- plicated by the presence of non-native species. These analyses apply only to the Upper Colorado River System in the interest of enhancing and managing habitat for recovering Colorado squawfish, humpback chub and razorback sucker. A similar study was conducted on four artificial backwaters in the Lower Colorado River for managing game fishes [4]. This study is part of the Colorado River Fishery Project conducted by the U.S. Fish and Wildlife Service and the Colorado Division of Wildlife, and funded by the Bureau of Reclamation, Bureau of Land Management and U.S. Park Service. NATURAL FEATURES The term backwater has been used to describe shallow shoreline pockets or indentations in the river channel, regardless of size or origin. A backwater, for the purposes of this discussion, forms either in a side channel at low 520 flow, or in a former river channel or intermittent tributary ~~ flooded by high flow. An opening to the river is usually at the downstream end of the flooded channel. Indentations at upstream and downstream ends of sand bars or small islands, usually formed by eddies, we shall call embayments. This distinction between backwaters and embayments is made because [he origin of these features greatly influences measures necessary to recreate them on the river. Backwaters and embayments appear to serve a similar purpose, but their origins are very different. Embayments are commonly called backwaters by many biologists afield since both are low-velocity habitats. Backwaters Backwaters in the Upper Colorado River vary in size and depth. Average size of five randomly-selected backwaters was 1.06 ha surface area, and average depth was 1.1 m (Table I). Water velocity in all of these backwaters was undetectable although wind and river circulation sometime occur, especial- ly near the outlet. The substrate of these five backwaters was silt and sand, or silt over gravel. These backwaters may be temporarily eliminated by high spring flows when [he side channel or tributary has continuous flow. Others exist only during high flow when flood waters inundate low-lying areas created by former river channels (Figure 1). Fish samples in these five backwaters from July to November 1980 yielded 5 native (17.4% numerical composition) and 10 non-native species (82.6%) (Table II). Red shiner Table I. Area, depth and substrate of five randomly-selected backwaters of the Upper Colorado River, located as river miles upstream from the Green River confluence. River Mile Area (ha) Depth Max. (m) Ave. Substrate 1.4 1.20 3.6 2.8 silt/sand 48.2 0.98 1.1 0.5 sand/silt 120.9 0.98 1.5 1.0 silt/gravel 125.7 1.01 1.5 1.1 silt/gravel 146.9 1.14 1.2 0.3 silt Means: 1.06 1.8 1.1 silt 521