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Figure 2. An embayment at low flow on the Upper Colorado River, river mile -16. Note the stable talus bank and large sand bar formed by a high-water eddy. Table III. Area, depth, and substrate of five randomly- selected embayments of the Upper Colorado River, located as river miles upstream from the Green River confluence. River Mile Area (ha) Depth Max. (m) Ave. Substrate 8.5 0.09 0.2 0.2 silt 16.9 0.11 0.3 0.2 silt 31.6 0.20 0.2 0.2 sand 38.3 0.10 0.2 0.1 silt 67.0 0.16 0.8 0.2 sand/silt Means: 0.13 0.3 0.2 silt Table IV. Percentage composition of native and non-native , fishes in five randomly-selected embayments of the Upper Colorado River (n=3757). Native Species Percentage Non-Native Percentage Composition Species Composition bluehead sucker 1.7 red shiner 56.1 Colorado squawfish 1.4 sand shiner 25.1 roundtail chub 0.6 fathead minnow 10.3 flannelmouth sucker 0.2 channel catfish 3.0 speckled dace 0.1 common carp 0.7 Total: 4.0 black bullhead 0.3 largemouth bass 0.2 green sunfish 0.1 `---- plains killifish 0.1 white sucker 0.1 Total: 96.0 Permanent populations of non-natives are precluded by the inundating and flushing effects of spring runoff. MANMADE FEATU RES Manmade features that resemble backwaters and embayments in the Upper Colorado River System are usually the coinciden- tal result of agricultural or industrial activities in or near the river channel. Flood control dikes on side channels, and gravel pits in islands or adjacent floodplains often provide habitat for large numbers of fishes. Use of these features by native and non-native fishes often varies sub- stantially. The intentional design and construction of backwaters as native fish habitat is known from only the Upper Yampa River. Flood Control Dikes Earthen dikes, constructed in side channels to prevent flooding and erosion of agricultural lands, often result in large backwaters during high flows. Four such cases are known from the Upper Yampa River where land owners blocked side channels to prevent erosion and flood damage to adjacent 524 ~ 525