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fields, and inadvertently created desirable native fish habitat. These backwaters are usually short-lived; they fill at spring runoff in May and drain with descending flows in late June or early July. One such backwater is located at river mile (RM) 59, just above Cross Mountain Canyon. The dike prevents a side channel from flowing and a backwater is formed by waters rising from the downstream end (Figure 3). The feature is about 1.5 ha in surface area and is 0.3-1.5 m deep, with a silt substrate. Two radiotagged Colorado squawfish, tagged 46 and 62 km upstream in early May, entered this backwater at peak runoff in early June 1981. Both fish freely swam the length of the backwater and periodically stayed at the outlet near the flooding river. The backwater, at this time, had relatively silt-free water and was warmer than the torrential river. One fish spent nearly 2 weeks in the backwater before moving downstream. One week after the fish left, the water level receded sufficiently to isolate the backwater from the river. .e _ .~.-_ f ; R. rw~1 a'~ ~ ~ .~ ~...- .. ,,,r- __ ,~, -~ .. - ~. ~s ~ ..- -,;. ..,. uw. ` ~ _ . ~a ± y - .;.-.DJ y ~Si " _ `VIM--.r _ ~X ~~y ! _ .tea .c ~ ~,, T ~ ° .~ Y_ f~` rcL... .Y:: SI ... .. _ . Y'.. .. ... .. . Figure 3. Aerial view of side channel on the Yampa River diked for flood and erosion control at river mile 59. The backwater formed by the large earthen dike is sealed from the main channel at low flow. A second diked side channel at RM 99, above Juniper Canyon, was also used by two radiotagged squawfish during runoff in early June 1981. This feature was about 1.5 m deep with a silt bottom and a 1-m deep narrow channel leading to' the river. The feature was drained by descending flows in late June, about 1 week after the squawfish left. A side channel at RM 64, diked to protect a low-laying alfalfa field, also developed into a backwater during runoff. Maximum depth was 1.5-2.0 m and the backwater was drained by descending flows in late June. One radiotagged squawfish entered this backwater briefly in mid -June for the high-flow period then proceeded downstream and entered the large backwater previously mentioned at RM 59. A natural backwater in this same area of the Yampa River was also used by these fish, suggesting that adult radio-tagged Colorado squawfish showed a strong tendency to find and occupy natural and manmade backwaters during runoff 1981 in [he Yampa River. The capture of numerous squawfish in large backwaters of the Colorado River during runoff also supports this contention. The flood control structures of the Yampa River demonstrate that usable backwaters can be intentionally created to enhance adult and possibly juvenile squawfish habitat during high flows. Design Backwaters Fish habitat enhancement efforts were attempted in the Upper Yampa River by private industry in the late 70's. Side channels were diked to create backwaters like those described in the previous section. But, the dikes were constructed of rock instead of earth and eventually breached to allow either continuous flow or flow only during runoff (Figure 4). The side channels with dikes that breached at high flow but sealed at low flow eventually filled with silt and sand and became unusable as native fish habitat. Those with some continuous flow provided good nurseries for native and non-native fishes (Table V). Non-native fishes made up 72.0% of the species composition in one breached backwater while native species made up 28.0%. Redside shiner (Richardsonius balteatus) and fathead minnow accounted for 68.8% of the composition. The most abundant native species was [he roundtail chub with 18.1% of the composition. Most roundtail chub were Large yoy while most other species were small yoy, suggesting that the native roundtail reproduces earlier, grows faster and has a size advantage on sympatric species. Colorado squawfish are not known to spawn at this location, but if they did they would likely spawn later and the young 526 527