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<br />.~ <br /> <br />approximate RM 155, 22 Colorado squaw- <br />fish, 18 of which were ripe and tubercu- <br />lated, were captured in the vicinity of a <br />dissected midchannel bar. The bar has <br />formed as a result of backwater caused by <br />a downstream contraction in the canyon <br />width, the result of deposition by a coarse- <br />grained tributary alluvial fan (Three Fords <br />Canyon West). Although no quantitative <br />physical data have been collected, it ap- <br />pears that this site meets the requirements <br />of the process-response model, and would <br />be a candidate for further investigation. <br />If the process-response model is correct, <br />it should also provide a basis for poten- <br />tially creating the physical requirements <br />for the "Yampa-type" Colorado squaw fish <br />spawning habitat. For example, the model <br />indicates that the Debeque Canyon reach <br />of the Colorado River could provide <br />spawning habitat for Colorado squawfish. <br />Access to the potential spawning habitat <br />has been blocked by low-head irrigation <br />dams that do not have fish bypasses. Unit- <br />ed States Fish and Wildlife Service records <br />(unpublished data) indicate that a signifi- <br />cant number of adult squaw fish have been <br />observed below the farthest downstream <br />dam. Downstream of Debeque Canyon in <br /> <br />the Grand Valley, the Colorado River has <br />a floodplain that is bounded on the south <br />side of the river by vertical outcrops of <br />Mancos shale, and therefore, under natural <br />conditions did not meet the macroscale re- <br />quirements of the model. However, the <br />combined effects of the shale cliffs, man- <br />made levees, and bank revetments have <br />modified the channel geometry locally and <br />caused backwater during high discharges <br />that has apparently created the macroscale, <br />mesoscale, and microscale conditions re- <br />quired for spawning habitat formation in <br />at least one subreach. Unpublished USFWS <br />records indicate that Colorado squawfish <br />spawning occurred in this sub reach in 1982, <br />which suggests that other requirements for <br />successful spawning such as water tem- <br />perature, and possibly chemoreception, <br />were also met. Considerably more inves- <br />tigation is required to confirm the poten- <br />tial for creating spawning habitat. The <br />Grand Valley observations suggest that it <br />may be possible to compensate to some ex- <br />tent for the loss of spawning habitat, pro- <br />vided that all the other factors required for <br />spawning and the temporal and spatial <br />constraints imposed by the habitat require- <br />ments of succeeding life stages are also met. <br /> <br />SUMMARY <br /> <br />A multidisciplinary data collection pro- <br />gram at a known Colorado squaw fish <br />spawning site at RM 16.5 (Cleopatra's <br />Couch) in the lower Yampa Canyon and <br />subsequent analysis of the field data en- <br />abled the development of a physical pro- <br />cess-biological response model for Colo- <br />rado squaw fish spawning habitat. The <br />process-response model indicates that high <br />discharges are responsible for the con- <br />struction of the spawning bar but not the <br />actual formation of the spawning habitat. <br />Downstream hydraulic controls cause a <br />backwater condition that results in the for- <br />mation of the bar as a heterogeneous mass <br />of sediments are deposited. Reduced tail- <br />water during recessional flows causes a <br />steepening of the local hydraulic gradient, <br />which in turn leads to bar dissection and <br />erosion of chute channels. Dissection of <br />the bar causes the fines to be flushed and <br />this is enhanced by reduced sediment de- <br />livery from upstream due to deposition in <br /> <br />the upstream pool. A clean cobble sub- <br />strate, with the constituent cobbles at in- <br />cipient motion, and suitable for egg ad- <br />hesion, is formed in the subaqueous tertiary <br />bars that are located within the chute chan- <br />nels. Spawning habitat at this spawning <br />bar is formed at discharges between about <br />400 and 5,000 cfs. The physical process- <br />biological response model appears to have <br />been validated by fish-capture data at this <br />spawning bar and at the Alternate spawn- <br />ing bar (RM 18.5) within the lower Yampa <br />River. Fish-capture data at Mathers Hole <br />in the lower Yampa River and at Three <br />Fords Canyon West on the Green River <br />indicate that these locations would be suit- <br />able sites to further test the process-re- <br />sponse model. <br />Similar m ul tidisci plinary investigations <br />could be used to develop physical process- <br />biological response models for habitat for- <br />mation for all life stages of the threatened <br />and endangered fish species of the Colo- <br /> <br />I M. D. Harvey et al. <br /> <br />129 II~ <br />