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. . I YY3 ~OJ0~y i ~{.Q\. ?137 @ 1993 by S.E.L & Associates . A Physical Process-biological Response Model for Spawning Habitat Formation for the Endangered Colorado Squawfish Michael D. Harvey Robert A. Mussetter Resource Consultants & Engineers, Inc. p.o. Box 270460 Fort Collins, Colorado 80527 Edmund J. Wick National Park Service 1201 Oak Ridge Drive, Suite 250 Fort Collins, Colorado 80525 ABSTRACT: The Colorado squawfish (PtychocheiLus Lucius), a federally listed endangered species, spawns at a limited number of sites within the lower Yampa River, Colorado, during the recessional limb of the annual snowmelt hydro- graph. A three-level, physical process-biological response model for spawning habitat formation was developed from field measurements, hydraulic modeling (HEC-2), and analysis of a known spawning bar at River Mile (RM) 16.5. Sed- iment deposition and bar formation occur at discharges greater than 10,000 cfs, a discharge at which downstream hydraulic controls cause backwater and re- duced transport capacity of the flows. Spawning habitat is formed by bar dis- section and erosion at a range of flows between 400 and 5,000 cfs when the local hydraulic energy is greatest because of reduced tailwater downstream, and sed- iment delivery to the chute channels is reduced by deposition in an upstream pool. The process-response model appears to be validated by fish-capture data at this, and another spawning bar at RM 18.5, on the Yampa River during both the 1991 and 1992 runoff seasons. KEY WORDS: Colorado River basin, endangered species, hydraulic modeling, sediment flushing. INTRODUCTION HistoriCallY the native cyprinids (Col- effects of habitat loss, regulation of flows, orado squawfish, humpback chub, proliferation of introduced competitors and bony tail chub, roundtail chub) and catos- predators, and other man-induced distur- tomids (razorback sucker, flannel mouth bances (Tyus and Karp 1989; Tyus 1992). sucker) were the dominant fishes in main- As a result, all of the named fishes have stream habitats of the Colorado River basin been designated as federally listed endan- and they were widely distributed and com- gered species. Recent proposals by the U.s. mon to abundant in major rivers of the Fish and Wildlife Service (USFWS) to des- basin (Tyus 1986; Tyus and Karp 1989). ignate critical habitat for most of the Col- However, with the exception of the round- orado River basin upstream of Lake Powell tail chub and flannelmouth sucker, all of have highlighted the need to develop sci- the above-named species are now threat- entifically defensible criteria for identify- ened with extinction due to the combined ing habitats for various life stages and de- I~ 114 Rivers . Volume 4, Number 2 Pages 114-131