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. . 8750; -- 1981 _~l ~ I ~ S250 I ~'l5OO &l is 1750~ _ ~SOWlGE m.<>EAAlUlE 0'-- --- -0 AL.GU3T APRJl ",",y ....., .ILLY 24500 -- 25 ! ~:. ._1983 :;:;J\lUlE/....\]...~.....:/.... = :~ 't ,. :... ...... .~ 10~ ~ 7000! /,:. ':.. .i . 3500~ ',._' .5 0: --....~-O APRJl Jl-"E AL.GU3T ",",y JlU FIGURE 1. Relationships between discharge, water temperature, and spawning period for Colo- rado squawfish, Yampa River, 1981 and 1983 (mod- ified from Tyus and Karp 1989). fining the physical processes and flow requirements for the formation of critical habitats for the affected species. Relations between flow, habitat, and fish production are not well understood. In- stream flow requirements for the endan- gered species can be determined only by the integration of life history information with instream flow needs (Tyus 1992). Streamflow requirements have generally been assessed by using the Instream Flow Incremental Methodology (IFIM), Physical Habitat Simulation (PHABSIM) models, or by empirical relations developed between limited fish-catch data and annual peak flows (Osmundson and Kaeding, USFWS, unpublished report). However, Tyus (1992) considered that both the IFIM and PHAB- SIM models are inherently unable to val- idate any relation between physical habi- tats and fish populations, and that empirically derived relations (regression) also are inherently unable to determine cause and effect. Life history information can be used to identify habitat use by var- ious life stages over the course of the an- nual hydrograph (Tyus and Karp 1989). However, validation of the relation be- tween habitat use and flow requirements ultimately requires that the basis for the relation be established with some form of physical process-biological response mod- el. Fish-habitat identification and charac- terization of habitat use by different life stages of various fish species obviously lies .'5 within the field of biology. Habitat is, how- ever, formed by the interactions between a range of flows of different magnitudes and the channel boundary materials. Therefore, determination of the factors re- sponsible for habitat formation lies pri- marily within the physical-process-based disciplines of fluvial geomorphology, hy- drology, and hydraulic engineering (Heede and Rinne 1990; Hill et al. 1991). Identi- fication of flows and of physical processes that form and maintain critical habitat for various species should be based on a mul- tidisciplinary approach. Adult Colorado squawfish (Ptychocheilus lucius) are distributed in the mainstream Yampa River from its mouth upstream for a distance of about 120 mi. A major Colo- rado squaw fish spawning migration with- in the upper Green River and the Yampa River was identified by Tyus and McAda (1984) and confirmed by Wick et al. (1983). In May and early June, Colorado squawfish begin migrating in the Yampa, Green, and White rivers to spawn in the lower Yampa Canyon (Tyus and Karp 1989; Tyus 1990). The only other confirmed spawning site in the Green River basin is in Gray Canyon at approximately River Mile (RM) 155 (Tyus and Karp 1989; Tyus 1990). To date, there are no physically based criteria for locating potential Colorado squawfish spawning sites. Currently known spawning sites have been located over the years by trial-and- error methods that involved collecting Colorado squawfish larvae (Haynes et al. 1984). Tyus and Karp (1989) and Tyus (1990) summarized the relations between dis- charge, water temperature, and spawning periods for the Colorado squawfish on the Yampa River (Figure 1). Colorado squaw- fish have been reported to spawn at a lim- ited number of sites on the Yampa River on the falling limb of the annual snowmelt hydrograph, when water temperature is between 17 and 270C. Timing of spawning varies somewhat with the runoff magni- tude. Spawning occurs earlier in low-water years and later in high-water years. In ad- dition to discharge and water temperature, other factors such as chemoreceptive im- printing-although not fully under- stood-are believed to be involved with spawning (Wick et al. 1983; Tyus and Karp 1989; Tyus 1990). 2!l 15 ~ "- :2 10~ I M. D. Harvey et al. 115 II~