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that the model proved to be highly predictive based on past experience with various fish species in the reservoir. The reservoir rated "low" in suitability for all warmwater fish species except stocked channel catfish (Table 4). While the thermal regime of the reservoir would preclude natural reproduction of channel catfish, a handful of catfish that had been stocked in the reservoir in the 1980s and subsequently sampled during surveys in the reservoir displayed good body condition (unpublished data). Despite the ability of smallmouth and largemouth bass to reproduce in the reservoir, neither species attained great abundance and age-growth data showed growth of smallmouth bass in the reservoir was slow, with the bass reaching 15 in. (380 mm) in 9 to 10 years (unpublished data). For the coolwater species for which the model's habitat scores have been defined, an enlarged Elkhead Reservoir ranked "low-medium" for white sucker and yellow perch, but low for northern pike (Table 4). While white sucker accounted for one of the highest percentages of fish species captured by a combination of seining, gill netting and electrofishing in 1987 (16% of 1,255 fish), it was outnumbered by native flannelmouth sucker (28%) and roundtail chub (21%), and smallmouth bass (21%) (Martinez unpublished data). Also, none of the white suckers captured in 1987 were of the larger sizes (> 400 mm) observed in other Colorado reservoirs suggesting less than favorable environmental conditions for this species. Yellow perch have never been introduced into the reservoir, and despite the higher habitat ranking for this species relative to the other species evaluated, the results of this modeling indicate that the performance of this species in the reservoir would be predictably poor. Rainbow trout, at one time regularly stocked in Elkhead, are not part of the current Lake Management Plan prepared for the reservoir (Elmblad et al. 1994). On the basis of temperature, Elkhead is certainly suitable for trout as surface waters only reach or exceed 70° F for only short periods during summer. This thermal regime contributed to the suitability score for this species of "low-medium" (Table 4). The ease of escapement and the turbidity of the inflowing water combine such that many trout spilled from the reservoir during runoff. This scenario has made it difficult to retain trout as more than a short-term component of the fishery in most years. An enlarged Elkhead would possess these same characteristics, but installation of a screen to minimize fish escapement (Miller and Laiho 1996) may make management of the reservoir with catchable rainbow trout more effective, whether the reservoir is enlarged or not. It is almost difficult to conceive of a reservoir which does not rank at least high-medium for some desirable sport fish or prey fish species. However, conditions in Elkhead reservoir, including a thermal regime that is neither optimum for coldwater or warmwater fish species during the summer months, is probably largely responsible for the marginal performance of species introduced to the reservoir to date. While the thermal regime might seem more favorable for coolwater species, the unique turbidity situation wherein there is surprisingly little sediment deposition or nutrient input, but rather a colloidal suspension during spring and other runoff events (R. Tenney, Colorado River Water Conservation District, personal communication), apparently restricts production. It also appears turbidity encourages escapement of some species (e.g. northern pike and rainbow trout). However, until 1992, smallmouth bass remained in the reservoir in fairly high numbers, although escapement of primarily young-of-year bass less than 75 mm has been documented (Martinez unpublished data, Miller and Rees 1995). In 1992, draining of the reservoir for repair of the dam resulted in flushing from the reservoir an apparently high proportion of sub-adult and adult smallmouth bass (Tyus and Saunders 1996, Appendix H). This flushing of bass from Elkhead reservoir is believed to be responsible for the decline of the reservoir's bass fishery and for the stark increase in the number of smallmouth bass collected in the Yampa River in recent years (B. Elmblad, Colorado Division of Wildlife, personal communication). 11