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44 1972; Stevens 1977; Newcombe 1981, Yermakhanov and Rasulov 1983; Stevens and Miller 1983; Crecco and Savoy 1984). Facultative riverine species frequently exhibited an inverse relationship between year-class strength and flow during spawning and hatching, while a direct relationship was often the case for obligate riverine species. Overall, for the years of observation, larval Colorado squawfish production (as measured by C/f) appeared to have been greater in the lower Yampa River than in the Colorado River. In the Colorado River, C/f values were quite low each year during 1981-83, making definitive assessment of yearly differences in larval production virtually impossible. However, trends were noted. In 1982 (highest C/f), the flow regime approximated an "average flow" year and the number of 20+ degree C days was 84. The earliest estimated spawning date was 7 July. In contrast, flow in 1981 (lowest C/f) was extremely low, while flow in 1983 was high. For 1981 and 1983, the number of 20+ degree C days were 83+ and 59 and the earliest estimated spawning dates were 3 July and 27 July, respectively. Hypothetically, in 1981, low flow reduced the areal extent and/or quality of suitable spawning and nursery habitats; hence, supressing spawning and/or larval survival. In 1983, prolonged high flow (513 m3/sec on earliest estimated spawning date) had a negative effect on larval production and/or survival through suppression of the thermal regime. In the Yampa River, C/f values suggested that larval Colorado squawfish production was greatest in 1983 and 1984 (both high flow years). Interestingly, although flow during those 2 years was substantially greater than in 1981 and 1982 (7-20 times greater in 1983), heat accumulation (in terms of summed 20+ degree C days) in 1983 and 1984 were comparable with 1982. Number of 20+ degree C days in 1983 and 1984 were 65 and 67, respectively, compared to 71 in 1982. In 1981, number of 20+ degree C days was 92, but was associated with very low flows. It would seem that relatively high flow in the Yampa River has a positive influence on spawning, possibly via increasing the areal extent and/or quality of suitable spawning sites, provided that rapid heat accumulation occurs with flow reduction and adequate water temperatures for spawning exist for a sufficient time period. Results from both rivers suggest an optimum range of flows exist that provides the best quantity and quality of spawning habitat as well as an optimal number of degree days for egg incubation and production of larvae. A lower range of flows provide an adequate thermal regime but only marginal spawning habitat. A higher range of flows provide suitable spawning habitat but an inadequate thermal regime. The data suggest the Colorado River has demonstrated both extremes, while the Yampa River has demonstrated only the low flow case. It should also be noted that "high" flows on the Yampa River would describe "average" flows on the Colorado River. Tyus (1985) suggested that large, long-lived riverine species like the Colorado squawfish may have developed a life strategy which relies on the production of large numbers of offspring during wet years. The Yampa River results appear to support this hypothesis, but documentation of the relative contribution of larval production to year-class strength during wet and dry ~ years in either riVer would better substantiate it. Unquestionably, dispersion of larval Colorado squawfish occurs via downstream transport. There are several possible reasons why stream fish larvae tend to be displaced or transported downstream, either by passive drift or active dispersal. Hoar (1953), Northcote (1962), and Lindsey and