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breeding requirements of G. cypha in the Yampa River. Interfaces between eddies and runs were similarly judged important in the Black Rocks area (below Grand Junction, CO) (R. A. Valdez and B. A. Nilson, Proc. Am. Fish. Soc., Bonneville Chapter, 1982, unpubl.). Adult G. cypha are primarily nocturnal (Fig. 1--5 of Valdez and Clemmer, 1982). During daylight hours in the LCR, they reside in deeper waters along cut banks with overhanging vegetation (primarily reeds), along sheer rock outcrops, or in deeper pools away from shore; they are active during crepuscular hours and in late evening (C. 0. Minckley, pers. comm.; M. E. Douglas and P. C. Marsh, pers. obs.). The greater numbers of G. cypha found in the Salt Canyon reach, when compared to Powell Canyon reach, sustain at least two alternative hypotheses. Increased habitat complexity in the Salt Canyon reach, with greater numbers of large travertine dams, eddy/run interfaces, and deep pools, may increase residency of G. cypha within this area. Alternatively, those G. cypha that move up the LCR may literally 'stack' within the upper reach, due either to a physical barrier at RKM 14.9, or a chemical one produced by high CO2, or other chemical content. Glen Canyon Dam and Gila cypha.---There is long-term residency by G. cypha within the LCR, particularly summer through winter. In fact, many adults apparently overwinter within the LCR, effectively using it as a warm-water refugium. Two hypotheses are presented to accommodate these data. One suggests residency is a pre-dam component of G. cypha's life history. The other proposes that it is a post-dam alteration. It is unclear which can be rejected; both are untestable in their present form. Long-term residency by adults may have always been an aspect of G. cypha's life-history. We know, for example, that it spawned within the pre-dam LCR during spring (Kolb and Kolb, 1914:127; Carothers and Brown, 1991:93). However, its duration of stay was unknown. If residency has always been a component of G. cypha's natural history, then our mark/recapture data simply define inherent behavior over evolutionary time. An alternative hypothesis is that the altered thermal regime of the mainstem has forced G. cypha to adjust its life history. It now accommodates lower mainstream temperatures primarily through avoidance (i.e., by increasing residency within the LCR). This hypothesis is anecdotally supported by three facts: First, movements into/from the LCR are primarily accomplished by larger (and presumably older) G. cypha (R. A. Valdez, pers. comm.). Gila cypha attains great age (20+ years; Minckley, 1991:150); larger adults may thus represent mainstem-adapted individuals from pre- 1968 cohorts (when Lake Powell filled). Secondly, larvae and juvenile G. cypha are often transported via flood into the mainstem, but adults less than 200 mm TL are seldom taken there 11