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Gila cypha was the last fish to be described from the mainstem Colorado River (Miller, 1946), the type specimen caught in 1932 by angling within Grand Canyon National Park (GCNP) [at Bright Angel Creek, now 141.3 river kilometers (RKM) below Glen Canyon Dam) (Carothers and Brown, 1991:95). The largest population of G. cypha is in the Marble Canyon section of GCNP, at the junction of the Little Colorado (LCR) and mainstem Colorado rivers, 99 RKM below Glen Canyon dam (Fig. 1--2b) While the life history of G. cypha is enigmatic (discussed in Douglas, 1993), the Grand Canyon population is least known of all. For example, G. cypha inhabiting the Colorado River at the LCR confluence were not even recognized as a reproducing population until 1975 (R. R. Miller, unpubl. field notes, Special Coll., Hayden Library, ASU). Even then their numbers were not considered substantial; the largest population of G. cypha at that time was believed to inhabit the Colorado River near Grand Junction (based upon 32 specimens captured in 1974; J. E. Johnson, Bur. Land Manag. Tech. Note 280, 1976, unpubl.). This study was undertaken to estimate numbers of adult G. cypha within the LCR, define the nature and extent of their movements within that river (where reproduction occurs annually), and test the hypothesis that both local movements and residency are restricted to the reproductive period. Habitat use is then discussed in context of Glen Canyon Dam, constructed in 1963 (completed in 1968) to impound Lake Powell at the northern extent of Grand Canyon. METHODS AND MATERIALS The study river.---The LCR drains 141,155 km2 of eastern and northern AZ and western NM, and flows 412 km from headwaters to confluence with the Colorado River (GCNP, Coconino Co., AZ; Figs. 2a, 2b). Unless in flood, the LCR is seasonally dry in much of its upper 390 km, a result of modern land-use practices and water impoundments initiated at the turn of the century (Miller, 1961). However, flow in the lower 21 km is perennial, from numerous groundwater springs which drain 72,520 kmZ of the Black Mesa north and east of Flagstaff, AZ. The largest of these (i.e., Blue SFrings, at LCR RKM 21; Fig. 1--2b) has a disch=--ge of 6.1-6.6 m /sec (Johnson and Sanderson, 1968). The LCR at base flow is saline (conductivity exceeds 5000 umhos/cm') and travertine-forming. Carbonate precipitates onto surfaces and in the water column, the latter giving the river a distinct turquoise color. Carbonate deposition (a function of C02 degassing and photosynthetic activity of algae and cyanophyceans) produces an intricate and confusing water chemistry (Kubly and Cole, 1979). Travertine accumulations over geologic time define pools, runs, and rapids, and generate scalloped waterfalls and 2