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2,800 cubic meters per second (cms) occurred in Cataract Canyon in 1983 and 5,600 cms in 1984. These 2 record water years were followed by 2 nonnal years (1985 and 1986), but peak and base flows remained high because of high soil moisture content. Flows in Cataract Canyon in 1985 peaked in May at about 1,750 cms, while base flows generally remained above 280 cms. In 1986, flows peaked in June at about 1,980 cms, and base flows also remained around 280 cms. Peak flows in May 1987 reached 1,130 cms; the peak in 1988 was 790 cms. Base flows approximated 140 cms during these dry years. During the higher water years, water temperatures did not reach 20. C until early June in 1985 and mid-June in 1986. In 1987 and 1988, this temperature was recorded by mid-May. head length divided by caudal peduncle depth (Minckley 1973),. Chubs longer than 175 mm TL were individually photographed on a centimeter grid board and measured afield for (I) total length, (2) fork length, (3) distance between insertions of pectoral and pelvic fins, (4) nuchal depth, (5) minimum caudal peduncle depth, (6) caudal peduncle length, (7) head length, (8) length of dorsal fin base, (9) length of anal fin base, (10) dorsal and anal fin-ray counts, (II) maximum caudal peduncle depth, (12) maximum body depth, and (13) snout length. These measurements were analyzed using principal components analysis (Humphries et al. 1981) and, together with the morpho- logical characters and ratios, were used to distinguish the three species of chubs-Gila cypha, G. elegans, and G. robUSla. Specimens that could not be definitively identified to species were classified as Gila sp. Methods Species Composition Six field trips were conducted annually in 1985 and 1986, eight in 1987, and nine in 1988. Sampling was conducted in every month, except during spring runoff in May-June and during winter in November-February. Access to the study area was by motorized rafts from launch sites at either Potash on the Colorado River or Mineral Bottom on the Green River. The takeout was at Hite Marina on Lake Powell, about 160 km downstream. Launch sites were alternated between the Colorado and Green rivers to systematically sample all five regions. Fish were sampled by a variety of means, including electrofishing, gill nets, trammel nets, seines, minnow traps, hoop nets, larval drift nets, kick screens, and dip nets. Native fish were counted and individually measured, weighed, and released. Nonnative fish were counted and individually mea- sured and weighed (when captured in small numbers) or subsampled for lengths and weights (when captured in large numbers). Catch per unit effort (CPUE) was computed for each species by gear type as number of fish/ I 0 h of electrofishing, number/30 m of gill or trammel netlloo h, number/lOO 1112 seined, or number/to h of minnow traps or hoop nets. Larval drift nets were assessed as number of fish/10m3 of water filtered, and fish captured by kick screens and dip nets were expressed as number captured by effort. All data were recorded on field data sheets using the upper Colorado River basin data base codes and later electronically stored as dBASE 111+ files. Endangered fish longer than 175 mm total length (TL) were marked with serially numbered Carlin dangler tags attached to the musculature at the base of the dorsal fin (Valdez et al. 1981). Humpback chub, roundtail chub, and suspected bony tail were distinguished by field examination of morpho- logical characters and field photographs and measurements used for later examination. Morphological characters were mouth overhang, jaw length, nuchal hump, skull depression, caudal peduncle angle, nuchal scalation, and breast scalation (Douglas et al. 1989). Morphological ratios included dorsal:anal fin-ray count, caudal peduncle length divided by head length, and Thirty-one species of fish, including 23 nonnative and exotic and 8 native species, were found in the study area from 1985 through 1988 (Table I). All four species of mainstem endangered fishes were captured, including humpback chub, bony tail, Colorado squawfish, and razorback sucker. These four species accounted for fewer than 6% of individuals captured in any given year of the study and about 3% overall. Of the eight native species, six were endemic-humpback chub, bony tail, rouildtail chub, Colorado squawfish, flannel mouth sucker (Caloslomlls lal;p;nn;s), and razorback sucker-and two were native but not endemic-speckled dace (Rhin;chlhys OSCU/IIS) and bluehead sucker (c. discobollls). The eight native species collectively ranged from less than Ito 19% of the total catch by region (Table 2). Numerically, nonnative species made up 95% of the fish encountered in all years combined, while the eight native species made up only 5% (Fig. 2). Five nonnative species-red shiner (Cyprillella llllrensis), sand shiner (Nolropis stramineus), channel catfish (lctalurus punctalus), common carp (Cyprilllls carpio), and fathead minnow {Pimephales promelas)-accounted for 90% of the catch. Red shiners outnumbered all other species in region I (Green River), region 3 (Confluence), region 4 (Cataract Canyon), and region 5 (Lake Powell). This species was abundant in shallow, protected areas with silt and sand substrate. Sand shiners were dominant in region 2 (Colorado River), where sand substrate was predominant. Nonnative species accounted for 80 to 99% of the fish caught by region (Table 2). The greatest percentage of nonnative species caught was in region 5, where both lacustrine and riverine species were abundant. Species that were present in region 5-but rare or absent in the riverine habitat of regions 1-4-included black crappie (Pomoxis nigromaclIlatlls), bluegill (Lepomis macrochirus), small mouth bass (Microplerus dolomieui), threadfin shad (Dorosoma pelellense), walleye (Sti'l.ostedion vitreum), and striped bass.