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point of maximum water depth, and at the midpoint toward either side. Total area and total volume of the habitat were estimated from these measurements. Primary and secondary substrate types were recorded at each depth point. Sub-surface temperatures (°C) and turbidity were ~ recorded at the midpoint of each transect. Turbidity was gauged on a subjective scale from 0 (> 6" visibility) to 1 (2" to 6" of visibility) and 2 (< 2" of visibility). The turbidity and temperature of the adjacent main channel were taken at the same time. Fish were collected from each habitat using a 1.2 m by 4 m seine, with 1.6 mm mesh size. The number of seine hauls varied from one to three with the size of the habitat. In general, ~ habitats wider than 4 m but less than 25 meters long were seined once, those between 25 and 50 m seined twice, and those > 50 m seined three times. In large habitats, the seine hauls were usually made at the width transects. Habitats less than 4 m wide and 25 m long were completely seined in one pass. Length, width and depth of each seine haul was measured, to estimate effort in area and volume seined. ~ Native fish were identified, measured and released in the field. Colorado pikeminnow that died during collection were preserved in ethanol during the first two years of the study, and were transferred to the Larval Fish Laboratory at Colorado State University (CSU) for future analysis of scales and otoliths. Non-native fishes were identified, separated into adult/subadult categories and counted. For each habitat, catch-per-effort (CPE) in #fish/100 m2 and #fish/100 m3• was ~ estimated by dividing the number of YOY pikeminnow collected by the area and volume sampled in all seine hauls. Mean CPE was estimated for each trip by dividing all pikeminnow collected by the total area and volume sampled. Additional information on water temperature was acquired using Ryan tempmentor temperature loggers set to record mid-column temperatures in the main channel and in selected ~ backwaters where pikeminnow had been collected. Temperatures were taken every two hours. Data from these recorders is not continuous due to equipment loss and failures. Habitat types were classified according to a scheme, developed in 1992 with hydrologist Dr. Jack Schmidt (USU) for a similar study on the Green River (Trammell and Chart 1998), that related types of habitats to the hydrological process that formed them (Table 1). Furthermore, ~ the development of each habitat was related to fluctuating flow levels and can be divided into three stages: areas with some flow (i.e. small side channels and shoreline areas), areas with no flowing water and one end open to the main channel (backwaters), and areas with no access to the main channel (isolated pools). Each sampling trip was coordinated with avideo-imaging overflight conducted by the Bureau of Reclamation (BOR). Individual habitats were mapped onto aerial video prints and overlays to evaluate their persistence through time. Habitat persistence was evaluated beginning with post-runoff habitats and ending in spring prior to the runoff. Habitats that were ephemeral within the year (sampled only once) were rated 0, those which were seasonally stable (sampled twice in one year) rated 1, and those annually stable (from summer to spring) rated 2. ~ Descriptive statistics were used to compare species composition and relative abundance between seasons and the two sections. Pearson's correlations were used to test for significant linear relationships between most variables, including peak runoff flows, sampling flows, habitat area and volume, catch rates ofnon-native cyprinids and Colorado pikeminnow, degree accumulations, total length of Colorado pikeminnow in the fall, and overwinter survival. ~ Multinomial analysis was used to test for habitat selection of Colorado pikeminnow and non- native cyprinids for geomorphically classified habitat types. 2