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areas were more limited, often consisting of flooded tributary mouths and canyons washes, and a few large backwaters. Additional areas not sampled that may harbor Colorado pikeminnow included smaller tributaries such as the Price, San Rafael, upper Duchesne, and Little Snake rivers (McAda et al. 1980; Wick et al. 1991; Hawkins et al. 1996; Cavalli 1999; Muth et al. 2000). Adult Colorado pikeminnow in those areas were thought relatively rare compared to larger main stem reaches because habitat size was small or occupied reaches were short and use was only seasonal. The Green River in Lodore Canyon upstream of the Yampa River was also known to support adult Colorado pikeminnow (Bestgen and Crist 2000; Kitcheyan and Montagne 2005). Supplemental sampling conducted there and in other reaches in some years when abundance estimation sampling was being conducted will be discussed. METHODS Sampling and fish handling.--Sampling for abundance estimation was conducted in spring in each year from 2000 to 2003 in the Yampa River, White River, and middle Green River study reaches, and in each year from 2001 to 2003 for the Desolation-Gray Canyon Green River reach and the lower Green River reach. We used Pollock's robust design to allocate sampling effort (Pollock 1982; Pollock et al. 1990). This was accomplished by conducting three or four short-term sampling occasions (usually 7 to 11 days each) through each reach at approximately the same time in spring of each year, and repeating that design over the remainder of the study years. Short-term sampling occasions were conducted in spring between the time when ice off occurred and prior to or during spring runoff before Colorado pikeminnow spawning migrations began. Using hard-bottomed boat (Yampa, middle Green, and lower Green reaches) or inflatable 17