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4 features -- a high degree of geologic control and a high degree of endemism- suggests that the two may be linked. This paper examines one element of that linkage by focusing on the way geology controls the distribution of age-0 Colorado squawfish Ptychocheilus lucius in the Green River. STUDY AREA AND GEOMORPHIC OVERVIEW OF GREEN RIVER This paper focuses on the Green River in Colorado and Utah, a study reach of approximately 650 km between Flaming Gorge Dam and the confluence with the Colorado River (Fig. 1). Flaming Gorge Dam alters the hydrology and sediment transport of the river (Andrews, 1986). Upstream from the Yampa River, mean annual discharge of the Green River is approximately the same as that of the Yampa, and the annual flow has not been affected by dam operations (Table 1). The magnitude of the Green River's 2-yr recurrence flood has been reduced by 60 percent upstream from the Yampa River, however, and the magnitude has been reduced by about 25 percent downstream from the Yampa. The Green River encounters geologic formations of differing erosional resistance because the river crosses regional geologic structures perpendicularly. Formations vary between hard, cliff-forming rocks such as the metasediments of the Uinta Mountain Group exposed in the Canyon of Lodore and erodible, slope-forming rocks such as the Mancos Shale exposed near the town of Green River (Table 2). Alluvial valley width and bankfull channel width alternate between narrow and wide segments which are controlled by the stratigraphic arrangement of resistant and erodible bedrock formations that in turn result from the region's geologic history. The alluvial valley of the Green River is typically more than 2 km wide where the river crosses erodible formations, but the valley may only be 200 m wide where resistant formations are crossed (Fig. 2). Channel width is greatest where the alluvial valley is wide, but the