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. . late Miocene sediments suggest a semi-arid climate with insufficient precipitation to support large rivers or lakes. Approximately 10 m.y. ago, the basin was uplifted and a pulse of tectonism occurred, increasing elevation and precipitation. Southeast of Glenwood Springs in Middle Park, Colorado, the oldest deposits indicated the presence of a large through-flowing river between 11.2 +/- 1.8 m.y. and 10.1 +/- 0.5 m.y. ago (Izett 1968). Therefore, it is believed that the Colorado River flowed in approximately its present position from Middle Park to Glenwood Springs about 10 m.y. ago. Channel downcutting was probably a gradual, erratic process for the upper Colorado River. If downcutting began 10 m.y., the river has cut down 1,050 m just east of Glenwood Springs, almost 350 m near State Bridge, and 400 m in the Eagle River gorge near Gypsum. About 700 m of downcutting had occurred on the Roaring Fork tributary by 8 m.y. ago, as indicated by a lava flow in the river valley. Downcutting was rapid from 10 to 8 m.y. ago, slow from 8 to 1.5 m.y. and more rapid again in the last 1.5 m.y. Therefore, the rate of erosion from 10 to 8 m.y. ago is estimated at 0.33 m/l,OOO yr, and 0.22 m/l,OOO yr during the last 1.5 m.y. These data indicate that the upper Colorado River became a through-flowing river only in the last 10 m.y. and supports lucchitta's (1972, 1979, 1988) proposed timetable for integration of the lower Colorado River. The Green River. The history of the Green River Basin drainage is similarly diverse. In Red Canyon, now impounded by Flaming Gorge Dam, the river cut through the resistant, ledge- forming quartzite, conglomerate and shales of the Uinta formation, which dips to the northeast (U.S. Bureau of Reclamation 1968). This profile suggests antecedent channel formation and subsequent uplift. The upper Green River basin may have only recently been captured by the Colorado River drainage because of local and regional Tertiary tectonism in southern Wyoming. This tectonism disoriented and redirected drainage, causing the continental divide to migrate eastward across the basin. A large endorheic basin occurs near the Green River's headwaters in southwestern Wyoming, and is as yet uncaptured by the Colorado, Columbia, or Mississippi systems. Analyses of post-Oligocene fish biogeography, neotectonics and paleoclimatology suggested that the ancestral Green River in Wyoming was a tributary of the North Platte River until 600,000 years ago, at which time it was captured by headward erosion of a southward flowing drainage (Hansen 1985). The capture of the Green River in the middle Pleistocene would have added greatly to the flow of the Colorado, especially as climate changed and deglaciation occurred. . . . . . . . . 8 .