Drainage Development of the Green River Basin In Southwestern Wyoming and its Bearing on Fish Biogeography, Neotectonics, and Paleoclimates

The Upper Green River flows southward out of the Green River Basin through a series of deep canyons across the Uinta Mountains in a course that post-dates the deposition of the Bishop Conglomerate (Oligocene); the ancestral Green flowed east to the Mississippi, probably by way of the North Platte. Basinal subsidence in Eocene time had produced 3 large, well-known, interconnected lakes on both sides of the Uinta Mountains--Fossil Lake and Lake Gosiute on the north side and Lake Uinta on the south. Lake-to-lake connections are indicated by a remarkably well preserved fauna common to all three lakes, the famous Green River fish fauna. Most of these fish had Mississippi River affinities, hence, the Continental Divide in Eocene time was west of the lakes. The Colorado River system did not yet exist. After the lakes disappeared, drainage was generally eastward across the present Continental Divide, until the Green River was captured near Green River, Wyo. by south-flowing drainage in middle Pleistocene time, ca., 600,000 years ago. Capture of the Upper Green River as recently as middle Pleistocene time, if a valid hypothesis, must take into account the marked differences between the endemic and indigenous fish fauna of the Green River and that of the North Platte. The Green is postulated to have been captured at the height of a glacial stage when the Green River Basin was a frigid semidesert and its fish population had been forced far downstream by the hostility of the glacial climate. Indigenous nonendemic cold-water fauna of the Green River/Upper Colorado River Basin - salmonids, cottids, and others - are geologically recent arrivals from the Columbia River Basin; they probably transferred to the Green during an interglacial stage, then spread throughout the Upper Colorado River system during the colder water phase of a succeeding glacial stage. Several possible transfer points exist along the Green/Bear and Green/Snake River divides.