Geologic Constraints on Snake-Columbia River Drainage Changes and the Associated Evolutionary Diversification of Cutthroat Trout over the Past 17 Million Years
Paul K. Link and Ernest R. Keeley
Abstract.—In the past 17 million years (myr), the topography and drainage systems of the northwestern United States were drastically modified by the Yellowstone–Snake River Plain (YSRP) hotspot and associated east–west extension of the Basin and Range Province. These geologic changes influenced distribution and diversification of Cutthroat Trout Oncorhynchus clarkii and allowed connections between Snake River, Colorado River, and Great Basin fish populations beginning in the late Miocene. Studies of detrital zircon grains in Miocene to Holocene fluvial sands of the Snake River document the eastward migration of the regional drainage divide from central Idaho to northwestern Wyoming. This migration was concomitant with the southwest migration of the North American tectonic plate over the YSRP hotspot. In the late Miocene and Pliocene, since 10 million years before present (Ma), the Chalk Hills and Glenns Ferry lake systems formed on the western Snake River Plain and were hosts to diverse fish fauna. The modern Snake River formed after 3 Ma with the cutting of Hells Canyon and integration of the Snake and Columbia River drainage. In the Great Basin south of the Snake River watershed, Lake Lahontan has a history that goes back to the Miocene. Connections between the western Snake River Plain and the Great Basin were recurrent over the past 10 myr. In southeastern Idaho, the Bear River has had a complex drainage interaction with the Snake River and Bonneville watersheds. Lake Bonneville, in northern Utah, grew during Pleistocene glacial climate regimes. The modern Bear River connection to Lake Bonneville was initiated about 50,000 years before the present. The integration of the Green River with the Colorado River occurred in the late Miocene, developing after breaking of Eocene connections between the Green River and streams draining to the Atlantic Ocean. In sum, geological constraints are compatible with patterns of fish fossils and genetic linkages and identify mechanisms of colonization and isolation of fish populations that have resulted in regional diversification of Cutthroat Trout.