The lower Bear River of Utah and Idaho was once a tributary of the Snake River that flowed along the NE edge of the Great Basin. Similar biological communities in the upper Bear and Snake Rivers attest to this phase of its history. Basalt flows in Gem Valley, Ida-ho, diverted the lower Bear River southward into the Bonneville Basin during the late Pleistocene [1] [2] [3]. However, there is no consensus about the timing, location, and mechanism of this major diversion [4][5]. Landscape and image analysis reveals previously un-mapped active normal faults and characterizes Late Pleistocene to Holocene(?) volcanic fields. Our discovery of a meandering basalt flow SE of Bancroft, Idaho, suggests that fault-guided basaltic volcanism created a “switching valve” for the Bear River where it enters Gem Valley from the east. This tectono-volcanic valve causes northward flow of the Bear River to switch abruptly to southward flow or back again, as basalt flows in the east-central part of Gem Valley flows into the river bed. We envision a mechanism in which flow of the Bear River and lava competed to occupy the same lowest terrain within graben of the East Gem Valley fault zone. Whenever a new basalt flow blocked the river’s northern route, the Bear River was diverted south to fill Lake Thatcher in southern Gem Valley. Eventually lava dams grew high enough for Lake Thatcher to overflow south into the Great Basin. Incision of Oneida Narrows was the final step in the transfer of the Bear River.