The upper Cenozoic deposits in Fisher Valley, Utah, record a long history of deposition, deformation, and geomorphic changes related to movement of the Onion Creek salt diapir. Apparently, several pulses of salt flowed into the diapir between about 2–3 and 0.25 m.y. ago, and the diapir may still be active. Fisher Creek once headed in the igneous rocks of the La Sal Mountains and flowed along the present course of Onion Creek to the Colorado River. The rising salt diapir impeded the flow of ancestral Fisher Creek, causing deposition of more than 125 m of basin-fill sediments, and eventually diverted the creek down Cottonwood graben to the Dolores River about 0.25 m.y. ago. Onion Creek has eroded headward from the Colorado River, through both the diapir and the basin-fill sediments, and is about to capture Fisher Creek, restoring the original drainage course.