ANCESTRAL COLORADO RIVER POTHOLES HIGH ABOVE HOOVER DAM

Two perched exposures of ancestral Colorado River gravel made of far-traveled clasts fill a river-carved bedrock channel and associated potholes that lie stranded 275 m higher than the Colorado River bed near Hoover Dam. One exposure of the perched riverlaid gravel and interbedded sand is > 9 m thick, and fills a bedrock paleochannel that flanks Sugarloaf Hill on an opposing side from that of the modern river canyon. Upstream 1 km, at a similar altitude, potholes and river-polished and fluted bedrock surfaces exposed at the rim of the modern inner canyon define the walls and flanks of a steep-walled paleochannel filled by fluvial gravel. The potholes were reported by Ransome in 1923. The river-sculpted bedrock surfaces retain original polish where they are most recently exhumed, and are more degraded at increasing height above the present upper limit of gravel, on which a carbonate-rich soil (stage III-IV) has developed. The bedrock paleochannel is more than 28 m deep, much of it no more than 25 m wide, incised a few meters below a low-relief erosional surface. Its slot geometry resembles deep inner gravel-filled channels found below the historic river bed by drilling in nearby Boulder Canyon and by drilling and excavation at Hoover Dam. Such gravel fills may be transitory and capable of occasional movement and bedrock abrasion during high discharge events, based on the sawn timber found deeply buried within the modern channel fill during the excavation to build Hoover Dam.

The perched ancient paleochannel deposits record an intermediate stage of the river’s incision, as they lie about 200 m below the pre-river topographic surface recorded by interior-basin fill including the Fortification Hill basalt flow (5-6 Ma). Since pothole formation, the river has incised hundreds of meters deeper. We are attempting to date the fluvial sediment in the paleochannel(s) in order to help evaluate history and rates of river incision in relation to such forcings as tectonics, climate, and sediment supply.