The how, when, and why of an abandoned bedrock meander of the Colorado River, Utah (U.S.)

Abstract

River meanders entrenched into bedrock are found worldwide, and they are famously well represented in the Colorado Plateau of the southwestern U.S. Meandering of bedrock streams can eventually lead to cutting off canyon loops, and these abandoned “rincons” are locations with high preservation of fluvial deposits and landforms. We document and luminescence date the fluvial terraces in and around the Jackson Hole rincon along the Colorado River downstream of Moab, Utah. Results indicate cutoff and abandonment of the rincon at ~200 ka and also record the rapid and unsteady incision in this region over the past 300 ky. A convergence of conditions contributed to the cutoff of the rincon, including alluvial-channel conditions at the onset of MIS 6 glacial-climate, which provided channel-bed cover and enhanced lateral erosion of weak strata. Also, a contemporaneous rock-avalanche partially obstructed the paleochannel just downstream of the breach, potentially creating a backwater that further enabled a flood to avulse across the neck. Although other studies show that bedrock-channel meandering and cutoff can generate unpaired strath terraces and short-term increases in incision rates, these are not evident in the record at the Jackson Hole rincon. This novel case study leverages the high preservation potential within abandoned bedrock meanders to illuminate the processes and controls of rincon formation during landscape evolution.