POTENTIAL KNICKPOINT RETREAT ON THE COLORADO RIVER DUE TO HISTORICALLY LOW LAKE MEAD LEVELS, GRAND CANYON NATIONAL PARK

Pool level at Lake Mead has dropped over 40 m in the past decade exposing 2717 ha of reservoir sedimentation in the Colorado River arm of the lake. This reservoir drawdown has led to pioneer riparian forest that later senesced with continuing lowering water levels, and the re-introducing of over 65 km of relatively free flowing stream channel. In the Grand Canyon, river kilometers (RKM) start at Lee’s Ferry and increase going downstream. Land cover change from 1998 to 2016 within water regions of the Colorado arm of Lake Mead, Separation Canyon (386 RKM) to South Cove (478 RKM), show that only 38% remained while the remaining land cover was dominated by riparian vegetation and sediment, dead vegetation, and lake bed exposure. Monitoring of the Colorado arm of Lake Mead is necessary to understand future changes in water storage, vegetation extent, and bank erosion. The newly “free flowing” reach of the Colorado River is experiencing high variability in sand bar extent and lateral locations and its incision through the exposed lake bed leaves 10s of meter tall bluffs of fine grained delta deposits leading to quick bank erosion and aeolian drift. Analysis of lateral channel migration within this reach shows a maximum change of ~70 m between 2002 and 2013. The Colorado River now flows over a highly lithified conglomerate ledge, not its original path, leading to the initiation and formation of a knickpoint, Pearce Ferry Rapid (451.9 RKM). This rapid generates a significant navigation hazard, impacts the migrating fish between the lake and river, and has created a new base level for up to 60 river km upstream. When this knickpoint migrates through the ledge it will lead to rapid bed incision through the remaining lake sediment plug, potentially causing a significant sediment evacuation from the newly flowing Colorado River reach of the western Grand Canyon. There will be strong geomorphic influences on rates of sediment remobilization and the evolution of the rapid on the river profile. This study will better grasp the mechanics of the rapid using high-resolution bathymetry results upstream of Pearce Ferry to allow for modeling the rapid changes that will occur within this highly erodible reach. Modeling Pearce Ferry rapid will bring clarity on upstream sedimentation and channel migration rates to help mitigate future problems.