ASSESSMENT OF THE POTENTIAL FOR LATERAL SHIFTING OF THE COLORADO RIVER CHANNEL, MOAB VALLEY, UTAH

The potential flood hazard at the Atlas uranium mill and tailings site has been a contentious and unresolved issue for nearly a quarter century. Situated on the Colorado River floodplain in Moab Valley, this hazardous waste site is located on the outside of a large meander bend. Previous workers have contented that the river channel has been migrating south and east away from the site and that this hypothesized migration is decreasing the flood hazard potential at the site. However, analyses of historic evidence of river migration, sediment input from drainage basins north of the river channel, the rate and character of salt dissolution beneath the Moab Valley, and the subsurface distribution of Colorado River gravels clearly show that this contention is not valid. (1) Precise registration of historic maps and aerial photographs documents an 80-year history of progressive northward and westward movement of the south and east bank of the active channel. This bank is now 150 to 300 feet closer to the mill site, and the channel has narrowed and deepened in this new position. (2) Analysis and direct observation of high energy flows from the largest local drainage north of the river channel demonstrate that these flash floods have deposited sediments on the south side of the Colorado River channel, and therefore, have actively contributed to the river?s northward migration in this area. (3) Although salt dissolution beneath Moab Valley is causing subsidence of the valley's alluvial fill, the Colorado River delivers far more sediment to the valley floor than could ever be accommodated by the valley's slow subsidence. Therefore, valley subsidence is not likely to affect the surface morphology of the valley or to significantly influence lateral migration of the river channel. (4) The geometry and position of ancient Colorado River gravels buried beneath the surface of Moab Valley show that the Colorado River has in fact shifted back and forth across mill and tailings site in the recent geologic past. These findings demonstrate that Colorado River channel has not migrated progressively south and east away from the Atlas tailings site during the past 80 years. Moreover, they provide no evidence to suggest that the channel is likely to shift away from the site in the immediate future or that the potential flood hazard at the site is decreasing.