TWENTIETH CENTURY CHANNEL NARROWING OF THE GREEN RIVER IN CANYONLANDS NATIONAL PARK, UTAH

Channel narrowing of the lower Green River since the late 1800s is well documented; however, the rate, timing, and magnitude, of channel change are not well understood. We analyzed aerial imagery, sediment exposed in a 50-m long, 2-m deep trench excavated in the floodplain, hydrology, and bathymetry to clarify the historical pattern of channel change and to understand whether or not the lower Green River continues to narrow today. Our results show that the contemporary, inset floodplain began forming in the late 1930s and continued to form and vertically accrete in the second half of the 20th century. Analysis of aerial imagery shows that the timing of channel change described in the trench using sedimentology analysis matches aerial imagery analysis covering the 61-km study area. The most recent phase of floodplain formation began during a period of drought that followed large runoff years between 1983 and 1985. Since that time, active channel width has decreased by 9%. Inset floodplain formation has been the primary mechanism of narrowing. Alternate bank-attached bars are colonized by woody riparian plants during multi-year periods when the annual spring snowmelt flood is small. These bars subsequently vertically aggrade, initially by a wide range of flows including rise and recession of the snowmelt flood. Eventually, only the peak of the snowmelt flood contributes to vertical aggradation, because no other flows can access this surface. Although typically of much higher suspended fine sediment concentration, late summer and fall floods of the monsoon season play a minor role in floodplain aggradation because these flows rarely inundate the aggrading floodplain. The most recent episode of channel narrowing may have been accelerated by changes to flood magnitude and timing resulting from water development and climate change.