THE RELATIVE INFLUENCE OF RARE SNOWMELT FLOODS, REGULAR SNOWMELT FLOODS, AND SUMMER THUNDERSTORMS TO FLOODPLAIN DEVELOPMENT ON THE GREEN RIVER IN CANYONLANDS NATIONAL PARK, UT

Inset floodplains continue to form and vertically aggrade along the lower Green River and have narrowed the river by 15% since the mid-1980s. Previous studies of the same river segment asserted that inset floodplain formation had ceased in the early 1950s. This most recent phase of narrowing began during a period of drought that followed the unusually high runoff years of 1983 to 1985. We interpreted the sedimentology and stratigraphy exposed in a 50-m long, 2-m deep trench in the floodplain to determine the timing and processes of floodplain formation. We precisely dated the stratigraphy using dendrogeomorphic techniques to date the germination and burial events of tamarisk (Tamarix spp.), an invasive shrub. Spring snowmelt flood deposits typically have a higher proportion of sand, and summer thunderstorm season flood deposits are predominately composed of mud. There have been 4 phases of narrowing since the late 1800s, reflected by paired sequences of sandy levees formed by snowmelt floods and intervening flat benches primarily composed of vertically accreted muds. Floodplain deposits develop when bank-attached bars are subaerially exposed and stabilized when the bar surfaces are colonized by tamarisk and sandbar willow (Salix exigua). During periods of low peak flows, vegetation establishes on bare sand bars and matures. These deposits are not easily scoured by subsequent floods and accrete on scales of 20 cm/year during moderate floods. The current episode of channel narrowing reflects continuing geomorphic adjustment to changes in streamflow and sediment supply resulting from water development and climate change.