VARIABILITY OF EDDY SANDBAR RESPONSE DURING TWO DECADES OF CONTROLLED FLOODING ALONG THE COLORADO RIVER IN GRAND CANYON

Sandbars are an iconic feature of the Colorado River in Grand Canyon National Park. Following the completion of Glen Canyon Dam in 1963, sediment deficit conditions developed downstream from the dam and eddy sandbars were eroded from much of the river corridor. In an effort to rebuild sandbars, controlled floods in 1996, 2004, and 2008 demonstrated that sand on the channel bed could be redistributed to higher elevations and that floods timed to follow tributary sediment inputs would increase sediment deposition in eddies. Since 2012, a new management protocol has resulted in 3 controlled floods following large inputs of sand from the Paria River. Flood response of 45 eddy sandbars shows that each controlled flood deposited significant amounts of sand and increased bar size. However, the magnitude of sandbar deposition varied from eddy to eddy, even over short distances where main-stem suspended sediment concentrations were similar. Here, we characterize temporal trends in sandbar size and sediment storage as a function of flow, channel, and vegetation characteristics that reflect the reach-scale hydraulic environment. We grouped the long-term monitoring sites using a combination of principal component analysis and regression models to predict differences in site-response variables from morpho-metric parameters measureable using remote sensing. Approximately half the sites are typical fan-eddy complexes where reattachment bars form in pools downstream from a debris fan. Within this group, narrow eddies are less-vegetated, stage changes rapidly with discharge, and sandbars remain dynamic. In wider settings, where stage-change during floods may be half that of narrow sites, reattachment bars have become stabilized by vegetation and vertically accreted, effectively transforming to floodplain. Other sandbar groups also reflect reach-scale hydraulic conditions, but form in different geomorphic settings. Bar-building during floods decreases through time where bars have stabilized and aggraded, but these sites also represent a significant proportion of fine sediment storage and riparian habitat. Ideally, a geomorphic-grouping framework can be used to assess whether the long-term monitoring sites are representative and to anticipate sandbar change along the 400 km river corridor in Grand Canyon.