EVALUATING CHANNEL MORPHOLOGIC CHANGES AND QUANTIFYING BED-MATERIAL TRANSPORT USING AIRBORNE LIDAR, UPPER COLORADO RIVER, ROCKY MOUNTAIN NATIONAL PARK, CO

Extreme influxes of sediment into a river, such as those caused by debris flows, typically have drastic effects on the morphology of the fluvial system. A 2003 debris flow in Rocky Mountain National Park, CO, obliterated bed- and planform morphology along the Colorado River and nearby tributary Lulu Creek, deposited a coarse unstable fan at their confluence, and caused extensive aggradation in a sensitive wetland. A decade later, two consecutive high-runoff years resulted in extensive reworking of the debris flow sediment along the river network. Channel morphology varies greatly between the Colorado River and Lulu Creek, with channel responses to high flows controlled by differences in water and sediment inputs and reach slopes. Unusually low step height-to-length ratios promote maximum sediment transport down Lulu Creek, but measurements at some sites suggest that morphology is adjusting to a more stable state. Along the Colorado River, variations in aggradation and degradation locations exist as the channel adjusts to a more single-thread geometry, yet aggradation appears more dominant in wide valley reaches. Differences between estimated and measured bed-material transport rates along the Colorado River, likely related to the overwhelming sediment supply and unsteady flow from upstream, has required the use of the ‘inverse’ method to quantify the bed-material transport through the site. Using repeat high-resolution topographic datasets (i.e. LiDAR) and observing changes in river channel morphology between 2004 and 2012, it is possible to quantify the flux of sediment required to produce these geomorphic changes. As the project moves toward restoration design, a more robust sediment budget will ensure designs adequately transport the delivered sediment load through the Upper Colorado River.