STREAM POWER AND GEOMORPHIC CHANGE DURING THE 2013 COLORADO FRONT RANGE FLOOD

The September 2013 flooding in and along the Colorado Front Range induced a range of geomorphic adjustments in many streams. These included localized streambank erosion, hillslope and terrace failures, reach-scale channel widening, sediment deposition, rapid downstream meander migration, and channel avulsions and braiding. This event provided an opportunity to assess geomorphic change from an extreme flood event in a semi-arid landscape and relate a categorical range in adjustment to peak flow unit stream power and valley confinement. Based upon observed impacts, six categories of geomorphic change were defined and 531 stream reaches over 226 km were classified for a wide variety of channel scales, slopes, and watershed areas. Generally, erosion of the channel margins drove geomorphic change in confined reaches, whereas a combination of erosion and deposition resulted in channel change in unconfined reaches. Unit stream power increased with the magnitude of geomorphic change. Additionally, for a given value of unit stream power, greater geomorphic response was observed in unconfined channels.

Based on a subset of the dataset, thresholds for geomorphic adjustment were defined using the 10^th and 90^th percentiles for the various adjustment categories. For channel slopes <3% there was a credible potential for substantial channel widening with unit stream power >250 W/m², a credible potential for avulsions, braiding, and loss of adjacent road embankments with power >500 W/m², and major geomorphic change was likely with power >700 W/m². Importantly, in channels where there was a large reduction in unit stream power as the valley form shifted from confined to relatively unconfined, large amounts of deposition-induced reach-scale geomorphic adjustment occurred at relatively-low unit stream power, substantially below these estimated thresholds. Additionally, alluvial channels with slopes >3% had greater resistance to geomorphic change, likely due to increased flow resistance induced from enhanced bedform flow dynamics. These results may likely be valuable for estimating expected adjustments of stream corridors within and along the Colorado Front Range during future floods, and may also be applicable in other semi-arid landscapes.