GEOMORPHIC IMPACTS OF THE NORTHERN COLORADO FLOOD OF 2013 ON BLACK CANYON CREEK AND NORTH FORK BIG THOMPSON

In September 2013, the Colorado Front Range experienced between 8 and 18 inches of rainfall during a weeklong rain event. This led to catastrophic flooding that triggered over 1,100 landslides and debris flows. Subsequent denudation of the surrounding hillslopes has led to accelerated sediment transport, bank erosion, and downstream sedimentation. The purpose of this study is to assess the degree, type, and spatial variability of geomorphic changes in mountain streams and their adjacent hillslopes in response to the 2013 flood. This research provides the foundation for a broader study to monitor these systems and to compare geomorphic and hydrologic recovery patterns between naturally evolving and human-restored streams. We studied two watersheds in Rocky Mountain National Park and the Arapaho-Roosevelt National Forest that are recovering naturally from the flood impacts: North Fork Big Thompson River and Black Canyon Creek. The North Fork Big Thompson River experienced a flow between the 50- and 100-year recurrence intervals, with an average peak flow of 1,700 cfs; this is estimated to be 11 times greater than bankfull discharge. Black Canyon Creek was inundated by multiple debris flows; the minimum peak discharge was 3,050 cfs, which is estimated to be 34 times bankfull discharge.

At both the reach and watershed scales, we identified sources of sediment by comparing flood-only affected regions to those also impacted by debris flows. Additionally, we derived basin and valley characteristics such as aspect, elevation, drainage area, and valley constraints from LiDAR to characterize the two watersheds. Characteristics of sediment deposits, streamside erosion, and large wood accumulations were assessed during field surveys. Preliminary results suggest that the degree of channel impact and sedimentation is heavily influenced by the presence and magnitude of debris flows.

Flooding disasters threaten the safety and well-being of at-risk communities. Thus, improved understanding of landscape response to increased precipitation and runoff is essential for effective construction and remediation of infrastructure damage.