COMPLEX RESPONSE OF CHANNELS IN FORESTED WATERSHEDS TO FLOODS: ASSESSING THE GEOMORPHIC IMPACTS OF THE 2013 COLORADO FLOOD ON NATIONAL FOREST AND PARK LANDS

In September 2013, the Colorado Front Range underwent catastrophic flooding during a week-long rain storm when between 200 and 450 mm of precipitation fell over the mountain front and neighboring plains. The flood caused considerable damage to property and infrastructure over a 3000 km² area, including substantial portions of the Arapaho-Roosevelt National Forest and Rocky Mountain National Park. This research assesses the degree, type, spatial variability of geomorphic changes in these largely forested watersheds using remote imagery, including National Agriculture Imagery Program (NAIP) images, available LiDAR, and historical aerial photos. Additionally, areas of channel erosion and large flood deposits were field-mapped to verify features identified on remote imagery and to clarify sources of sediment, remnants of large wood jams, and the extent of flood-related features.

In this presentation, we focus on streams on federal lands near the town of Glen Haven, Colorado (primarily tributaries to the North Fork Big Thompson). Prior to flooding, channels were single thread and narrow (2-6 m), with a largely forested riparian area. During the flood, streams widened, entrenched, aggraded, and avulsed in response to high discharges, increased sedimentation, and large wood loads. In one debris flow impacted watershed, channels downcut an estimated 5-7 m, with debris flow levees and massive large wood accumulations on floodplains further downstream. In other watersheds, streamside failures were common in confined valleys, and eroded colluvial fill, terraces, and alluvial fans became significant sources of sediment. Both living and downed trees were stripped from riparian areas and transported downstream, forming large, instream wood jams that caused temporary channel blockage. Channel avulsions (complete abandonment and channel relocation) occurred in wood obstructed reaches, often following a hypothetical “plug-fill-avulse” sequence: 1) the channel was plugged at pinch points by transported large wood, 2) ponding and coarse sediment infilled behind the plug, and 3) a new channel carved around the obstruction, leaving the sediment plug in the pre-flood channel. Multiple avulsions, along with the release of flow from behind failed jams, likely contributed to irregularity in flood flows downstream.