Instream wood loads and log jam spacing on the Upper Colorado River, Rocky Mountain National Park

Van, Breanna; Merrill, Jay

In-channel wood in log jams is a key feature of healthy mountain river systems. Geomorphically-effective log jams promote sediment and organic material deposition, water ponding in pools, groundwater and surface water exchange, channel avulsions, and channel floodplain connectivity. The slower flow that these jams induce allows nutrients to accumulate and provides essential habitat for aquatic biota including fish and other wildlife (Wohl, 2000). Rocky Mountain National Park (RMNP) hosts the headwaters of the Upper Colorado River, a major water resource for numerous western US states. As such, the condition of the Colorado River is of primary interest, especially with regard to water quality and riparian and aquatic habitat condition. The in-channel wood along the Colorado River is especially important for promoting a healthy fluvial ecosystem. In May 2003, the Grand Ditch in RMNP breached, causing a large debris flow that altered channel morphology within the Upper Colorado River and its tributaries, and effectively wiped out 20,000 trees (Rathburn et al., 2013). This event deposited 36,000 m3 of sediment in a debris fan along a tributary channel called Lulu Creek (Rathburn et al., 2013). This breach, along with the pine beetle kill of 2006-2008, altered the abundance and characteristics of in-stream wood and channel morphology of the surrounding area. This analysis assessed the post-debris flow in-channel wood in log jams and the effects of the beetle kill to understand the role of debris flows in altering log jam characteristics. In addition, the data may provide information for future channel restoration efforts. Our hypothesis is that the 2003 debris flow altered wood volume and wood distribution in log jams within the Colorado and Lulu Creek tributary. We hypothesize that our study reaches have; greater longitudinal spacing between jams, greater wood recruitment levels, and lower overall wood volumes contributing to geomorphically-effective jams as compared to instream wood within reference sites.