RECENT GEOMORPHIC CHANGES IN RESTORED OR RECONFIGURED COLORADO, USA, RIVERS

Channel reconfiguration, or restoration, to mitigate a variety of riverine problems has become a widespread practice in the Western United States. Numerous stream and river channels in Colorado have been reconfigured; however, geomorphic response to, and the effectiveness of, these modifications over a period of time have not been assessed in a consistent manner. Long-term monitoring and geomorphic analysis of reconfigured channels can reveal how and why a particular reconfiguration design may have remained functional or failed due to annual streamflow variability. The USGS Reconfigured-Channel Monitoring and Assessment Program includes multiple surveys, photographs, and sediment data for monitored reconfigured reaches on several western Colorado Rivers. Data and photographs from these sites periodically are updated on a website: http://co.water.usgs.gov/projects/rcmap/rcmap.html.

Greater than bankfull discharge from mountain snowmelt in 2005 provided an opportunity to observe geomorphic responses at several of the monitored reconfigured reaches in Colorado. The Uncompahgre River had a steep, braided, gravel channel and was reconfigured to a single channel with boulder and log stability structures (cross vanes, J-hooks, log groins, rip-rap). The 2005 runoff peak discharge had a recurrence interval of approximately 4 years and caused bank erosion, channel realignment, and damage to log structures. The North Fork of the Gunnison River had a steep, cobble bed with mid-channel bars and was reconfigured to a single channel using very few boulder stability structures. The 2005 runoff peak discharge had a recurrence interval of approximately 8 years and caused bank erosion, some channel realignment, and sedimentation at boulder structures. The Lake Fork had a steep cobble and boulder bed and the formerly shallow channel was narrowed and deepened using numerous boulder structures. The 2005 runoff peak discharge had a recurrence interval of approximately 2.4 years and had little observed geomorphic effect. Boundary shear stress from the 2005 discharge peaks and the critical shear stress for sediment at various locations on the channel boundary were explored as reasons for observed geomorphic changes, channel instability, and sediment erosion and deposition.