EFFECTS OF THE 2003 GRAND DITCH BREACH ON LULU CREEK AND THE COLORADO RIVER, ROCKY MOUNTAIN NATIONAL PARK, CO

In May 2003, a breach in Grand Ditch in Rocky Mountain National Park initiated a debris flow that entrained approximately 36,000 m³ of sediment. The debris flow entered Lulu Creek, a tributary of the Colorado River, compromising geomorphic function of the pre-existing single thread, step pool channel, and formed a fan at the confluence with the Colorado River that contained approximately 20% of the total sediment load. As a result of the debris flow, up to 2 m high unstable debris flow berms and banks of sand to boulder-sized sediment formed adjacent to Lulu Creek. Other effects to Lulu Creek include a 10-fold increase in channel width, scour of the channel bed, and complete destruction of the narrow floodplain and riparian zone. Once the debris flow entered the Colorado River, sediment sorting occurred downstream due to subsequent stream flow, with the majority of the sand fraction deposited 2.5 km downstream in the Lulu City wetland. Deposition along the Colorado River occurred as overbank deposits, lateral bars, and mid channel bars, which caused additional bifurcation of the Colorado River. Numerous wood debris dams formed along the Colorado River, buttressing large quantities of sediment on the upstream side. Previous historic debris flows, both natural and anthropogenic, make estimates of aggradation difficult along the Colorado River reach. Six years after the debris flow, flow releases from Grand Ditch into Lulu Creek are irregular and unpredictable, but peak flows of 1.2-1.4 m³/s during snowmelt of 2008 and 2009 mobilized step-forming boulders on Lulu Creek, which now reoccupies a single channel. Erosion of 1 m of sediment from unstable banks was measured in repeat cross section surveys. On the Colorado River, maximum sediment transport of 85 g/s was measured with bed material load dominating the total load. Fining of the bed material D₅₀ is occurring over time. Lesser changes to channel cross sections on the Colorado River indicate most of the sediment transport is derived from upstream sources along Lulu Creek. Increased frequency of debris flows and decreased energy availability due to flow regulation combine to increase the channel recovery time of sediment-laden disturbances down gradient from Grand Ditch.