Evaluating Watershed Response and Increases In Sediment Loading To Willow Creek and Willow Creek Reservoir Due To East Troublesome Fire

Once a dam is constructed on a waterway, it begins to trap and store sediment to some degree. All Reclamation reservoirs are currently filling with sediment at various rates and will eventually reach the end of their useful life. Most Reclamation reservoirs were planned and constructed with a 100-year design life, meaning at the end of 100 years of operation, sediment will have reached the level of the lowest dam outlet. Costs of dam decommissioning are high and new reservoir sites are becoming more difficult to find as most of the best locations have already been used. The occurrence of a wildfire within the watershed has the potential to significantly increase the rate of sedimentation, effectively reducing its useful life and negatively impacting Reclamation’s water management and water delivery mission. With climate change, the risk of wildfires and extreme hydrological events is increasing. As hydrology becomes less predictable, water resources become less reliable. Reclamation needs to plan for increases in sediment loads due to wildfire in their design and operations.

Willow Creek Dam and Reservoir are located on Willow Creek in Grand County in north-central Colorado approximately 2.5 miles upstream of the confluence of Willow Creek with the Colorado River and 5 miles north of the town of Granby, CO. Operated by the Northern Water as part of the Colorado Big Thompson Project, Willow Creek Dam captures about 33,700 acre-feet of excess Willow Creek flows annually for diversion to Lake Granby for storage.

In October and November 2020, the East Troublesome Fire burned 193,812 acres surrounding Willow Creek Reservoir including more than 91 percent of the watershed that feeds the reservoir. The second largest fire in Colorado history, it grew more than 150,000 acres in a single 24-hour period driven by near-hurricane-force winds as it burned the majority of the 55-percent-forested watershed.

Monitoring sediment deposition in the reservoir over multiple years is underway to determine and possibly predict the watershed and reservoir response to large-scale wildfires. Repeat hydrographic surveys (including above and below water data collection) and sediment sampling following spring runoff, are being used to measure the volume and distribution of sediment deposits from the burn area and assess the effectiveness of sediment management activities undertaken prior to runoff. Potential impacts to the Colorado-Big Thompson Project and operations at Willow Creek Reservoir will be identified through this research.

Reclamation’s Sedimentation and River Hydraulics (Sedimentation) Group is conducting repeat hydrographic surveys of Willow Creek Reservoir following the 2021, 2022, 2023, and 2024 spring runoff. The Sedimentation Group is collecting the bathymetry using multibeam sonar integrated with RTK GPS while Northern Water contractor River Science repeats November 2020 and fall 2021 and 2022 UAS surveys to capture the above water changes, anticipated to be primarily in the upper delta. The Sedimentation Group then processes the bathymetry data and combines it with the UAS data to generate a continuous surface of the reservoir bottom up to the dam crest elevation. Surface and core samples are also being collected in the reservoir delta to determine the thickness, gradation, and source of sediments delivered to the reservoir.