The Worst Fire Season in Colorado History: Its Impact on Dams

Last year Colorado experienced unprecedented wildfires in terms of size and intensity. On July 31, 2020, the Pine Gulch Wildfire ignited and became the largest fire in Colorado history. Unbelievably, this was surpassed in October by both the East Troublesome and Cameron Peak fires, burning through 193,800 and 208,900 acres, respectively. These wildfires engulfed basins containing dams. Twenty-five jurisdictional dams were impacted by the wildfires, thirteen of them classified as high hazard. Wildfires alter vegetation, soil properties, and runoff dynamics. One of the most concerning impacts to dams is a significant increase in the potential for overtopping failure from flashy runoff, a result of denuded basins, altered soil properties, and reservoirs experiencing extreme sediment loading. During and after wildfires numerous groups are gathering and producing data. Collecting, understanding, and using the data, and avoiding duplication of efforts, required continuous coordination and outreach. Assessing wildfire impacts to dams required an understanding of the individual structures and drainage basins. Inspection efforts involved coordinating with entities we, the Colorado Dam Safety Branch, typically do not interact with on a routine basis: the United States Forest Service (USFS), Health Departments, and local nonprofit watershed protection groups, among others. Analyzing the data for how it impacted dams included learning about Burn Area Emergency Response (BAER) reports and how burn severity and runoff are estimated. We developed GIS maps to calculate percentages of drainage basins burned and burn severity within those areas. Some basins were 100% burned, with up to 40% categorized as High Burn Severity. This information was then used to develop mitigation activities dam owners could enact to reduce risk. These activities varied based on equipment availability, physical access, money, and time. One tool we recommended was coordinating with the National Weather Service to understand threshold warnings for rainfall in the burned areas. These thresholds can be incorporated into Emergency Action Plans, along with specific actions to undertake when the thresholds are met. While the dams themselves exhibited resilience following the fire, the real test will come during spring runoff and summer thunderstorms. The impacts will likely be felt for years. We hope by sharing our experiences during and immediately after the wildfires may help others be prepared and know where to seek information. The unfortunate reality is that dams will continue to be impacted by wildfires well into the future.