United States Department of Agriculture

Umpqua National Forest Burned Area Emergency Response Summary – Thielsen Fire October 22, 2020

The Thielsen Fire was detected on September 8, 2020 at approximately 8:30 a.m. after lightning passed through the area. The fire was on the Diamond Ranger District of the Umpqua National Forest, approximately 5 miles north of Crater Lake National Park. It burned within one mile of the Diamond Lake Resort, along both sides of Oregon Highway 138, and a small portion of the Mount Thielsen Wilderness. The fire (as of 10/6/2020) encompassed 9,951 acres primarily in a mixed conifer forest. The fire burned a mosaic pattern through most of the area, and the majority burned with low and moderate severity.

The Forest Service assembled a Burned Area Emergency Response (BAER) team on September 26, 2020. This team of experts in soils, geology, hydrology, engineering, botany, recreation, archaeology, wildlife, fisheries, and GIS, began assessing the post-fire effects to critical values on Forest Service lands. BAER critical values include Life and Safety, Natural Resource, Cultural Resources and Property. The team developed a Soil Burn Severity (SBS) map to document the degree to which soil properties had changed within the burned area. Fire damaged soils have low strength, high root mortality, and increased rates of water runoff and erosion. Using the SBS map, BAER team members ran models to estimate changes in soil erosion, stream flows (hydrology) and debris flow (geology) potential. The models compared pre-fire conditions to predicted post- fire conditions to determine relative changes, which are then used to determine the relative risk to different critical values and recommendations to address those things determined to be an emergency. Below is a summary of the findings of each resource area.

SOILS An estimated 48% of the burned area within the Thielsen Fire had high or moderate soil burn severity and may have developed water repellent soils as a result. However, of that 48%, only 5% was mapped as high severity. Overall, the BAER team does not expect large scale erosion response on the Thielsen Fire burn area due to: 1) minimum amount of high soil burn severity; 2) field observations of soils response after a few local rain events; 3) mostly moderate soil erosion hazard ratings; and 4) gentle terrain in much of the fire. Based on these findings, the probability of damage/loss was unlikely. Due to

Forest Service Burned Area Emergency Response United States Department of Agriculture those considerations, the BAER team does not recommend any soils treatments.

GEOLOGY Fire increases the potential for debris flows, partly due to the removal of vegetation. The biggest hazards associated with the Thielsen Fire are debris flow and rockfall. A major highway (Highway 138 Scenic Byway) runs through the burned area, exposing it to various hazards. Debris flows could impact the highway via inundation or by blocking culverts that cross beneath the road. Other Forest Service roads (NF 4792, NF 4793, NF 100, NF 005) may be at risk due to localized rockfall events and debris flow. Some landslides may occur along stream segments in the Lake Creek and Thielsen Creek watersheds.

HYDROLOGY Hydrologic effects anticipated from the Thielsen fire include moderately increased runoff and peak flows that may result in increased sediment movement near or in streams. Increased annual peak flows and runoff velocities may cause stream bed erosion, streambank instability, and instances of channel migration. High in the watershed, minor scour and incision in low order channels is expected. In contrast, further downstream the primary effect is the deposition of soil and debris from higher up the slope. Under peak flow conditions, fine-grain volcanic sediment will likely settle out in either deeper pools or when they reach Lemolo Reservoir. These responses are expected to be most noticeable early on and during large storm events but will become less evident as vegetation is reestablished and soils stabilize. Localized and short-term stream temperatures are also likely to increase following this wildfire, primarily from removal of the overhead vegetation. The estimated vegetation recovery time is expected to be approximately 2-5 years. Some high severity spots with poorer site conditions may take longer.

ENGINEERING The Thielsen Fire includes 14.62 miles of National Forest System Roads and 24.6 miles of National Forest System trails. Post-fire conditions, in combination with the expected watershed response, indicate there will be increased risk of road and trail failure due to rock fall and debris flow. It is expected that debris flows originating from the Thielsen Creek watershed could cause material to flow onto roadways and clog/block culverts beneath roadways, which could cause localized flooding. As ditches, culvert inlets, and roadway dips become compromised, they become vulnerable to failure and may result in loss of the property and/or structure. Highway 138 is a heavily used Scenic Byway, but it is not managed by the Forest Service.

Forest Service Burned Area Emergency Response United States Department of Agriculture

The largest risk to trails is the numerous hazard trees along the trails, loss of tread, and debris flows. It is expected that the threat of hazard trees will continue throughout the into the Spring of 2021. The probability of damage along these roads and trails is likely and the magnitude of consequence of property loss is major. BAER treatments include hazard tree removal to support culvert and ditch cleaning, road hazard sign installations, trail warning signs, and storm inspection and response.

BOTANY Aggressive invasive plants are present within the burned area, primarily on travel routes. These infestations are within or adjacent to burned areas, and several survey and manage fungi species important for old growth forest structure. Invasive plants are highly adapted to take advantage of conditions created after fire and can out compete native plants for resources. The primary corridor (Hwy 138) used for accessing the fire contained populations of diffuse and spotted knapweed. There is risk of loss of the wilderness character for this natural area. Spread of invasive plants into un-infested areas due to conditions created by the fire pose direct competition for resources including water, nutrients, and above and underground growing space. BAER treatments include the survey, treatment, and monitoring of invasive species infestation in 145 acres of at-risk land.

CULTURAL RESOURCES Three resources were identified as values at risk within the Thielsen Fire. In the first, a culturally modified tree site likely associated with Native American use, additional features were present outside of the previously documented boundary. This site was severely burned, and damage occurred to all the resources observed. They suffered irreversible loss of archaeological data because of the fire. One of the historic sites, a Civilian Conservation Corps wooden stave culvert, was unaffected by the fire as it was in an area not burned. The remaining historic site, a can and bottle refuse associated with dispersed camping and a Humboldt bridge stream crossing, was observed in a mosaic of moderate to high burn severity area but was largely unaffected by the fire.

Forest Service Burned Area Emergency Response United States Department of Agriculture

Upon evaluating all the cultural resource values, the probability of damage or loss was found to be likely due to the burn severity observed and the loss of data from the resources as described above. No treatment recommendations were made to the three resources, either due to limited effects from fire or due to the unlikelihood of success for any reasonable treatment. It is recommended that the current observable extent of the multicomponent site be documented, and monitoring occur in the spring of 2021.

CONCLUSION The BAER team has identified imminent threats to values at risk based on a rapid scientific and engineering assessment of the area burned by the Thielsen Fire. Despite taking significant precautions to minimize exposure to COVID-19, the assessment was conducted using the best available methods to analyze the potential for flooding and debris flows. The findings provide the information needed to prepare and protect against post-fire threats. The Forest Service will continue to provide information and participate in inter- agency efforts to address threats to public and private values at risk resulting from the Thielsen Fire.

Forest Service Burned Area Emergency Response