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The Sourdough Fire is burning on the western slope of the North Cascades mountains, which is known for high levels of annual rainfall, dense evergreen forests, high biodiversity, and rugged backcountry. Visitors will see old-growth trees like western hemlock and red cedar, which create a multi-layered canopy that intercepts water during heavy rain events. The tiny, needle-like leaves of these lush trees create thick layers of organic material, or duff, that cover the soil below them as they drop from their branches. Other trees, like bigleaf maple, grow deep and expansive root systems that stabilize the soil by physically holding it in place.

When a wildfire burns through areas of dense vegetation like these, it removes stabilizing elements in the landscape and the effects of a storm become much different than what was present before. More water hits the ground at once, too fast for the soil to absorb it, which can cause flooding. This is exacerbated by a reduction of the duff layer, which act as a sponge to hold some of the rainwater. With higher water flow and fewer root systems to stabilize the soil, mudslides and debris flow become more likely as rain dislodges soil. Flooding, mudslides, and debris flows pose a threat to visitors, residents, and employees in the Sourdough Fire area. Flooding can damage structures and carry away vehicles, but soil and debris exacerbate the risk. As soil and debris become mobilized, they carry more weight and strength than water alone. Debris like large logs, boulders, and fencing can be carried by water along a drainage. The Stetattle Creek drainage, where the Sourdough Fire is burning, has a high concentration of large boulders that are over two feet wide. These heavy elements can strike people during a flood event, trap vehicles, and destroy structures in their path.

The Sourdough Burned Area Emergency Response (BAER) team was deployed to evaluate and address threats to human life and safety, many of which stem from flooding risks. Hydrologists started the assignment by collecting data from satellite images, called Burned Area Reflectance Classification (BARC). BARC provides data about how severe the soil burns in a wildfire and separates them into four classes: high, moderate, low, and unburned. Hydrologists used BARC imagery to create a first draft of a soil burn severity map, which they then improved using field data. 

Hydrologists hiked into the burn area to ground truth the estimates provided by satellite imagery. They took measurements to understand how deeply the soil was charred, the condition of the plant root systems, the impact to the duff layer above the soil, and the pre- and post-fire behavior of the soil. Hydrologists conducted water repellency tests, which measure the rate at which water penetrates and is absorbed into the soil, in unburnt and burnt areas. Soils were water-repellant in both types of areas, causing hydrologists to speculate that the water repellency was likely due to recent dry weather and not the Sourdough Fire. 

Back at the office, hydrologists plugged the field data into models. They modeled future runoff and debris flows by simulating different levels of storms and predicting changes in peak flows, runoff volume, erosion, and sediment movement. This data enables land managers in the Sourdough Fire area to predict and assess potential hazards so that they can plan mitigations in at-risk areas. 

The BAER team is here to support safe and effective post-fire management so that residents, employees, and visitors remain safe in a post-fire-changed landscape. These risks will likely last for years after the fire is contained. As vegetation grows back in the area and as infrastructure like trails are rehabilitated, the public should expect that recreation in the area may be limited for some time.