When fires burn up vegetation, the charred remains become hydrophobic—meaning they repel away any water. The soil is also very dry, which counterintuitively makes it harder for water to infiltrate. … Fires can also destroy the natural clumps in soil, increasing their erodibility. Altogether, this means that water is hitting the ground with more force and the soil is unable to suck it up.
- Northern California Water Association: Blog: When the smoke clears: Taking action for healthy headwaters