A wildfire dramatically alters the terrain and ground conditions of the watershed. Soils are charred, barren and unable to absorb water, creating conditions ripe for flash flooding and mud flows. In some cases, burned soil can be as water repellant as pavement. Flood risk remains high until vegetation is restored which can take up to 5 years after a fire. Flooding after a fire is often more severe than other flood events because debris and ash left from the fire can form mudflows. Mudflows are rivers of liquid and flowing mud on the surface of normally dry land, caused by a combination of brush loss and subsequent heavy rains.  

A flash flood is a rapid flooding of a low-lying area in less than six hours. A moderate rain event after a fire is enough to trigger flash flooding. These floods are known to roll boulders, tear out trees and destroy buildings and bridges. Additional info for tree hazards and rock falls can be found in the links to the left.


Important: Even areas that are not traditionally flood-prone are at risk because of changes to the landscape caused by wildfire. Locations downhill and downstream from burned areas are very high risk especially near steep terrain.  

Flood Insurance: Residents and business owners are urged to purchase flood insurance now to guarantee financial protection from flooding. There is typically a 30-day waiting period before flood insurance takes effect. No home is completely safe from flooding. Just one inch of water in a home can cost more than $25,000 in damage, flood insurance can be the difference between recovery and financial devastation. 

Tips to reduce chance of flooding or damage from flooding: 

  1. Pay attention to weather reports, specifically flood watches and warnings. It takes a lot less rain to trigger flash flooding after a wildfire. Sign up for Alert Napa County to get notifications to your email or phone.
  2. Ensure that drainage infrastructure is clear of debris. Inspect culverts and drains, both inlet and exit, and ensure water can flow through with no obstructions. Be thorough in your inspection as plastic culverts can melt completely but still appear as if a culvert is present.  
  3. Clean out gutters and downspouts. 
  4. Don’t let anyone sleep in basements during storms. Flash floods and debris flow can fill basements with water and mud in a minute, blocking doors and trapping people.  
  5. Bring important documents above basement level when there is a storm threat.  
  6. Always be prepared to evacuate. 


See the Post-Fire Watershed Recovery Frequently Asked Questions for more information.

Click here to return to the Post-Fire Watershed Recovery homepage.