King County has nine high hazard dams that would cause widespread flooding if they were to fail. When dams fail, a large flood wave travels rapidly downstream that can cause loss of life and property. King County has 87 smaller dams that pose a similar threat.
History of dam failures
King County has experienced three dam failures over the past 100 years. The first incident occurred in 1918 near North Bend and the most recent in 1997 in Shoreline. Seven deaths have been attributed to dam failure. One of King County's most notable dam related incidents occurred in 2008-2009 when structural deficiencies were discovered on the federally-owned Howard Hanson Dam. While repairs were being made, the United States Army Corps of Engineers warned that a substantial release from the dam may be required to protect the dam from failing. This release would have caused widespread flooding in the Green River Valley, and economically devastated portions of Auburn, Kent, Renton, and Tukwila. The dam has been reinforced and is currently fully operational.
While the failure of any King County dams is unlikely, it's important to be prepared:
- Sign up for emergency notifications at ALERT King County
- Pack an emergency supply "go" bag with important documents, prescription medication, and other essential items.
- Identify an evacuation route to higher ground from your home or business, and address any mobility challenges.
- Teach all family members how, where, and when to turn off utilities.
- Plan a meeting place outside of the hazard area.
- Learn how to prepare for flooding which typically accompanies dam failures.
There are also things you can do now to reduce the risk and impacts of flood damage:
- Consider purchasing flood insurance. Your standard homeowner's insurance doesn't cover flooding. King County residents receive a 45% discount - one of the best in the country. You will receive a cheaper rate if you are outside of the 100-year Flood Plain, which many homes and businesses in dam failure zones are.
Install backflow valves on wastewater pipes to prevent sewage from backing up.
Keep valuables and important documents at higher elevations (second story, if possible).
Store chemicals off the floor, above flood levels.
Elevate utilities, such as the water heater, furnace, and electric panel.
Seal cracks in your building with urethane-based caulk.
Know where to get sandbags to keep water from entering garage and door openings.
- Follow the directions of emergency alerts and law enforcement.
- When told to evacuate, do so immediately. If you have mobility concerns call 911. Failure to evacuate creates additional safety risks for yourself and first responders.
- Grab your emergency supply "go" bag and take your family and pets to high ground.
- Once out of the area, stay out until emergency personnel say it's safe to return.
- Never drive or walk through flooded areas. It is the number one cause of flood related deaths. You can be knocked off your feet by as little as six inches of moving water.
The City of Seattle cannot guarantee the accuracy of the information contained in this map. Each user of this map is responsible for determining its suitability for his or her intended use or purpose. The inundated areas shown on this map reflect unknown, potential events of an unlikely possibility and are not in any way intended to reflect upon the integrity of the Tolt Dam within King County. Because of the methods, procedures, and assumptions used to develop the flooded area, and the inherent uncertainty of dam break simulations, the limits of flooding and data values shown are only approximations. In the event of an actual failure, the affected areas will depend on specific failure conditions and will likely differ from mapping shown. The information contained herein is provided “AS IS, WITH ALL FAULTS” and no warranties of any kind, including accuracy, validity, fitness for a particular purpose or merchantability, accompany this product.
- Flood hazards and preparedness - King County Emergency Management
- Flood Hazard Reduction Services - King County Water & Land Services
- King County Flood Warnings and Alerts
- Dam Safety Research - Federal Emergency Management Agency (FEMA)
- National Flood Insurance Program
- Association of State Dam Safety Officials
- Washington State Department of Ecology Dam Safety Office
- United State Army Corps of Engineers
Dams are human-built structures that are designed to hold back water. You might picture the giant concrete Hoover Dam when we talk about dams, but most dams in King County are earthen dams. Earthen dams are composed of impermeable materials at their core, reinforced with other natural materials to hold back water.
The 127 dams in the county serve many beneficial purposes. Most dams protect our communities from regular flooding, others serve as a source of clean drinking water, and only a few produce electricity for our cities.
Many of the dams in King County are owned by local governments or private parties. Only a few are owned by the State and Federal government.