Mossyrock
Mossyrock is an arch dam located in Lewis County, Washington, built in 1968. It carries a High hazard potential classification and has a unsatisfactory condition assessment.
About Mossyrock
The primary purpose of Mossyrock is hydroelectric. The dam generates hydroelectric power by channeling water through turbines, converting the energy of flowing water into electricity. The dam is owned by City Of Tacoma (public utility). It impounds the Cowlitz River near Toledo, Wa.
The dam stands 606 feet tall and stretches 1,750 feet across, has a maximum storage capacity of 1,900,000 acre-feet, and collects water from a drainage area of 1,042 square miles. Completed in 1968, the structure is well into its service life at over 50 years old.
This dam carries a high hazard potential classification, which means that a failure or uncontrolled release of water would likely cause loss of human life. This classification reflects the consequences of failure, not the likelihood — it indicates that people live or work in the area downstream. Its condition is rated unsatisfactory, indicating serious safety deficiencies exist that require immediate attention or remedial action. An emergency action plan is in place for this dam. The most recent inspection on record was 08/20/2025.
At 606 feet, this dam is taller than 99% of all dams in the United States.
Living Downstream
If you live or work downstream of this dam, consider taking these steps: learn your local evacuation routes, check whether your county emergency management office has a dam failure inundation map, and sign up for local emergency alerts. The Washington dam safety program can provide more specific information about emergency planning for this structure.
Search for dams near your address to see all dams in your area with hazard classifications and safety data.
Location
| State | Washington |
| County | Lewis |
| Nearest City | Toledo, Wa |
| River/Stream | Cowlitz River |
| Coordinates | 46.53460, -122.42500 |
Physical Characteristics
| Dam Type | Arch |
| NID Height | 606 ft |
| Dam Height | 366 ft |
| Dam Length | 1,750 ft |
| Year Completed | 1968 (58 years old) |
Storage & Hydraulics
| NID Storage | 1,900,000 acre-ft |
| Max Storage | 1,900,000 acre-ft |
| Normal Storage | 1,700,000 acre-ft |
| Drainage Area | 1,042 sq mi |
| Max Discharge | 280,000 cfs |
| Spillway Type | Controlled |
Ownership & Safety
| Purpose | Hydroelectric |
| Owner | City of Tacoma |
| Owner Type | Public Utility |
| Hazard Classification | High |
| Condition Assessment | Unsatisfactory |
| Last Inspection | 08/20/2025 |
| Emergency Action Plan | Yes |
Identifiers
| NID ID | WA00151 |
| Federal ID | WA00151 |
| More Info | State Dam Safety Program |