Dam operators store water behind Pipestem Dam during periods of high runoff, most often during extreme storms when rain or snowmelt fill the reservoir. When flows in rivers and streams below the dam are lower, a gated outlet structure in the dam’s earthen embankment releases water into Pipestem Creek. Operators release stored water during normal operations for recreation and to support fish and wildlife.
The outlet is designed to release up to 2,300 cubic feet of water per second (cfs). For perspective, the largest release of water as a result of flooding was approximately 1,420 cfs from the outlet structure in 2009. (One cubic foot of water is equal to 7.5 gallons.)
The spillway’s role
If reservoir levels from extreme storms or snowmelt exceed the outlet’s release capacity and water rises high enough in the reservoir, water will flow through the spillway to reduce the possibility of water flowing over the dam embankment. This overtopping can lead to dam failure and intensify downstream flooding, putting thousands of lives at risk and impacting critical infrastructure.
The Pipestem Dam spillway is designed to pass up to 110,000 cfs. This large but intentional flow is a ‘non-breach’ release. The reservoir has never been high enough for spillway flow. It is important to understand that the dam is designed to release far more water when necessary.