From an Update from the Department of Water Resources
On the first Friday of each month at noon there will be a test of the Emergency Siren at the Oroville Spillway.
You may hear a loud siren. This is only a test. The siren test will last less than one minute.
The purpose of the siren is to alert construction workers about a worksite emergency, should one arise.
The next test will be on August 4 at noon.