The Grant County Sheriff's Office said the discovery was made as part of the drawdown at Wanapum Dam.
Many areas upstream are normally underwater, but are now exposed as officials are relieving pressure on the dam.
Deputies say the bones are not recent, but they cannot determine how old they are.
Authorities believe the bones may be those of an early settler or a Native American.
The Wanapum Indian Tribe has been notified, and Grant County Sheriff's Office is handling the investigation.