The water level in the reservoir behind the central Washington dam has been lowered to take pressure off a crack in a spillway pillar and many upstream areas that are typically underwater are now exposed.
The sheriff's office says a person walking along the river at Crescent Bar on Tuesday afternoon found the bones in such an area.
Detectives don't know how long the bones have been there. Since it's possible the bones are those of a Native American, leaders of the nearby Wanapum Tribe visited the scene.
The sheriff's office and Washington State Patrol Crime Lab plan to collect the remains Wednesday for analysis.