The U.S. Army Corps of Engineers says a turbine at John Day Dam spilled less than 25 gallons of oil into the Columbia River on Wednesday.  Corps Public Affairs Specialist Tom Conning says crew members were performing maintenance on the unit and observed the spill after they restarted the system.  He says the sheen was small, dissipated quickly and the crew could not recover any oil.  Technicians secured the unit, and identified the leak came from a failed turbine guide bearing cooling unit.  The unit remains out of service and isolated from the river.  Conning says the Corps Portland District is investigating the spill to determine the cause of the failure and the actual quantity of oil discharged.  He adds the amount is likely less than reported.  Corps officials notified partner agencies, including the National Response Center, Oregon and Washington emergency management offices, and the Columbia River Intertribal Fish Commission.