The Army Corps of Engineers estimates about 100 gallons of oil leaked into the Snake River last week at Ice Harbor Dam near Burbank.
Oil was being transferred from one tank to another and filtered as part of routine maintenance when a valve was left in the wrong position, said Corps spokesman Bruce Henrickson. The leak was discovered May 7 when a sheen of oil appeared on the Snake River.
About 300 gallons leaked into the powerhouse and an estimated 200 gallons had been recovered still in the powerhouse during cleanup efforts that lasted until Saturday. That left an estimated 100 gallons that gradually leaked into the river, Henrickson said.
The oil in the river was unrecoverable and it was allowed to disperse in the water, he said.
The Corps is looking into procedures and making changes to prevent a spill from a similar cause, he said.
In early 2012, about 44 gallons of transformer oil leaked into the river at Ice Harbor because of human error. At the time, work was under way to repair the dam's cooling units after pinhole leaks in the units allowed 1,680 gallons of transformer oil contaminated with PCBs to leak into the river during many months.