Dam operators from the U.S. Army Corps of Engineers confirmed the loss of approximately 53 gallons of oil from one of the generating units at The Dalles Dam.  The Corps says it is in the process of determining the exact cause of the spill, and partner agencies, including the National Response Center, Oregon and Washington emergency management offices and the Columbia River Intertribal Fish Commission, have been notified.  Earlier this year, the generating unit lost approximately 470 gallons of oil due to a faulty blade seal.  The seal was replaced, and the unit returned to service, and the Corps has been monitoring oil levels at the dam more frequently since then.  After fluctuations in the oil level were observed in the unit, Corps technicians removed it from service and initiated spill prevention protocols May 31.  As a precautionary measure the unit was dewatered and isolated from the river, and the Corps says it will remain that way until a solution is determined.