The U.S. Army Corps of Engineers says about 470 gallons of oil was lost from one of the generating units at The Dalles Dam.  The Corps initiated spill prevention protocols on Wednesday and is in the process of determining what happened to the oil.  Although there is no evidence the oil went into the Columbia River, the Corps will continue to monitor the waters downstream of the dam.  Initially, the generating unit sent an indication of low oil to the control room on March 18.  A power plant mechanic inspected the unit and added oil to it.  Because of the low oil notification and as a precautionary measure, Corps staff began monitoring the Columbia River on March 19 for any evidence of oil.  Last Thursday the oil level in the unit appeared to be lower, and as a result, workers removed it from service.  On Friday, Corps workers isolated the unit from the river with bulkheads and began de-watering it.  The next day, workers inspected the tube and did not observe any oil sheens.