[1] Prior to the 10 August 1996 outage, 500-kV line sections from the lower Columbia River to load centers in Oregon had been put out of service by tree faults. One on the Keeler-Allston line [circle] severed a 500-kV path west of the Cascades and led to a power surge on lines east of the mountains. Improper controls and relay actions at some plants aggravated the situation, causing the system to separate into islands.