Regarding potential flooding from failure of the Howard Hanson Dam, remember that most of the West end of the Northern Pacific Stampede Pass line, & all of the Valley Main between Seattle & Tacoma, was designed and built in the days of routine flooding from the Green River. Of course the realignment around the dam is built above the lake level, & crosses well above the Green River at Kanasket. With the development of farmland in the Auburn - Kent Valley since 1962, the bigger issue is the loss of these floodwater overflow areas, & the potential for the river & creek system to backup into now built - up areas.
(Even though I live some 300 feet above the valley floor, my Auburn address prompted FEMA to send a scare letter warning me to buy flood insurance.)