I always heard from my Dad about the Vanport flood in 1948. It also buried my farm area in Rainier, OR for nearly 2 months when the dike broke! That flood came in May when there was a sudden snowpack meltoff (heavy rain+warm weather) and the Columbia River dams got so backed up that they had to release a wall of water (twice!) Vanport near Portland was completely wiped out and mostly not rebuilt.