Spectacular floods radically changed the landscape of Oregon and Washington some 17,000 years ago when Ice Age glacial dams failed near Missoula, Montana. These floods unleashed an inland sea and geologists believe they are the greatest floods ever known to humankind. Evidence is most easily seen from the air and includes ripple marks and dry falls. The floods dropped huge boulders and deposited mountains of topsoil in the Willamette valley.