“…there were a few double falls each member of which receded at approximately the same rate, so that the island in mid-channel became very much elongated, like a great blade, as the falls receded and the canyons lengthened.” J Harlen Bretz (1928)
A tall, narrow basalt ridge, coined “The Great Blade” by J Harlen Bretz, parallels Lower Grand Coulee east of Lake Lenore. The blade is the product of Ice Age floods that repeatedly rampaged Grand Coulee as recently as 15,000 years ago. Most of the floods appear to have come from sudden outbursts from glacial Lake Missoula. During flooding the coulees on either side of the Great Blade were filled with up to 800 ft turbid water. The largest floods also overtopped the Great Blade, submerging the site under at least another 100 ft of floodwater.
On the west side of the blade, where Lake Lenore is located, lies the Lower Grand Coulee, which ultimately migrated 10 miles northward – all the way to Dry Falls. On the east side of the blade is the higher East Lenore Coulee, which migrated a shorter distance (~3 mi) to Dry Coulee.
Like a gigantic rib the Great Blade is tallest and narrowest at its south end, widening to the north. The blade extends for almost four miles from where the head of East Lenore Coulee intersects Dry Coulee. In places the blade narrows to as little as 800 ft wide.
Video produced by Bruce Bjornstad for Ice Age Floodscapes