MISSOULA MEGAFLOODS AND PRE-WISCONSIN AGE OF MOSES AND UPPER GRAND COULEES

Pleistocene Okanogan-lobe ice dammed Columbia River to pond glacial Lake Columbia and divert the river south across a low spot along a 230-km-long divide. When enormous Missoula floods from the east engorge the lake, water spills through a few such divide sags. The grandest spillway became Grand Coulee, a gigantic flood cataract that eventually receded back into Columbia valley. Stratigraphic details of flood beds interbedded with varves show glacial Lake Columbia early in the late Wisconsin as small (the coulee open) as it was late. Upper Grand Coulee seems to have opened before, and changed little during, the late Wisconsin.

Giant Pangborn gravel bar 200 m above Columbia River near Wenatchee accumulated during immense late Wisconsin flood(s) down Columbia valley. Ice-rafted erratics register the upper limit of megaflood 325 m above the river. Fancher bar 45 m higher than Pangborn bar also has tall foreset beds, but its gravel is partly rotted and capped by 2 m of calcrete. And western Quincy basin holds such partly rotted basaltic gravel capped by calcrete whose foreset beds dip east into the basin. These areas show great pre-Wisconsin flood(s) down the Columbia that backflooded into Quincy basin. Similar field evidence for the late Wisconsin is lacking.

Columbia tributary Foster Creek heads at a divide saddle into Moses Coulee. When advancing Okanogan ice blocked the Columbia but not also Foster Creek, glacial Lake Columbia stood at a high 640–690 m level before upper Grand Coulee completed. Such a high level made Moses Coulee easier to invade by a large Missoula flood. This seems the scenario causing deepest Missoula flood(s) and most erosion in Moses Coulee. But once the head of Grand Coulee opened, Moses Coulee became hard to flood. During the late Wisconsin, only when Okanogan-lobe ice has blocked the Columbia but not also Foster Creek can floodwater rise deep enough to overflow into Moses Coulee—and then only in largest floods. Five such late Wisconsin Missoula floods poured down Moses Coulee. But Moses Coulee’s main excavation story lies in pre-Wisconsin glaciations—before upper Grand Coulee’s great cataract had eaten into Columbia valley.

New-generation 2-D modeling of the Scabland floods (Denlinger and others, adjacent paper) confirm such very different flooding results under differing Pleistocene ponding scenarios.