Timing, Topography and Type (“Rock Type” that is)
Most of north-central Washington State is covered by lava flows associated with the Miocene Columbia River Basalts. The source of these lava flows came from a series of large fissure eruptions in northeastern Oregon, southeastern Washington and west-central Idaho (similar to the eruptions currently occurring today in Hawaii). These massive outpourings of lava eventually forced the course of the Columbia River to the north and west into what we call today the “big bend”. The nature in which the basalts cooled and cracked, and the structural folding and faulting of the basalts during and after emplacement (Yakima Fold Belt), allowed the Ice Age floodwaters to sculpt large Coulees (Grand and Moses Coulees) and the channeled scabland of eastern Washington.
During the Pleistocene (Ice Age) over a period of about two million years, climatic conditions allowed the formation of principally two large continental ice sheets in North America. East of the Rocky Mountains was the Laurentide and to the west of the Rockies was the Cordilleran. Three principal large glacial “lobes” flowed south from the Cordilleran Ice Sheet in Canada into the Pacific Northwest. Those ice lobes are known today as the Puget, Okanogan and Purcell Trench Lobes. The Purcell Trench and Okanogan Lobes played a vital role in controlling ice age flooding and timing of flooding in north-central Washington.
During the most recent ice age advance (about 20-12 thousand years ago) the Purcell Trench Lobe was first to arrive blocking predominantly all the drainages in northwest Montana forming a large glacial lake known as Glacial Lake Missoula (GLM). Periodically the ice dam holding back GLM would break allowing Ice Age floodwaters to catastrophically escape. The initial flooding from GLM flowed principally in two directions; to the southwest down the Cheney-Palouse scabland tract, and to the west/northwest into the preexisting Columbia River drainage. During the initial flood/floods the Okanogan Lobe of the Cordilleran had not advanced far enough (south) down the Okanogan River valley to block floodwaters flowing down the Columbia River around the big bend and eventually reaching the Wenatchee area where maximum floodwater depth was almost 1,000 feet deep.
Advance of the Okanogan Lobe into the Columbia River valley (Bridgeport/Brewster, WA area) prevented floodwater from flowing the natural course into the Columbia valley north of Wenatchee, deflecting floodwaters into the existing Moses Coulee drainage and possibly down the beginning of Grand Coulee. Floodwaters flowing down Moses Coulee formed a large bar across the Columbia River valley south of Wenatchee, Washington, forming a large temporary lake in the Wenatchee valley until water became deep enough to erode down through the bar and empty the lake. Evidence in the Moses Coulee bar deposit reflects four distinct “basalt” dominated flood beds indicating four separate flooding episodes.
Note that about the same time as the Okanogan Ice Lobe advanced into the Columbia valley another glacial lake started forming behind this ice dam, known today as Glacial Lake Columbia (GLC). As GLC filled, flooding continued down Moses Coulee and became more prominent down the beginning of Grand Coulee. Eventually the Okanogan Lobe advanced up onto the Waterville Plateau and blocked the upper portion of Moses Coulee thus ice-age floodwaters could no longer flow down Moses Coulee.
With the blocking of Moses Coulee, GLC filled and flowed down the Grand Coulee drainage. During this time, when the Okanogan Lobe blocked the Columbia River drainage around the big bend, natural flow of the Columbia River was down the Grand Coulee drainage. Flood waters flowing down the Grand Coulee went over two separate folds in the basalt. An extension of the Beezley Hills anticline allowed a southern waterfall or “cataract” to form near Soap Lake, Washington. Another fold in the basalt, the Coulee Monocline, located just a few miles north of Coulee City, Washington, was the starting site of the upper falls or cataract. These two cataracts were simultaneously being eroded and “prograded upstream” with each subsequent Missoula Flood down the Grand Coulee drainage.
During this time when GLC was at its maximum depth and flowing naturally down Grand Coulee, floods out of GLM would overtop lower divides and flow into the Telford and Wilson Creek floodways eventually meeting up with floodwaters down Crab Creek. As the Purcell Trench Lobe waned and flooding episodes became smaller and more frequent, these smaller flooding events were still able to overtop the lower divides along the south side of the Columbia River drainage due to the high water level in GLC.
Eventually the upper cataract in Grand Coulee “breached” the existing Columbia River valley drainage near Grand Coulee, Washington. This breaching lowered the northern opening into Grand Coulee close to 500 feet lower than other divides to the east, making it the predominate flood pathway for the remainder of smaller and more frequent GLM floods. These later smaller GLM floods were not large enough in volume to overtop previous divide drainages to the east of Grand Coulee. The terminus of the lower Grand Coulee is marked by “Dry Falls” cataract.
Eventually the Purcell Trench Lobe retreated north (melted back). With no ice dam blocking the drainages in northwest Montana, GLM could no longer exist. The Okanogan Lobe still blocked the Columbia River drainage west of Grand Coulee and GLC existed for about another 300 years after the last GLM flood based on glacial lake sediment (glacial varves) evidence.
Eventually the Okanogan Lobe melted enough that one last catastrophic ice age flooding event from the emptying of GLC occurred down the Columbia River valley into the Wenatchee, Washington area and south along the preexisting Columbia River. It is believed that West Bar south of Trinidad, Washington, was formed from the demise of GLC. A small bar adjacent to West Bar may be evidence for one last small flood, after the demise of GLC, down the Columbia River drainage.
The existing topography, the timing and position of the Purcell Trench and Okanogan Lobes, and the unique nature in which the basalt eroded has played a significant role in the development of Ice Age flood pathways in north-central Washington.
By Brent Cunderla, Wenatchee Valley Erratics Chapter President