The National Park Service commissioned an Environmental Assessment, which concluded that Option 3 - creation of a "National Geologic Trail—designating the Floods pathways managed by the National Park Service, with an Interagency Technical Committee representing the federal, tribal, and state agencies and a Trail Advisory Committee to assist the Trail Manager and staff" was the preferred option. As a result, the Ice Age Floods Trail was established by the Omnibus Public Land Management Act of 2009, in which Congress authorized establishing the Ice Age Floods National Geologic Trail in parts of Montana, Idaho, Washington, and Oregon states, and established NPS administration of the Trail.
Geologic basis for the trail
At the end of the last Ice Age, a branch of the Cordilleran Ice Sheet moved out of Canada into the Idaho panhandle region. There it formed a -high ice dam that blocked the mouth of the Clark Fork River, creating glacial Lake Missoula, which impounded greater than of water. The lake extended up the valleys eastward for over. The periodic rupturing of the ice dam resulted in the Missoula Floods - cataclysmic floods that swept across Idaho and Eastern Washington, and then down the Columbia River Gorge approximately 40 times during a 2,000 year period. The flood front swept in a wave across Idaho and Washington at speeds approaching, and Glacial Lake Missoula drained in periods as short as 2 days. The Columbia River channel downstream was blocked by the Okanogan lobe of the Cordilleran, impounding water in Glacial Lake Columbia. As a result, the floods could not continue down the Columbia River, being forced instead to flood over the highlands of Eastern Washington, vastly transforming the landscape by forming the Grand Coulee, Moses Coulee, the Channeled Scablands, Dry Falls, Palouse Falls and many similar features. The cumulative effect of the floods was to excavate of loess, sediment and basalt from the channeled scablands of eastern Washington and to transport it downstream. Over a period of 2,000 to 2,500 years, the repetition of ice dam failure and flood was repeated 40-60 times, leaving a lasting mark on the landscape. There are a number of characteristic features that illustrate the effect of these ice dams and power of the resulting floods:
Large portions of the scabland region of Eastern Washington were stripped bare of topsoil and left as the base basalt rock.
The landscape is extensively eroded, with deep sheer-walled canyons in the Grand Coulee, Moses Coulee, the Wallula Gap, and the Columbia River Gorge.
There are giant ripple marks high and gravel bars over long and high, which would have required extreme currents to form.
There are erratic boulders which originated in Canada and Montana, found well up valleys such as the Willamette River valley far south of any evidence of glacial activity; these boulders were ice rafted in and deposited as the ice was stranded & melted in backwaters of temporary lakes.
The scablands formed include deep kolks formed by local turbulence, which plucked basalt boulders out and excavated pits in the middle of relatively smooth plains.
Backwaters of temporary lakes formed during the floods show extensive & periodic deposits of silt and sand.
Probable features of the trail
The trail, which is now being developed, will comprise routes throughout the Pacific Northwest region where the Ice Age Floods left a lasting impact. Features likely to be included are:
The Glacial Lake Missoula region in Montana in which the waters were initially impounded
Temporary lakes such as Lake Lewis in Washington as well as Lake Condon and Lake Allison in Oregon and the evidence for these temporary lakes as seen in the ice rafted erratics and in the deposits of the Touchet Formation
