A crescent-shaped ridge of glacial debris stretching across the Waterville Plateau marks the farthest point that a continent-spanning ice sheet reached in central Washington. The Withrow Moraine is the terminal moraine of the Okanogan Lobe...
A crescent-shaped ridge of glacial debris stretching across the Waterville Plateau marks the farthest point that a continent-spanning ice sheet reached in central Washington. The Withrow Moraine is the terminal moraine of the Okanogan Lobe of the Cordilleran Ice Sheet, a wall of ice that stood roughly 3,000 feet thick when it blocked the Columbia River and created Glacial Lake Columbia -- one of the key players in the Ice Age Floods saga. The moraine itself is an impressive ridge of unsorted boulders, gravel, and clay bulldozed into place by the glacier's slow advance southward. When the Okanogan Lobe blocked the Columbia, it diverted the river's entire flow into the Grand Coulee, carving one of the most spectacular canyons in North America. This humble ridge of glacial rubble thus marks the trigger point for a chain reaction of geological events that reshaped the entire Pacific Northwest. Visit the Withrow Moraine and you stand at the exact boundary between the ice age world and the landscape we know today.
The Withrow Moraine and Jameson Lake Drumlin Field is a National Natural Landmark in Douglas County, Washington, but is predominantly privately owned. The two principal public-access points are WDFW lands: the Dormaier Wildlife Area Unit (off Road 5 NE) and the Chester Butte Wildlife Area Unit, both requiring a Discover Pass for non-licensed visitors. The moraine itself is best appreciated by driving SR-172 across the Waterville Plateau between Mansfield and Withrow, the road crosses the moraine and the surrounding drumlin field.
Withrow Moraine is the southernmost terminal moraine of the Okanogan Lobe of the Cordilleran Ice Sheet, the actual maximum-extent line of glacial ice on the Columbia Plateau. The moraine itself is 200+ ft high and ~2 miles wide near the village of Withrow. The relevance to the Missoula floods is sequential rather than direct: the Okanogan Lobe blocked the Columbia River north of Wenatchee at roughly 18.5–18 ka, creating glacial Lake Columbia and forcing subsequent Missoula floods south down Moses and Grand coulees (the routing that produced the Channeled Scablands as we know them). Without the Okanogan Lobe and its terminus at the Withrow Moraine, the flood story would look completely different. Balbas et al. (2017) dated boulders on the Withrow Moraine itself: the Okanogan Lobe had retreated from the moraine by 14.5 ± 0.3 ka, and was on the Omak Plateau by 14.1 ± 0.3 ka, finally reopening the Columbia drainage and ending the Lake Columbia / late-Missoula flood sequence. Geomorphologically the moraine is unusual: 2024 GSA work argues it is largely glaciotectonically excavated basalt diamicton rather than the kame-and-kettle till assumed in older interpretations.
Balbas et al. (2017) remains the principal cosmogenic chronology. A 2024 GSA Cordilleran/Rocky Mountain Section abstract (Lillquist and colleagues) reinterprets the Withrow Moraine as glaciotectonically excavated basalt rather than ice-marginal till, which would change the standard textbook description if formalized in peer-reviewed publication. A separate 2024 Geology paper (Waitt and others) on Okanogan-lobe tunnel channels and subglacial floods into Moses Coulee adds a related (but distinct) subglacial-discharge story for the lobe.
The IAFI Ellensburg and Wenatchee Valley Erratics chapters lead field trips on the Waterville Plateau that stop at the moraine; the Central Washington University Geography Department has a published Waterville-Plateau field-trip guide. There is no permanent IAFI panel or visitor facility at the moraine itself; the NNL is privately owned, and signage is minimal.
Best May through October (winter snow closes some roads). SR-172 across the plateau gives the long view of the moraine and drumlin field. Pair with a stop at Banks Lake / Dry Falls to the south to see what the floods did downstream of the Okanogan Lobe terminus.
Flood-extent overlay shows the maximum reach of the Missoula Floods.
The site in its place along the flood path, with the maximum flood extent drawn over the modern map.
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