Ice Age Floods National Geologic Trail
Ice Age Floods National Geologic Trail
Lake Pend Oreille is the largest lake in Idaho -- 43 miles long, over 1,150 feet deep, and holding roughly 86 million acre-feet of water -- and it exists in its current form because the Cordilleran Ice Sheet gouged this basin to extraordinary depths during the Pleistocene....
Lake Pend Oreille is the largest lake in Idaho -- 43 miles long, over 1,150 feet deep, and holding roughly 86 million acre-feet of water -- and it exists in its current form because the Cordilleran Ice Sheet gouged this basin to extraordinary depths during the Pleistocene. During the ice ages, a 20-mile-long tongue of glacial ice filled the lake basin, and when the broader Purcell Trench Lobe blocked the Clark Fork River at the lake's east end, it created the ice dam that impounded Glacial Lake Missoula. Nearly all of the water from those catastrophic floods -- estimated at 500 cubic miles per event -- burst from the south end of Lake Pend Oreille and surged across the Rathdrum Prairie toward Spokane. The lake's exceptional depth is a legacy of glacial scouring, and its steep-sided basin preserves sediment layers that record both glacial advance and catastrophic flood drainage. Today the lake is renowned for its trophy-class Kamloop rainbow trout and Gerrard rainbow trout, fishing in a basin that was once the bathtub drain for the largest floods on the continent.
Open year-round; Idaho's largest lake (148 sq mi, 1,150 feet deep — the fifth-deepest lake in the U.S.). Multiple public access points: Sandpoint City Beach, Farragut State Park (south end), Hope and Clark Fork on the east side. No single fee — costs depend on which park or marina.
This is the keystone site of the entire trail: the lake basin held the ice dam that created Glacial Lake Missoula. The Purcell Trench Lobe of the Cordilleran Ice Sheet advanced south down the trench from Canada and met Green Monarch Ridge across the Clark Fork River outlet on the lake's east side, forming an ice plug roughly 2,000 feet tall and 30 miles wide. When that dam failed — repeatedly between 18.2 and 14 ka (Balbas et al. 2017) — up to 500 cubic miles of water surged west through the Rathdrum Prairie and on across the Columbia Plateau. The lake basin was also scoured by ice; Lake Pend Oreille's 1,150-foot depth (bottom at roughly 900 feet below sea level) reflects glacial overdeepening followed by ~400 feet of post-flood sediment fill. The bedrock floor lies roughly 1,500 feet below the modern lake surface.
Balbas et al. (2017) refined the Purcell Trench Lobe retreat to ca. 15.5 ka. Cohen et al. (2022) and subsequent core studies of Lake Pend Oreille sediment continue to refine flood timing through varve and tephra correlations. The U.S. Navy's Acoustic Research Detachment at Bayview continues to publish bathymetric and acoustic studies that incidentally inform lake-floor geomorphology.
The Coeur du Déluge Chapter (Sandpoint) is centered on the lake and runs the most active Ice-Age-Floods interpretation program in north Idaho — chapter field trips visit Green Monarch Ridge, Cabinet Gorge, and the Clark Fork delta. NPS interpretive signage is in place at multiple lake-edge viewpoints.
Best June through September. The east-shore drive (SR 200) from Sandpoint to Clark Fork passes the ice-dam abutments, the modern Clark Fork delta (formed by post-glacial sediment), and several IAFI/NPS interpretive pullouts. Best single overview: Green Monarch Ridge pullout (above) and Hope city overlook.
Every site along the trail will receive the full Terrain360 capture treatment: ground-level 360° panoramas, drone aerial imagery, and photogrammetry-based 3D models that visitors can spin in their browser. This page reserves the slots; the imagery flows in as field capture completes.
Ground-level 360° panorama, every step along the feature, captured by Terrain360 field crews.
Drone flyovers reveal the geometry of catastrophe — ripple marks, gravel bars, and scour patterns invisible from the ground.
Photogrammetry and Gaussian-splat models let visitors rotate, measure, and inspect features in detail-page WebGL viewers.