The Okanogan lobe of the Cordilleran Glacier moved down the Okanogan River valley and blocked the ancient route of the Columbia River, backing up water to create Lake Spokane.
Initially water discharged from Lake Spokane by running up through the head of Grand Coulee and down through Foster Coulee to rejoin the Columbia River.
As the glacier moved further south, Foster Coulee was cut off and the Columbia River then discharged through Moses Coulee, which runs southward slightly to the east of the ancient and current course of the Columbia.
As the Okanogan lobe grew, it blocked Moses Coulee as well; the Columbia found the next lowest route through the region which was eroded to become the modern Grand Coulee.
Flowing across the current Grand Coulee & Dry Falls regions, the ice age Columbia then entered the Quincy Basin & joined Crab Creek, following Crab Creek’s course southward past the Frenchman Hills and turning west to run along the north face of the Saddle Mountains & rejoin the previous and modern course of the Columbia River just above the main water gap in the Saddle Mountains, Sentinel Gap.
The Okanogan lobe of the Cordilleran Glacier moved down the Okanogan River valley and blocked the ancient route of the Columbia River, backing up water to create Lake Spokane. Initially water discharged from Lake Spokane by running up through the head of Grand Coulee and down through Foster Coulee to rejoin the Columbia River. As the glacier moved further south, Foster Coulee was cut off and the Columbia River then discharged through Moses Coulee, which runs southward slightly to the east of the ancient and current course of the Columbia. As the Okanogan lobe grew, it blocked Moses Coulee as well; the Columbia found the next lowest route through the region which was eroded to become the modern Grand Coulee. Flowing across the current Grand Coulee & Dry Falls regions, the ice age Columbia then entered the Quincy Basin & joined Crab Creek, following Crab Creek’s course southward past the Frenchman Hills and turning west to run along the north face of the Saddle Mountains & rejoin the previous and modern course of the Columbia River just above the main water gap in the Saddle Mountains, Sentinel Gap.
Drumheller Channels National Natural Landmark