Okanagan Lake is a large, deep ribbon lake in the Okanagan Valley of British Columbia which formed in a tunnel valley from the Okanogan lobe of the Cordilleran Ice Sheet. The lake is long, between wide, and has a surface area of . Northern Idaho and Montana show evidence of tunnel valley formation under the Purcell lobe and the Flathead Lobe of the Cordilleran Ice Sheet. Tunnel valleys in southeast Alberta form an interconnected, anabranching network comprising Sage Creek, the Lost River and the Milk River and generally drain southeast.