The Skagit River was highly influenced by the repeated advance and retreat of the Puget Lobe of the Cordilleran Ice Sheet. Ice and gravel moraines repeatedly blocked the Skagit, causing it to pool into lakes and forcing it to drain south into the future North Fork Stillaguamish River. After the ice retreated the Skagit breached the moraine dam near Concrete, Washington, finding its present course. The Sauk River and Suiattle River continued to drain into the future North Fork Stillaguamish River until eruptions of Glacier Peak choked the rivers with debris, causing the formation of an alluvial fan near present-day Darrington, Washington. The debris forced the two rivers north to join the Skagit.