Skagit River - Geology

Geology

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.

Above Newhalem, Washington, the Skagit flows through a deep gorge, contrastingly strongly with the glacial valley below Newhalem. There are several theories about this anomaly, one of which is that the upper Skagit once drained northward into Canada and the growth and retreat of successive Cordilleran ice flows brought about the reversal. Each advance blocked the river, forcing it to find new routes to the south, in the process carving deep gorges. Eventually the Skagit gorge was so deep that even after the Cordilleran ice retreated for good the river continued flowing south instead of north into Canada.

The Skagit watershed is made up of high peaks and low valleys. The highest points in the basin are two volcanoes: Mount Baker (10,781 ft (3,286 m)) and Glacier Peak (10,541 ft (3,213 m)). Most of the basin lies above 2,000 feet (610 m). The river completes its course at sea level where it meets the Puget Sound.