Hood Canal and the rest of Puget Sound were created about 13,000 years ago, during the Late Pleistocene, by the Puget Lobe of the Cordilleran Ice Sheet.
The Skagit River was highly influenced by the repeated advance and retreat of the Puget Lobe of the Cordilleran Ice Sheet.
During the Vashon Glaciation, the Cordilleran Ice Sheet grew and advanced southwards at a rate of about 135 metres (443 ft) per year.
Cordilleran ice sheet (Wikipedia) The Cordilleran ice sheet was a major ice sheet that periodically covered large parts of North America during glacial periods over the last ~2.6 million years.