Colonial Peak is a steep North Cascades mountain above Thunder Arm of Diablo Lake on the Skagit River in Washington. Named for the Colonial mining claim on its slopes, it stands at the north end of a chain of major summits extending south toward prominent climbing peaks. Its significance lies in the dramatic relief that defines it and the surrounding range, rising sharply from deep glacially carved valleys and rugged ridges. First climbed on July 31, 1931, by William Degenhardt and Herbert Strandberg, it became part of the region’s classic mountaineering landscape. Its wet marine climate, heavy snow, and avalanche danger reflect the broader North Cascades environment.