Some of the primary rivers in the North Cascades are the Skagit, Nooksack, Stillaguamish, and Stehekin. Lake Chelan is the largest lake in the North Cascades. Although now termed a reservoir, it was originally a natural lake, but enlarged by a dam. The urban areas of Seattle, Tacoma and Everett are all west of the North Cascades. The Columbia River, Washington's largest river, forms the east and south boundary.