July and August are peak season with nearly 5,900 reservations each month and 19–20% of bookings made 6+ months in advance — beautiful weather but zero margin for flexible planning. May offers the most last-minute opportunity (22.6% of bookings) and lighter crowds, though some campgrounds and facilities may not be fully open. September is counterintuitively the hardest month to get a site last-minute at 18.0%, driven by high-planning campers who want fall color and cooler temperatures without the summer crowds — if that's you, book in January.
State Route 20 (North Cascades Highway) is the primary corridor connecting Newhalem-area campgrounds to the Colonial Creek and Diablo Lake campgrounds — it typically closes for winter between November and April, effectively ending the camping season at those sites. Stehekin and its campgrounds (Purple Point, Harlequin, Lakeview) are entirely inaccessible by road and require travel by ferry, floatplane, or on foot; once in Stehekin, the valley road is served by a seasonal shuttle bus. Hozomeen is only accessible via a long unpaved road from Hope, British Columbia — do not plan to reach it from a US highway.
