Why the North Cascades Highway Closes Every Winter

In November, Washington State Department of Transportation crews close the gates on State Route 20 near the Ross Lake trailhead, east of Diablo Lake, and again a few miles west of Mazama. Between the gates lies the highest part of the North Cascades Highway, and it will not be plowed again until spring.

The highway is the northernmost paved crossing of the Cascade Range in Washington state. It follows the Skagit River past Diablo Lake and Ross Lake, two reservoirs created by Seattle City Light dams, then climbs to Rainy Pass at 4,855 feet and Washington Pass at 5,477 feet before dropping into the Methow Valley. It is the only road through North Cascades National Park. The rugged middle section opened in September 1972, after a four-year construction push through the gap between Ross Lake and the Methow Valley.

A highway under avalanche slopes

The stretch that closes sits in terrain that is steep on both sides. Above the road, bare rock bowls and gullies collect snow for months at a time. The highway corridor contains dozens of avalanche paths, chutes down which slides run to the roadbed, and more than one crosses the pavement. A slide can carry snow, ice, and timber over the road and into the valley below.

Snowfall in this part of the Cascades is measured in feet per storm, not inches. By late winter the snowpack on the road commonly stands ten to twenty feet deep. Plowing a highway under those conditions is not a matter of driving a grader; the crews would be working beneath slopes that can release without warning. The state keeps the lower passes open — Snoqualmie Pass on I-90 and Stevens Pass on US 2 — because they carry steady traffic and connect communities that need them year-round. The North Cascades Highway carries almost nothing in winter, and its slopes are not safer than those at the other passes; there is no practical way to keep it clean and safe for a handful of vehicles.

Closure built into the road's design

The shutdown is planned, not an emergency measure. Each autumn crews clear culverts and ditches, take down weather signs, and lock the gates before the first heavy snow. In most years the highway closes between early November and early December. The exact date depends on the forecast; the state does not wait for the snow to arrive before closing the road.

The road can stay closed because no community depends on it. The Methow Valley, with the towns of Winthrop and Twisp, has a year-round route to the rest of the state: State Route 20 east over Loup Loup Pass to Okanogan, and from there along the Columbia River toward Wenatchee. The closure takes away the shortest route, not the only one. Through drivers heading from western Washington to the Methow or the Okanogan use US 2 or I-90 in the winter months, adding time but not cutting anyone off.

The unplowed highway is not empty. It becomes a winter corridor for skiers and snowshoers, with access to backcountry routes around Washington Pass and Rainy Pass. The pavement serves as a trail until the state reopens it.

Spring reopening takes weeks

The reopening is a separate operation that begins in April. Crews arrive with explosives and avalanche-control teams, deliberately triggering slides on paths that could release onto the road or onto workers below. Once the hanging snow is down, graders and snow blowers move in from both ends of the closure. In most years the highway reopens in late April or early May, depending on how much snow fell and how quickly the weather warms.

Until then, the gates near Ross Lake and outside Mazama stay locked, and the North Cascades Highway does what it has done every winter since 1972: it waits for the snow to leave.