The trailhead parking lot sits empty at 6am. Snow covers everything above 5,000 feet. Three alpine lakes wait beneath glaciers where ice forms patterns nobody sees for months at a time.
Thornton Lakes in North Cascades National Park freezes solid from November through May. The trail climbs 2,750 feet over 5.2 miles to Lower Thornton Lake at 4,500 feet. Middle and Upper lakes sit higher in the same basin, another 1.5 miles and 1,000 feet up unmarked trail. Winter transforms turquoise water into thick ice beneath jagged peaks.
The approach through frozen forest
The first two miles follow an old logging road through Douglas fir. Elevation gain stays minimal here. Snow depth reaches 3-4 feet by mid-January. Snowshoes sink 6 inches with each step.
At mile 2, the trail turns steep. Switchbacks climb through dense forest where exposed roots hide under ice. Crampons replace snowshoes on the hardpack. The temperature drops to 15°F as you gain elevation. Wind cuts through the trees above 4,000 feet.
The ridge crest at 5,000 feet opens to views. Mount Triumph’s glaciers catch morning light. Big Devil Peak and Snowfield Peak rise to the southeast. The descent to Lower Thornton Lake drops 500 feet down a steep slope where ice axes become necessary. One slip here means a long slide.
Three frozen basins under glaciers
Lower Thornton Lake spreads across the basin floor. Ice thickness reaches 2-3 feet by January. Snow covers most of the surface. The silence here feels absolute until ice cracks echo across the bowl.
What freezes first and stays longest
Lower Thornton Lake freezes in October. Upper lakes follow by early November. The ice holds until late April or early May depending on snowpack. Winter 2024-2025 brought above-average snow to the North Cascades. Depths at 5,000 feet exceeded 10 feet on nearby Highway 20 before the November closure.
Middle and Upper Thornton Lakes sit in a higher cirque. The unmarked trail gains another 1,000 feet through steep terrain. Most winter visitors turn back at Lower Lake. The upper basin sees maybe 10 people all winter. Mountain goat tracks cross the snow between frozen pools.
The glacial context that shapes everything
Mount Triumph’s glaciers feed these lakes in summer. Winter buries the melt channels. The basin becomes a study in white and gray. Rock walls rise 1,500 feet on three sides. Avalanche debris piles up in runout zones below chutes.
The North Cascades hold more glaciers than any range in the lower 48 states. These frozen lakes sit at the edge of that ice. Summer water temperatures barely reach 50°F. Winter ice forms fast and stays thick. A backpacker who swam Lower Lake in August 2019 would find 30 inches of solid ice covering the same spot by January.
Five more frozen alpine destinations
Lake Ann below Mount Shuksan freezes at 4,800 feet. The 7.2-mile round trip from Maple Pass gains 2,200 feet. Ice shelves form against the basin walls. Shuksan’s Fisher Peak headwall looms above. Winter access requires snowshoes and moderate avalanche skills. The approach sees low traffic but stays more accessible than the massive snowfield beneath 8,000-foot peaks where backcountry silence replaces lift lines.
Blue Lake at 6,300 feet near Rainy Pass shows persistent turquoise even under ice. The 4.4-mile round trip gains 1,100 feet. Glacial flour suspended in water creates the color. Snow coverage reaches 4-5 feet by mid-winter. Corteo, Arriva, and Frisco peaks of the Mount Logan massif ring the basin. This ranks as the shortest approach of the six destinations.
Copper Lake in Glacier Peak Wilderness sits at 5,900 feet. The multi-day approach covers 20-plus miles from Kennedy Hot Springs or White Pass. Glacier Peak dominates views from the frozen basin. Mountain goat tracks appear regularly in the snow. Winter visitors number in single digits. Permits run $5-15 per night through recreation.gov. The remoteness here exceeds even the Washington island where orcas hunt salmon from free cliffside parks.
Hidden Lake at 6,700 feet requires snowmobile or ski access after Highway 20 closes in November. The 9-mile round trip gains 3,000 feet. High avalanche danger marks the steep chutes above the basin. The lookout tower sits in ruins. Snow depths reach 10-15 feet. Only experts with avalanche training attempt this between December and March.
Lake Chelan-Sawtooth Wilderness holds the highest frozen destinations. Surprise Lake and nearby pools sit above 6,800 feet. Multi-day approaches from Stehekin cover 25-plus miles. Permits cost $5 per night with quotas. The area sees less than 1% of park visits. Winter access demands expedition-level skills. This compares to the Montana hot spring where snow falls on 7,150-foot pools but with extreme isolation.
Planning for winter alpine conditions
North Cascades National Park requires no permits for winter day use. Overnight camping needs advance registration but quotas don’t apply off-season. Northwest Forest Pass costs $5 per day at trailheads. Glacier Peak and Lake Chelan areas use recreation.gov for permits.
Gear rentals in Marblemount or Winthrop run $20-40 per day for crampons and ice axes. Avalanche beacons, shovels, and probes rent for $15-30 daily. AIARE Level 1 avalanche training costs around $400. The Northwest Avalanche Center provides forecasts at nwac.us. Winter ratings typically run moderate to high for these basins.
Temperatures at 5,000-6,800 feet range from -5°F to 25°F during the day. Nights drop to -20°F. Storm cycles dump 2-3 feet in 24 hours. Sunrise comes around 7:45am in January. Sunset hits by 4:30pm. The short days amplify the isolation. Cell service disappears at trailheads. Satellite beacons become essential. Helicopter rescue takes 12-48 hours if weather allows.
The North Cascades recorded 40,351 total visits in 2023. Winter months see fewer than 100 visitors. Summer backcountry use peaks around 11,000 annually. These frozen lakes represent the park at its most severe. Similar winter activities draw crowds to the Victorian town where 175 inches of snow empties streets by 8pm. Here the crowds never arrive.
Your questions about Thornton Lakes answered
When do these lakes freeze and thaw?
Thornton Lakes freeze between October and November. Ice holds through April or May. Blue Lake and Lake Ann follow similar patterns. Higher destinations like Sawtooth lakes freeze by November and stay frozen into June. Snow depths peak in February and March. Avalanche danger remains high through April.
Do you need avalanche training for winter access?
Yes for all six destinations. Steep terrain surrounds every basin. AIARE Level 1 certification provides minimum skills. Hidden Lake and Sawtooth areas demand Level 2 experience. The Northwest Avalanche Center issues daily forecasts. Moderate ratings still mean significant risk. Groups should carry beacons, probes, and shovels. Solo travel in winter alpine terrain is not recommended.
Which frozen lake offers easiest winter access?
Blue Lake near Rainy Pass has the shortest approach at 4.4 miles round trip. Highway 20 closes in November which blocks vehicle access. Before closure, the trail stays moderate. Lake Ann ranks second at 7.2 miles with good snowshoe access from Maple Pass. Thornton Lakes requires 10.4 miles and steep terrain. Copper Lake and Sawtooth destinations demand multi-day expeditions.
Dawn light touches the ice at Lower Thornton Lake around 8am. The frozen surface glows pale blue for maybe ten minutes. Then clouds move in and snow starts falling again. The basin disappears into white.