Backpack Trail Guide

Trails / Washington / North Cascades

Thornton Lakes

North Cascades National Park, off State Route 20 (east of Marblemount)

Thornton Lakes is a short, steep overnight to a chain of three lakes below Mount Triumph, on the west side of North Cascades National Park. The Park Service lists 5.2 miles one way to the lowest lake, where three designated campsites are the only camping allowed.

Because it’s short and close to Highway 20, the camp fills quickly, and the Park Service names Thornton Lakes among its busiest permit areas.

The hike

The trailhead is at the end of Thornton Lakes Road, a rough and steep 5-mile spur that leaves State Route 20 about 11 miles east of Marblemount. NPS doesn’t recommend it for low-clearance vehicles or trailers.

The first 2 miles follow an old road grade that the Park Service says is often brushy. Hiking Project puts the road-grade section at about 2.3 miles through new-growth forest. The trail then climbs steep switchbacks through hemlock forest and a wet meadow into heather and huckleberry meadows near the ridge. NPS lists 2,524 feet of gain to the ridge, where the view opens to Triumph Peak and lower Thornton Lake. Hiking Project mentions views of the Skagit Valley as well.

From the ridge, the trail drops 460 feet to the lake. The Park Service calls the descent steep, muddy and slippery, and it ends with a scramble across the lake outlet over large boulders and floating logs. “Use caution when crossing the outlet on boulders and logs!” the park warns. A July 2025 WTA trip report found the scramble down to the logjam harder than the crossing itself, and described crawling down a steep, tilted rock to reach it.

Camping is allowed only at lower Thornton Lake. The Park Service says there’s no camping on the ridge, at Trappers Peak, or at the second lake. A primitive climbers’ path leads past the lower lake to the two upper lakes and on to Triumph Col; climbers headed for Triumph can get a permit for the Triumph Col cross-country zone.

From the ridge junction, a side route climbs 1.4 miles and 889 feet to Trappers Peak (NPS). The Park Service rates it strenuous and calls the upper part a steep scramble, with views of all three lakes.

The same 2025 trip report described a downpour on the way back from Trappers Peak and a constant swarm of mosquitoes at camp. The writer wore a head net, treated clothing and DEET.

For a longer trip in the park’s northern corner, see the Copper Ridge and Chilliwack River Loop.

Tips

Camping

A park backcountry permit for a designated site is required for every overnight stay, and permits are limited. Reservations are made on Recreation.gov; about 40% of sites are held for walk-up permits issued the day of or day before your start, at the Wilderness Information Center or the Glacier Public Service Center. Lower Thornton Lake has three sites. Fires aren’t allowed anywhere in the area. Pets aren’t allowed on the ridge, at Thornton Lake, or at Trappers Peak.

Inside Intel: In a July 2025 WTA trip report, a camper who found site 2 taken chose small site 3, near the lake, noting “I was a little worried about the smell from the outflow as site 1 is right along the creek.”

Trail facts

The Lowdown

The slippery 460-foot drop to Thornton Lake:

After 2,524 feet of gain to the ridge, the trail drops 460 feet to lower Thornton Lake on a steep, muddy, slippery descent, then crosses the outlet on boulders and logs. See how poles help on all of it.

Get the trekking pole tips →

Where this comes from

This page draws on the sources below, along with other guides and trip reports. Links go to the original pages.

Before you go: This page is based on published trail guides, trip reports, and reviews, along with official park and agency information where we could find it. It’s meant to help backpackers discover new trails, not to replace current, official information. Conditions, closures, permits, campsites, and water sources change, and some details here may be out of date or wrong. Confirm everything with the managing agency and check current conditions before you head out, and don’t rely on this page alone for safety or navigation. Backcountry travel involves risk.