Diablo Lake

Late June through early October is the primary season — the North Cascades Highway (WA-20) is closed by snowfall typically from November through late April; late spring and early summer (May and early June) can bring lingering snowpack that covers the higher trails. The glacier flour color is most intense from late June through August (peak glacial melt season); the turquoise color is most vivid and most reliably present in July and August. Summer (July and August) is the peak visitor season; the campgrounds fill quickly and WA-20 can be congested on weekends. The finest combination of timing is late June (glacier color at maximum, wildflowers peaking in the subalpine, and the falls in full roar from snowmelt) or early September (the crowds diminish, the weather is often stable and clear, the North Cascades peaks have fresh snow on their summits, and the lake color remains intense). Check WA-20 road status at WSDOT before any spring or fall visit (the highway can close suddenly for early-season snow or late-season storms).

Diablo Lake and the surrounding Ross Lake NRA support the characteristic wildlife of the western North Cascades. American black bears are abundant throughout the Skagit Valley watershed; proper food storage is required at all campgrounds and trailheads. Bald eagles nest along the Skagit River corridor (the Skagit River just downstream of Gorge Dam is one of the most important bald-eagle wintering areas in the Pacific Northwest — the largest concentration of wintering bald eagles south of Alaska, feeding on spawned-out salmon in the river below the dams in December and January). Common loons nest on the quieter coves of Diablo and Ross Lakes — their haunting calls across the turquoise water are one of the finest North Cascades lake-sounds. Osprey fish the lake surface. Belted kingfishers, common mergansers, and harlequin ducks are present on the water. Mountain goats inhabit the high rocky terrain of Colonial and Davis Peaks above the lake.

The North Cascades Highway (WA-20) can close suddenly for snow, rockfall, or slides at any time of year above 3,000 feet; check WSDOT road conditions before any trip (especially in spring and fall). Swimming in Diablo Lake is extremely dangerous — the water temperature (fed by glacier melt) is typically 40-50°F year-round; cold-water shock and hypothermia incapacitate swimmers quickly; no swimming in the lake. Canoeing and kayaking in Diablo Lake: the lake can develop significant afternoon whitecaps from afternoon valley winds; paddle in the morning and be off the water by noon. Black bears: never leave food unattended at any campsite or in any vehicle (bears have learned to associate vehicles with food; store all food and scented items in the campground food-storage lockers). The Sourdough Mountain trail is a serious hike with 5,000 feet of gain; full mountain preparedness required.

Diablo Lake is a reservoir — it was created by the impoundment of the Skagit River behind Diablo Dam (completed by Seattle City Light in 1930), part of the Skagit Hydroelectric Project that comprises three dams (Gorge, Diablo, and Ross) built between 1924 and 1949 to supply electrical power to Seattle. The construction of the Diablo Dam in the deep Skagit River gorge was a significant engineering achievement of its era; the Diablo Townsite (a company town built by Seattle City Light for dam workers and their families) survives as a nearly intact 1930s hydroelectric-project community and is maintained by Seattle City Light as a historic district. The North Cascades Highway (WA-20) was completed in 1972, transforming access to the Skagit Valley and the Diablo Lake area from a remote mountain road to a major scenic highway. North Cascades National Park was established in 1968; the Ross Lake National Recreation Area (which includes Diablo Lake) was established at the same time, with the complex purpose of managing the existing hydroelectric infrastructure within a national park framework.

Diablo Lake’s turquoise color is entirely the result of its glacial origin — the Colonial, Snowfield, and Pyramid glaciers (and numerous smaller glaciers on the surrounding peaks of the Colonial Creek watershed) grind the underlying Cascade granite and metamorphic bedrock to an extremely fine powder as the ice moves over the rock; this rock flour (particles less than 4 micrometers in diameter) is carried in the glacial meltwater streams (notably Colonial Creek) into Diablo Lake, where it remains in suspension in the cold water. The suspended glacier flour preferentially scatters light in the blue-green wavelengths (a phenomenon called “Mie scattering”), giving the water its distinctive turquoise color. The surrounding mountains are composed of the Skagit Gneiss and Skagit Migmatite (Precambrian to Paleozoic metamorphic rocks — some of the oldest rocks in the Cascades), intruded by Cretaceous granodiorite (the light-colored intrusive rock of the high Cascade peaks); the glaciation of these rocks produces the specific mineral flour that colors the lake.

Diablo Lake’s ecological character is defined by the interaction between the hydroelectric reservoir system (Gorge, Diablo, and Ross dams regulate the Skagit River flow throughout the valley) and the surrounding old-growth forest and glacier ecosystem. The old-growth western red cedar, Douglas-fir, and western hemlock forest of the Colonial Creek drainage — accessible on the Diablo Lake Trail — is one of the finest intact old-growth Cascade forest stands in Washington State; the trees reach extraordinary dimensions (western red cedars exceeding 10 feet in diameter, Douglas-firs over 200 feet tall) in the wet, cold climate of the North Cascades. The glaciers that color Diablo Lake are retreating rapidly due to climate change — the Colonial and Snowfield glaciers have lost significant volume since the mid-20th century; the lake’s turquoise color will diminish as the glaciers shrink and the glacier-flour supply decreases.

Diablo Lake holds an iconic place in the visual culture of the Pacific Northwest — the turquoise lake against the dark-green forested canyon walls and the gray granite peaks is one of the most widely reproduced and most immediately recognizable landscapes of Washington State; the Diablo Lake Overlook view has been photographed millions of times and is the image most associated with the “hidden gems of the North Cascades Highway” travel narrative. The Skagit Hydroelectric Project (the three dams and their associated infrastructure) represents one of the most significant public-utility achievements in Pacific Northwest history; Seattle’s growth as a major city was substantially enabled by the Skagit River power supply. Diablo Lake is a cherished natural and engineering icon of Washington.

Diablo Lake is accessible from the North Cascades Highway (WA-20), approximately 125 miles east of Seattle via I-5 north to Burlington then WA-20 east through the North Cascades. The Diablo Lake Overlook pull-off is on the north side of WA-20 approximately 4 miles east of the Diablo Dam (well-signed; large parking area; the overlook is immediately adjacent to the road). The Colonial Creek Campground (NPS, on the south shore of Diablo Lake) is approximately 2 miles east of the overlook on WA-20. The Diablo Townsite (Seattle City Light; canoe rentals; restrooms) is accessible from the Diablo Dam road. WA-20 is typically open from late April through November (check WSDOT for current status). Winthrop and Twisp (east side) and Burlington and Sedro-Woolley (west side) are the gateway towns with full services. No entrance fee for Ross Lake NRA.

The National Park Service manages Ross Lake National Recreation Area; Seattle City Light operates the dams. The North Cascades Complex (including North Cascades NP, Ross Lake NRA, and Lake Chelan NRA) is one of the least-visited NPS units in the western United States, a fact that contributes to its exceptional ecological integrity; support visitation patterns that distribute pressure throughout the complex rather than concentrating it at the most popular spots. Camp only in designated campgrounds (the old-growth forest floor is extremely sensitive; no dispersed camping in the old-growth areas). Black-bear food storage (hang food 12 feet high and 6 feet from the tree trunk, or use a certified bear canister) is required throughout the NRA. The glaciers coloring Diablo Lake are retreating due to climate change; their loss will alter the lake’s iconic color permanently.

No entrance fee for Ross Lake NRA. Camping at Colonial Creek Campground: NPS fee; reserve at recreation.gov in peak season (fills quickly for summer weekends). No swimming in Diablo Lake (enforced; water temperature hazard). Boating on Diablo Lake: motorized boats must follow speed limits; canoe and kayak rentals from Seattle City Light Diablo Townsite (summer only; check for current availability and pricing). Black-bear food storage required throughout the NRA (bear canisters or use provided bear boxes). No campfires in designated areas (check current fire restrictions for the North Cascades Complex). Washington State fishing license required for fishing. Pack out all trash.

Ross Lake (North Cascades NP; accessible by water taxi from the Diablo Townsite — a 24-mile reservoir stretching to the Canadian border with remote backcountry campsites accessible only by boat or trail; one of the finest remote lake-wilderness experiences in Washington), North Cascades National Park (the park unit proper — accessible by trail from the highway corridor; the Cascade Pass Trail is the finest single-day hike in the park), the Methow Valley (east of the North Cascades Highway via WA-20 — a sunny, dry valley with the charming Western-themed towns of Winthrop and Twisp, excellent cross-country skiing at Methow Valley Ski Trails, and the finest road-cycling in eastern Washington), Newhalem (the Seattle City Light company town just west of Diablo, with the North Cascades Visitor Center and the Skagit Gorge Trail), and the Skagit River Bald Eagle viewing area (along WA-20 west of Newhalem in December and January) define the region.

Visit the Diablo Lake Overlook at sunrise (or in the last 90 minutes before sunset) for the finest light on the turquoise water — the morning sun striking the glacier-flour-colored lake from a low angle, with the Colonial Creek drainage and the glacier-capped peaks in the background, is one of the most photographed landscapes in the Pacific Northwest for excellent reason. Rent a canoe from the Seattle City Light facility in Diablo Townsite and paddle the turquoise lake in the morning (before the afternoon valley winds create whitecaps); the experience of being on the turquoise water — the glacier-peak reflections, the old-growth cedar forest on the shoreline, and the extraordinary color of the water beneath the canoe — is the finest way to experience Diablo Lake. Check the WA-20 road status at wsdot.wa.gov immediately before any North Cascades trip; the highway can close without warning for rockfall or snow.