Diablo Lake is the colour that nobody believes in photographs. The water is a milky, luminous turquoise, as though someone has dissolved a gemstone in it, and it is not a trick of the light. Glaciers on the surrounding peaks grind the rock beneath them into a fine powder, rock flour, which washes down in meltwater and stays suspended in the lake, scattering blue and green light back to the surface. The effect is strongest in late summer, when the melt is at its peak and the sun is high.
The lake is a reservoir, formed by Diablo Dam on the Skagit River, built by Seattle City Light in the late 1920s to produce electricity for the city. The company ran the project, building a company town at Newhalem and an incline railway to haul equipment up the mountain, and it still runs tours of the dams. The construction was a tremendous effort in this rugged, steep valley. Above the lake, the Skagit climbs further to Ross Lake, an even more remote body of water that reaches north to the Canadian border.
State Route 20, the North Cascades Highway, was finished in 1972 after decades of argument, and it passes the lake at an overlook that is among the most popular stops on the route. The road here is easy: a wide, smooth, well-engineered highway climbing in sweeping bends through forest, with great views of jagged peaks, hanging glaciers and cascades of water down the cliffs. It is a motorcyclist's road, and a slow one thanks to the speed limit and the number of stopping places. The pass is closed in winter by snow, usually from about November until April or May, and conditions can change quickly in the mountains. Rain is common on the western side, and cool mist drifts up the valley in the morning.
Marblemount, on the west side, is the last place with petrol, and Winthrop on the east has the next, a gap of around ninety miles, so fill up before you start. Newhalem has a small store and a visitor centre, and a campground sits nearby. Take warm clothing, and watch for deer and the occasional bear.