Thunder Knob Trail

Thunder Knob is an out-and-back from Colonial Creek to a rounded knoll above Diablo Lake. Plan around 3.8 miles round trip from the signed campground access, following the main trail to the summit area and back. Published estimates range from about 3.3 to 3.8 miles; use the longer figure for planning. Allow about two and a half hours, with time to sit at the benches. You get a good stretch of forest and a proper climb, but the payoff is that wide lake-and-peak view without committing to a high-country pass.

From the signed north-side access, the route passes through the Colonial Creek campground to several seasonal footbridges over creek branches. Soon you reach a broad, open fan of stones and woody debris. Floods in 2003 and 2006 shifted Colonial Creek away from this old channel, and the trail now crosses the former creek bed. The footing is loose and uneven here. Cairns mark the route across the rock, so slow down and keep an eye out for them rather than cutting across the brush.

The route changes quickly after the creek fan: shady, moist forest replaces the exposed rock. Moss covers the boulders, and a small clear creek crosses the trail under Douglas fir, western redcedar and western hemlock. White dogwood flowers brighten the woods in spring; vine maple turns orange in fall. It’s a welcome cool stretch before the climb opens up again.

At about a mile, the trail reaches a sunny clearing with a bench and a view of snowy Colonial Peak. From there it climbs through knobby, dry ground with lodgepole pines and low kinnickinnick, then skirts a small pond and a marsh before the final gentle rise. The marsh can hold reflective water earlier in the season and dry later. This upper section is more open and sun-baked than the forest below.

At the knoll, several viewpoints and benches look over Diablo Lake and the surrounding peaks: Davis Peak to the west, Colonial and Pyramid peaks to the south, and Jack Mountain to the northeast. A short side spur reaches the lake-facing benches; return to the main tread after the view. The lake’s striking blue-green color comes from fine glacial rock flour suspended in the water. The color can look bluer earlier in the season and greener later. Diablo Dam, visible in the distance, was completed in 1930 and was then the world’s tallest dam. From the benches, leave a little forest or shoreline in the frame so the lake’s scale shows.

The access is at Colonial Creek off State Route 20 near milepost 130. The signed start is on the highway’s north side by the campground; park instructions also direct hikers to roadside spaces or the large lot beyond the Colonial Creek South entrance building. If you use the South lot, allow time to reach the signed north-side start on foot and follow the posted access signs. The route is hiker-only, and leashed pets are allowed. The seasonal footbridges may be removed outside the summer hiking season, so check the crossing before setting out. Bring water and snacks for the hike; campground drinking water and restrooms are available near the access.