Letts Lake (named Upper Letts Lake on federal maps and on the Forest Service's dam repair records) is a 35-acre reservoir in the Mendocino National Forest, managed by the Grindstone Ranger District as part of the Stonyford Recreation Area. It sits at roughly 4,500 feet in western Colusa County, ringed by ponderosa pine and mixed conifer, with views of higher ridges that often hold snow into spring. The Forest Service lists swimming, trout fishing and non-motorized boating as the main uses of the lake.

The lake is about 19 miles west of Stonyford by Forest Road M10. Letts Lake Campground wraps around the north and east shores in four loops (Main Letts, Saddle Camp, Spillway and Stirrup), and its roads handle trailers up to 30 feet. Day visitors can also use Big Springs Picnic Ground across the water, reached by a side road about a quarter mile past the Letts Lake turnoff. Parking there is limited, and there is a separate small lot by the fishing pier on the east shore.

The usual place to get in the water is the small beach on the north shore, reached by a short paved spur off the campground's north road between the Stirrup and Spillway loops. Because motors are banned, the only boats you'll share the lake with are canoes, kayaks and float tubes. There are no lifeguards. The Forest Service doesn't publish depth or water temperature figures, but at this elevation expect the water to stay cool into early summer. Grass and reeds line much of the shore, so the beach and the trail edges are the easiest places to wade in.

Swimming and day use are free. Big Springs Picnic Ground charges no fee and is open 6 a.m. to 10 p.m. It has vault toilets and picnic tables but no drinking water. Camping costs $12 per night from Memorial Day to Labor Day and $6 per night off-season. All 42 sites are first-come, first-served, paid at the kiosk or with the Recreation.gov Scan and Pay app. Check-in is 2 p.m., check-out 11 a.m., and quiet hours run 10 p.m. to 6 a.m. Drinking water is only turned on from May through September, and trash pickup is summer-only.

Check road conditions before you go. Forest Road M10 between Fouts Springs and the Letts Lake turnoff has had weekday closures for slide repair, so look at the Mendocino National Forest alerts page first. The basin burned in the 2018 Ranch Fire. The Forest Service has since repaired the damaged dam and reforested around the campground, but it still warns of downed trees, dropped limbs and stump holes, and asks visitors not to park under dead standing trees. Water and trash service end in late September, when the campground's back loop also closes. The day use area and front loop stay open through winter when snow allows. For current conditions, call the Stonyford Work Center at 530-963-3128.

The one-mile Letts Lake Loop Trail (the Basil 'Doc' Raines Memorial Foot Trail) circles the shore, with a paved, accessible section on the east side leading to a fully accessible fishing pier. The lake is stocked with trout and also holds bass. Anglers 16 and older need a California fishing license. Mill Valley Campground is a mile away, the area's OHV trail network starts nearby (riding is not allowed in the campground itself), and trailheads into the Snow Mountain Wilderness are a short drive away.