Loon Lake sits in a fold of the Oregon Coast Range about 20 miles east-southeast of Reedsport, and it exists because a hillside gave way. Roughly 1,400 years ago a slab of sandstone broke loose and slid into the Lake Creek valley, piling up blocks and boulders deep enough to plug the drainage. The water backed up behind the rubble and never left. What you swim in is a natural landslide dam still doing its job: a lake two miles long, about four-tenths of a mile across at its widest, and close to 190 feet deep in the middle. Lake Creek feeds it, and Mill Creek drains it north toward the Umpqua River and the Pacific.

The Bureau of Land Management's Coos Bay District runs the day use area at the lake's northwest corner, down by the outlet. This is the public side of Loon Lake, and it is not the same place as the privately run lodge and RV park about two miles along the road on the east shore. The BLM entrance is marked by a brown sign set among boulders on Loon Lake Road, with a staffed booth and a self-service fee station just inside. Day use costs $5 per vehicle, or $3 if you walk in, plus $3 to launch a boat, and runs 8 a.m. to 8 p.m. The season is Memorial Day weekend through September 30.

The beach is the reason to come. It is a real stretch of sand, uncommon in this part of the Coast Range, and the water off it warms up well by midsummer. A buoy line marks the designated swimming area, which keeps boat traffic out and gives kids an obvious boundary. Dogs and glass containers are not allowed on the swim beach, though there is a separate dog day-use stretch close by. Picnic tables and grills sit back in the shade of Douglas-fir, bigleaf maple and red alder, and an accessible fishing dock, a boat ramp, horseshoe pits, a playground and a small amphitheater are spread along the shoreline.

When you have had enough of the water, a quarter-mile trail leaves the south end of the day use area and climbs through old growth to a waterfall on Elliott State Forest land. Ospreys work the lake most summer afternoons, and Steller's jays are everywhere in the campground, which has 53 reservable sites behind the beach. Two things are worth knowing before you drive in. There is no cell service anywhere at the lake, and the site was closed from 2019 until a snowstorm rebuild was finished, so older write-ups may describe facilities that are gone. The way in is 13 miles east on OR-38 from Reedsport, then 7 miles south on Loon Lake Road.