Diamond Lake sits in the Umpqua National Forest at 5,138 feet, a 3,000-acre oval of water flanked on the east by Mount Thielsen — the spired lightning rod of the Cascades at 9,182 feet — and on the west by Mount Bailey, a broad shield volcano that provides the gentler counterpoint. The combination of lake and peak is one of the more composed landscapes I've found in the Oregon mountains.
The lake is the most productive trout fishery in Oregon by most measures, stocked annually with 300,000 rainbow trout. Anglers arrive in numbers throughout the fishing season, which runs from late April through October. The trout can be caught from the shore in several locations, from the campground banks, or from boats launched at the Thielsen View boat ramp.
Three national forest campgrounds ring the lake — Diamond Lake, Thielsen View, and Broken Arrow — with 450 combined sites. Thielsen View on the western shore has the most dramatic mountain view: you're looking directly at Mount Thielsen's spired summit across the water. The paved 11.6-mile bicycle path circles the entire lake and is one of the better easy cycling routes in the Oregon Cascades.
Diamond Lake is also the gateway to Crater Lake National Park, 25 miles to the south on Highway 138. Most visitors come through here on the way to or from Crater Lake, and most of them drive past too quickly. The lake earns a longer stop. In the evening, when the light drops on Thielsen's spire and reflects off the water, it's very clear why people camp here rather than just passing through.