Mirror Lake (Boulder)
Drainage: Boulder
Approaches: Approximately 21.5 miles round trip on Upsidedown Creek Trail #26, with roughly 4,100 feet of climbing. The last mile is off trail.
Elevation: 9,784 feet
Fish: Rainbow trout, common and self-sustaining. Yellowstone cutthroat, common and stocked with some natural reproduction. Montana FWP.
Coordinates: 45.27744, -110.14900
Bugs: Mosquitoes, just bad enough to be annoying.
Getting there
On the Lake Plateau in the headwaters of Rainbow Creek, reached on USFS Trail #26. Ten and three quarter miles from the Upsidedown trailhead, nine and three quarters of that on trail.
We came at it across the plateau from a camp at Wounded Man, by way of Diaphanous, Owl, Pipit, the Rainbow chain and Squeeze. Nine and a half hours of walking, and worth every hour.
The hike
This was the best camping of the trip and my favourite spot on the plateau. FWP puts the sites near the outlet with some firewood nearby, and that is where we stayed.
Mirror is 16.4 acres and 80 feet at the deepest, with a 34-foot average. That makes it far and away the deepest water on the plateau — Wounded Man is 55 and nothing else comes close.
The fishery is the middle of an experiment. In 2011 FWP began stocking Yellowstone cutthroat at high density, deliberately, to try to swamp out the rainbows. They put 11,750 in during 2018 and the same again in 2019.
It has not worked, or not yet. Their 2025 sampling with nets and a rod caught rainbows, cutthroat and a few hybrids together. The numbers tell it plainly: 10 rainbows to 3 cutthroat in 2025, and in 2021, 50 cutthroat to 14 rainbows. The rainbows are self-sustaining and the cutthroat need restocking.
Fish run 9 to 11 inches on average, with some to 14.
Tips & safety
Navigation: Trail #26 from Upsidedown, off trail the last mile. Coming across the plateau it sits west of the Rainbow chain.
Camping: The best on the plateau. Sites near the outlet with firewood nearby.
Fishing: Rainbows and cutthroat together, plus hybrids, mostly 9 to 11 inches with fish to 14.
Bugs: Mosquitoes, annoying rather than bad.
Depth: 80 feet, which is deeper than anything else up here by a wide margin.
