Rimrock Lake
Drainage: East Rosebud
Approaches: Approximately 11 miles round trip from the East Rosebud trailhead, with roughly 1,400 feet of climbing.
Elevation: 7,603 feet
Fish: Rainbow trout hybrids, common and self-sustaining. Montana FWP.
Coordinates: 45.14494, -109.68671
Bugs: Not a problem in August.
Getting there
Rimrock is the third lake in the drainage, counting East Rosebud Lake at the trailhead and Elk above it. FWP counts only what you pass after leaving the car and calls it the second lake on the main trail. Either way, it is where the East Rosebud stops being an easy walk.
FWP puts it at 6 miles from the trailhead. Our track read closer to 5.6 to the shore, so call it somewhere in there and plan on a real day either way.
The work is in the middle. Our GPS had Elk at 6,722 feet and Rimrock at 7,602, so the trail picks up roughly 880 feet in the stretch between them after loafing along for the first three miles. FWP's description of this section is short and accurate: the valley is narrow and steep here.
We reached it in the late afternoon carrying overnight packs, better than five hours out from the car, and were still moving past it toward Rainbow. As a day-hike destination it is a full outing, not a stroll.
The hike
After Elk, Rimrock is a relief. This is an actual lake. FWP measures it at 33.6 acres and 25 feet at the deepest, more than four times Elk's surface and five times its depth, held in a tight steep-sided pocket of the valley.
We swam it. At 7,603 feet in late August that is a short, bracing swim and a good one.
The fish here are the interesting part. FWP describes the population as colorful hybrids, a cross of golden trout, rainbow and Yellowstone cutthroat, and their sampling backs it up: 21 fish in 2015 averaging 10.3 inches with the largest at 15.7, and 13 fish in 2007 averaging 10.1. It has not been stocked since 1948, so everything swimming there now got there on its own. The crew also logged an otter in the lake in 2015, which tells you something about how well the fishery is doing.
Tips & safety
Navigation: No route-finding. Trail #15 the whole way, third lake in the drainage and the second one you walk past. The climb between Elk and Rimrock is the steepest ground on the lower trail.
Camping: FWP is blunt about this one, saying there is no firewood and no good places to legally camp. We did find one established site just off the trail near the lake, and there may be more back from it. Take FWP's warning seriously and know the current wilderness camping regulations before you count on staying here, because the tight valley leaves very little ground that works.
Fishing: Self-sustaining hybrids running about 10 inches on average, with fish past 15 inches in FWP's nets. No stocking since 1948.
Bugs: Nothing that stood out.
Swimming: Deep and cold, and worth it if you have the day for it.
