From Bend, OR, take the Cascade Lakes Highway 27 miles west. About 6.5 miles west of Mt. Bachelor Ski Area turn left into the Devils Lake Campground. Park at the end of the camp loop. Make sure you start out on the South Sister Climber's Trail and not the more obvious trail that ducks through a culvert under the highway. The correct trail goes over a footbridge just before crossing the highway. Elevation here is about 5450. The first 1.5 miles are uphill through a forest. You emerge from the trees and there is a 4-way trail junction. Go straight ahead and the next part of the hike is nice and easy through thin trees and open tableland. There is a spot where the trail forks but the paths rejoin soon after. To the right of the trail is the Moraine Lake Valley and at the head of the valley a trail joins from below.
From the head of the Moraine Lake Valley the path goes a bit left and starts climbing up a steep rocky path. After 1.2 miles you will come to a sandy saddle at the base of the Lewis Glacier and a small cirque lake at about 8850 feet. This is also the point where a climbers trail joins from the right from Green Lakes. From here, follow the climbers path along the ridge to the left. It gets even steeper here but it's only a bit over a mile to the lip of the crater up a huge cinder ridge. Once at the crater rim, follow it around to the right to the high point with a benchmark but no climbers' register. You can walk around the entire crater rim for incredible views in all directions. Descend the same route. Total distance is 5.75 miles to the summit with an elevation gain of just under 5000 feet.
None needed in the summer but bring plenty of water (or bring a filter/purifier and fill up at the small lake at the base of the Lewis Glacier). For snow ascents you may want ice axe and crampons.
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