Follow the Peru Creek Trailhead directions to the turnoff at 2.1 miles for Chihuahua Gulch Road 263. If you have 4WD with good clearance, you may continue north on the road, otherwise, you may as well park in the vicinity of the turnoff. We found the road in mid-July to have numerous potholes and muddy spots. The road heads generally north, gaining about 500 feet in elevation to where the Chihuahua Basin levels out for a while. At Pt. 10,841 the road crosses to the west side of the creek and continues north, skirting the marsh and willows and staying just inside the trees. There's a possible campsite just before this first creek crossing. Then the road crosses abruptly back to the east side of the creek. It continues north skirting the edge of forest and comes to an end at 1.9 miles from the Peru Creek road at what the Forest Service describes as an old "buck and rail" fence. We think that's at an intersections at 11,155 ft., just by a pair of ponds. The right fork heads up into Ruby Basin, on the north flank of Cooper and Ruby Mtn. The left fork continues more as a trail to Chihuahua Lake at 12,180 ft. This lake is unnamed on the USGS map. Coordinates for the intersection are: N 39° 37' 09.25" W 105° 50' 23.04". If using 4WD, you must park in the vicinity. Continue walking north up the road/trail, then depart the road on the left at these coordinates: N 39° 37' 20.39" W 105° 50' 24.55". Work your way in a northerly direction uphill through open forest, then make a turn to the west to cross a more open slope of tundra and boulders. Cross the slope walking generally west to reach a bench-like area of open tundra and willows. You should come out on the north side of the middle SE ridge of Lewanee.
At about 12,000 ft., gain the ridge and begin following it upward. The next 400 feet of ridge offers some real entertainment with some Class 2+ scrambling on tilted slabs of rock and great blocks to work around, up or over. Some may feel this reaches brief Class 3 scrambling in places. At one point, just on the north side of the ridge crest, we followed a ramp-like fissure for some distance. The going was steep, but it never feels too exposed and offers a great opportunity for some dynamic photos. At about 12,400 ft., the sharp ridge moderates and gives way to a wider, tundra-covered ridge with large blocks of rock to work around. Watch for some nice displays of Old-Man-On-the-Mountain and some primrose. The remaining Class 2 ridge continues on mostly tundra until a couple hundred feet below the summit where it turns to broken rubble/talus. When we climbed this in mid-July, the summit was still capped by a large snowbank that presented a steeper face we had to kick-step up and over. Ice axe was handy for this short section.
The view from the top of Lewanee is a nice one. To the east, you have a dramatic view of Grays and Torreys and can survey the ridge off Torreys over to Grizzly. All of Chihuahua Gulch lies below to the east and to the west, you can gaze down upon A-Basin Ski Area. The tundra-covered summit holds some nice flowers as well. It should be noted that Lewanee also has a southern summit. Technically that summit could be as high as 13,199 ft., making it only 5 feet lower than the marked, northern summit. When you're standing on the north summit looking over to the south one, it "appears" to be as high. It has an extrapolated elevation of 13,180 ft. The connecting ridge goes at Class 3 according to some reports and can require some time to navigate. We have not done it ourselves so have provided a link below to a useful report.
To descend, we suggest altering your descent route a little to enjoy a pristine little tarn. Head back down as you came, but about 600 feet down the ridge, turn south and find a route to drop down into the cirque-like basin that lies between the two Lewanee summits and faces east. At 11,860 ft., there is a small, picturesque tarn. The basin behind it between the two summits has classic, glaciated, cirque-like features. In places it is quite rugged sporting some nice cliffs and crags and steep couloirs filled with snow. At the tarn, you can enjoy a great view of Grays & Torreys and also Cooper and Ruby to the east. A beautiful, crystal-clear little rivulet of a stream cascades and meanders into the tarn from above and there is some lush grass and more wildflowers tucked in between large boulders nearby.
From the tarn, find a route down through willows and then forest to reach the Chihuahua Gulch road. There could be any number of ways to do this. Our route that shows on the Google Earth image is only an approximation. You may want to consider swinging/contouring back north and trying to rejoin your ascent route from earlier in the day at the bench-like area. Return to wherever you left your vehicle. We completed this hike in about 5.5 hours from the intersection of the Peru Creek Road and the Chihuahua Gulch Road.
Possible alternate Route: While driving up the Peru Creek road, before arriving at the turnoff for Chihuahua Gulch road, there is a trail that takes off on the north side of the road. This trail heads generally north along the side of the massive SSE ridge of Lewanee Mountain. This ridge terminates at the southern summit. On the USGS map, the trail is named the "Argentine-North Fork" trail. On the White Rive NF map, it's trail #34 and leads past the American Eagle Mine. On some internet sites, it's referred to as the "Lewanee Trail." This trail leads over to the top of one of the lifts that operates in A-Basin. If you follow the trail the entire way to the lift, you would then need to follow the ridge NE then east to the south summit of Lewanee. From there, you would need to navigate the 3rd class connecting ridge over to the north summit for the official finish. See the link provided below for a description of this connecting ridge.
Links to other information, routes & trip reports for this peak that may be helpful.