The upper portion of this route is referred to as the SE Gulley in Rocky Mountain National Park, The Complete Hiking Guide.
The rugged south side of Hallett Peak and the glaciated west slope and long northeast ridge of Otis Peak form the beautiful cirque that define Chaos Canyon, a secluded gorge that receives few visitors. Chaos Canyon and the route described herein can be accessed from the Glacier Gorge TH or, preferably, the Bear Lake TH in Rocky Mountain National Park.
Bear Lake Trailhead
This trailhead is at 9,450 feet and provides access to the Lake Haiyaha Trail, used to access Chaos Canyon, and the Emerald Lake Trail, which you will use on your return journey. From the Beaver Meadows Visitor Center on US 36 just west of Estes Park, drive 1.1 miles to the entrance station. continue on 36 and turn south (left) at the first road toward Bear Lake. Drive 9.2 miles to the large Bear Lake parking lot.
Keep in mind that this route takes you up a steep couloir, and a descent of this couloir is not recommended. Your exit depends on going over Hallett Peak and either to the Flattop Mountan Trail or down the Tyndall Glacier, as described herein. As always, be prepared.
From the parking lot, pass the ranger station and begin hiking on the asphalt trail toward Bear Lake. It gets confusing here, as you need to locate the trail to Lake Haiyaha. Basically, follow the Emerald Lake Trail for 1 mile to a junction with the Lake Haiyaha Trail. Turn southwest (left) and follow the Lake Haiyaha Trail as it climbs through the forest and traverses the eastern flank of Hallet Peak (although you won't know this is Hallett Peak). If the trail is snow-covered, which is typical through June, follow the path of least resistance toward the southwest.
Another option is to hike to Bear Lake and then immediately head southwest (left) through the forest, rather than hiking to the right around the lake. This shortcut saves about 1/4 mile but you must find your way around (or over) some cliffs. Generally, just stay south of Hallett's east ridge. Lake Haiyaha is southwest of Bear Lake. Use common sense. Although this is only a mile or so, do not attempt this without a map and compass.
A note: Chaos Canyon and Tyndall Gorge look very similar. They are both forested gorges with steep mountains, pretty lakes, and high glaciers. It's easy to get confused.
Arrive at Lake Haiyaha on its northeast slopes, which offers great views from elevated boulders. Now, climb through these huge boulders toward the southern shores and make your way to the westernmost tip of the lake. More huge boulders confront you, but don't worry. They are easy and fun to scramble over. From here, look to the northwest and gaze upon your destination, the only major couloir on Hallett's SE ridge that faces into the gorge, just west and north of Lake Haiyaha. Behind you, the blocky west face of Longs Peak graces the skyline. At the end of the gorge, the steep Chaotic Glacier prevents simple escape. This glacier is about a 60 degree incline and rated AI2 Steep Snow. Think twice about attempting this.
Chaos Couloir is an unoffically named couloir that offers a relatively steep snow climb with some exposed rock about halfway up. It reminds me of Flattop's Dragon's Tail couloir. The couloir starts at about 10,700 feet and is wide at the base. It narrows as it goes up with steep cliffs on both sides. You are exposed to potential rock fall in warmer temperatures. Keep on the right side of the couloir as you ascend. There is about 600 feet of climbing.
About halfway up, you reach the rock step. Be prepared for a few difficult and very exposed moves to get up and over the rock. The rock step is about 30-40 vertical feet before more steep snow takes you out of harm's way from the rock fall hazard. The couloir seems to go on and on and on, as it widens again at the top and continues to ascend a broad talus field toward Hallet's southeast ridge. This ridge is exposed to high winds and thunderstorms. There is no quick evacuation route, so be certain a large window is open for the ridge traverse and summit climb. Hallett Peak is the rounded mass to the west. Climb it for about 300 vertical feet. It may be snow covered, and this climb is an ascending traverse of 30-40 degree, north-facing snow. Be certain it is stable, because the drop-off into Tyndall Gorge is huge. Although it won't appear large as you climb, the ridge finally finishes at a rather large summit, from which you can actually see Grand Lake (not to mention everything else in Colorado). Tyndall Glacier is to the north and obvious from here, so head towards it. You will probably see a few hikers on the expanse of the Flattop Mountain Trail. If you descend via this trail, the way is quite obvious. Don't get blown away by the gale-force winds.
Once at the glacier, pick your line of descent. The northern aspects (skier's right) are the steepest, the southern aspect (skier's left) is the mildest at about 35 degrees. Be wary of the crevasses at the top of the glacier and walk, ski, or glissade down the glacier. On June 5th, there was evidence of two recent avalanches on the steeper sections of this glacier. Basically, getting down from here is by way of the path of least resistance, just be cautious of avalanche and rock fall hazards, primarily from the north facing slopes of Hallett (on your right). Enjoy the rest of your hike to Emerald Lake and then follow the Emerald Lake Trail back to the trailhead. Total distance is about 6.5 miles.