Overview
The Chasm Lake Trail reaches one of the most dramatic mountain settings in Colorado: a glacially carved cirque at 12,760 feet directly below the Diamond, the 1,700-foot vertical east face of Longs Peak (14,259 ft). The lake sits in a bowl of granite, surrounded on three sides by cliffs and permanent snowfields. On a clear day, looking up at the Diamond from the lake shore is arresting - a wall of rock that fills your entire field of view above.
The trail is 8.4 miles round trip with 2,350 feet of gain. It shares its lower section with the Longs Peak summit route, one of the most popular 14er approaches in Colorado, before diverging at the Chasm Lake cutoff about 3 miles from the trailhead. Because most Longs Peak summit hikers are focused on the summit and not on the lake, Chasm Lake receives notably fewer visitors than the full summit route despite sitting in the same corridor.
This trail is located within Rocky Mountain National Park, not within Arapaho-Roosevelt National Forest, though it sits at the forest's northern boundary and is included here as the northern gateway to the Indian Peaks corridor. RMNP rules apply: no dogs, timed-entry vehicle permits required during peak season, and the standard national park entrance fee or America the Beautiful pass. See our national forests vs. national parks guide for context on the differences in management between these two designations.
The Route
Miles 0 to 1.5: Longs Peak Trailhead to forest edge. The trail leaves Longs Peak Trailhead (at 9,400 ft) heading southwest through lodgepole pine and Engelmann spruce forest. The grade is consistent and moderate. The paved trailhead area gives way quickly to natural trail surface. Early morning departures in July and August put you in the forest during the cooler, quieter pre-dawn hours when the Milky Way is visible above the peaks ahead.
Miles 1.5 to 3.0: Goblin's Forest and Timberline Junction. The trail climbs through the krummholz transition zone - the gnarled, wind-shaped trees that mark the upper limit of forest growth at around 11,000 feet. Timberline Junction is signed and marks where the trail opens into alpine tundra with views of the Longs Peak-Mount Meeker massif ahead. The terrain above is open and exposed.
Miles 3.0 to 3.5: Chasm Lake Junction. Watch for the signed junction for the Chasm Lake spur trail. Summit hikers continue straight; Chasm Lake is to the left (southeast). If you miss this junction and continue toward the Boulderfield, you're on the Longs Peak summit route.
Miles 3.5 to 4.2: Chasm Lake Approach. The spur trail climbs steeply toward the lake, gaining the remaining elevation across rocky tundra and granite slabs. A short scramble (using hands in one section on the rocky approach ledges) leads to the lake basin. The lake appears suddenly as you crest the final shelf. The Diamond rises directly above. The snow and ice at the base of the wall often persist through summer. The cirque is sheltered but cold and windy.
At the lake: The basin is exposed and often windy. The noise of water draining through the talus and the sound of rock fall from the cliff above are common. Give climbers on the Diamond a wide berth below - rock fall is a hazard in the cirque.
Descent: Return the same route. The spur trail junction back to the main Longs Peak trail is clearly signed.
Permits and Fees
Rocky Mountain National Park timed-entry vehicle permit: Required from late May through mid-October for the Longs Peak area, typically covering 5 AM to 6 PM. Permits are free and reserved through Recreation.gov. They go quickly - book well in advance for summer weekends. The entrance fee to RMNP ($35 per vehicle, as of 2026) is also required. An America the Beautiful pass covers the entrance fee.
Verify all current permit requirements at nps.gov/romo before your trip. The RMNP timed-entry system has changed year to year since its introduction, and current requirements may differ from prior seasons.
No additional permit is required for a day hike to Chasm Lake beyond the RMNP entrance fee and the timed-entry vehicle permit.
When to Hike
Mid-July through September is the main window. The approach above Timberline Junction is exposed and can be snow-covered and icy into early July. The Chasm Lake cirque holds snow well into July and may not be fully accessible until mid-month in heavy snow years.
Start very early. The standard advice for Longs Peak area hikes is a 3 to 5 AM departure. Reasons: (1) the timed-entry permit applies after 5 AM, so arriving before sunrise allows trailhead access before the permit window opens; (2) afternoon thunderstorms above treeline are a serious hazard, and the Chasm Lake approach is fully exposed for miles; (3) parking at Longs Peak Trailhead fills rapidly on summer mornings. Most hikers aiming for Chasm Lake leave the trailhead well before dawn.
September: Excellent month. Crowds are lower, morning weather is often stable, and the light on the Diamond at sunrise is exceptional. Temperatures at the lake in September can be in the 30s even in morning sun.
What to Bring
Water: Carry water from the trailhead. Alpine streams are available for filtering on the lower approach, but above Timberline Junction sources are limited. Many hikers bring 2 to 3 liters; individual needs vary with pace and conditions.
Layers: Chasm Lake cirque is cold and frequently windy even in July and August. A wind shell, insulating layer, and gloves are worth carrying.
Headlamp: An early start (before sunrise) requires a headlamp on the approach through the forest section.
Navigation: Download an offline map. No cell service at the trailhead or on the trail. The Chasm Lake junction is signed but easy to miss in low light.
Trekking poles: Useful for the rocky approach to the lake and for the descent.
Sun protection: UV intensity at 12,000-plus feet is high. Sunscreen, sunglasses, and a hat.
Altitude Considerations
The trailhead is at 9,400 feet, and Chasm Lake is at 12,760 feet. The altitude gain over the hike is substantial, and the thin air at these elevations affects pace and effort significantly. Visitors arriving from low elevations should allow time to acclimate at intermediate altitudes. Individual responses to altitude vary; symptoms such as headache, nausea, and shortness of breath can occur. Consult a physician before planning high-altitude trips if you have cardiovascular or respiratory concerns.
Trailhead Access
Longs Peak Trailhead is 11 miles south of Estes Park on CO-7, a scenic highway that runs along the eastern boundary of Rocky Mountain National Park. From Denver, the drive to Estes Park is about 90 minutes (65 miles via US-36 through Boulder), then 20 minutes south to the trailhead. Total from Denver: approximately 1 hour 50 minutes. From Boulder: about 1 hour 15 minutes.
The trailhead parking area has vault toilets, an information kiosk, and a ranger station (staffed seasonally). The lot is large but fills before dawn on summer weekends. There is no cell service at the trailhead.
For those exploring the broader Indian Peaks region, the Arapaho and Roosevelt National Forests trailheads at Brainard Lake are about 30 miles south of the Longs Peak Trailhead on CO-7 through Allenspark and Ward, offering a full range of alpine hiking options within the national forest system.