8 very adventurous loop runs & hikes in the Colorado Rockies

For the first time since 2013, this year I didn’t race and I won’t log at least 3,000 miles. Nagging injuries, lack of race entry luck, and a growing guiding program all played a role, as did an absence of desire — whatever I’d been running towards in my mid-30’s, I felt like I’d reached it.

Somewhat ironically, however, this year had a disproportionate share of favorite and memorable trail runs — out of my all-time Top 10-ish, this year scored five, which on this page I’ll highlight (and recommend three more). Since I wasn’t on a strict training plan or beat up from the last race, I was free to run where, when, and how far I pleased, and to do it at whatever pace felt right.

The commonality: Pfiffner Traverse

Due to their proximity and quality, I focused my efforts on Rocky Mountain National Park and the Indian Peaks Wilderness, both in the Colorado Front Range. From Boulder, the east-side trailheads are about an hour away; the west side trailheads are 2.5 hours, but conveniently my work put me in Grand Lake for a few September days.

Intentionally, these loops included sections of the Pfiffner Traverse, which is a 76-mile high route that parallels the geographic Continental Divide between Milner Pass (on Trail Ridge Road in Rocky) and Berthoud Pass (on US-40 near the boundary of the James Peak Wilderness). The route is 40 percent off-trail, gains or loses 750 vertical feet per mile, and approaches Class 3 in difficulty. Four of the eight loops on this page are recommended section-hikes.

By including sections of the Pfiffner, I was able to edit my Pfiffner Traverse Guide, gather data for a potential FKT attempt, and elevate the difficulty of each outing, making them true adventure runs. But most importantly, the Pfiffner helped to create loops in a range that otherwise has very few.

1. Across Onahu

  • 17.2 miles (3.5 miles off-trail) with 5,000 feet of gain
  • +6.8 miles on Trail Ridge Road if you don’t shuttle
  • Strava (with Tonahutu TH finish)

The heart of this run is between Timber Lake and Haynach Lakes, which are separated by two 11,800-foot passes that provide access to and out of upper Onahu Creek. Onahu is a gem: it’s one of just two trail-less valleys on the west side of the park, features a rarely visited alpine lake, and serves as a quiet refuge for elk and moose.

Several options exist for reaching and returning from Timber Lake and Haynach Lakes. A shuttle or hitch is necessary for all of them.

The shortest variation of this run starts at Timber Lake Trailhead and finishes at Green Mountain Trailhead, which are separated by seven miles along Trail Ridge Road. Alternatively, start at Milner Pass (which is about the same distance but has less climbing) and/or finish at North Inlet Trailhead (+2.4 miles). The net vertical uphill/downhill difference between running northbound and southbound is generally negligible, unless you start at Milner Pass.

Timber Lake, as seen from Timber Lake Pass, looking north

2. Continental Divide Trail Loop

  • 25.8 miles (no off-trail) with 4,400 feet of gain
  • Strava

The west side of Rocky Mountain National Park has just one all-trail loop — this one. Because of that, it’s already popular with backpackers and trail runners, and is actually the only route on this page with an established FKT (4 hrs 2 min), at least currently.

The highlight of this loop is in the middle, the roughly seven miles between upper Tonahutu Creek and upper Hallet Creek. This section is entirely above treeline and hovers for several miles at around 12,000 feet while crossing Bighorn Flats. Early in the season, the waterfalls are also exceptional.

Start at the North Inlet/Tonahutu Creek Trailhead, and pick a direction. I prefer clockwise, so that the flat and boring 5.5-mile section along Big Meadows is at the start. If you have access to a shuttle or are willing to hitch, you can shortcut the route by 2.4 miles by starting or ending at Green Mountain Trailhead. Climb steadily to Ptarmigan Point (elev. 12,200+) and then descend into North Inlet and back to the trailhead.

A herd of cow elk at Sprague Pass, just off the Continental Divide Trail

3. North & East Inlets

  • 21.3 miles (3.3 miles off-trail) with 6,100 feet of gain
  • +1.4 miles without shuttle between North Inlet TH and East Inlet TH
  • Strava

The North Inlet and adjacent East Inlet both drain into Grand Lake, the largest natural lake in Colorado. Hiking trails go far up both watersheds, but strangely do not connect.

Fortuitously, however, these glacier-carved valleys can be connected with just 3.3 miles of off-trail travel, most of it along a Sound of Music-worthy alpine bench perched below the Ptarmigan-Andrews ridge and high above North Inlet. If you don’t see elk, you will surely smell them; the wildflower show usually peaks in July.

I strongly recommend running this loop clockwise, entering the park from North Inlet Trailhead and exiting at East Inlet Trailhead, so that you have gravity on your side for the light bushwhacking between Beak Pass and Spirit Lake.

The trailheads are separated by 1.4 road miles, which can be used as a warm-up or skipped with a shuttle or hitch.

Run up North Inlet, and take a right/south turn towards Lake Nokoni and Lake Nanita. When this trail terminates at the latter lake, follow the Pfiffner Traverse to Spirit Lake, where you will intersect an unmaintained use trail. Turn right/west and follow this track to the mapped end of the East Inlet Trail, which will take you back to the trailhead.

Wildflowers and Ptarmigan Mountain

4. Triple Bypass

  • 23.3 miles (3.1 miles off-trail) with 7,500 feet of gain
  • Strava

The popularity of the 28-mile Pawnee-Buchanan Loop (aka Double Bypass Loop) is nearly on par with the famed Aspen Four Pass Loop. It gains 8,000 vertical feet and crosses two 12,000-foot passes on the Continental Divide, making it a fantastic long run or weekend backpack trip.

Triple Bypass is an adventurous play on this Colorado classic. Rather than dropping to the 8,800-foot junction of the Buchanan Pass Trail and Cascade Creek Trail above Monarch Lake, it “shortcuts” the route by heading off-trail for three miles up a deep valley and over a Class 3 pass.

This loop can be run in either direction, starting and ending at Brainard Lake. But by doing it counterclockwise, you can follow the guidebook description for the off-trail section. Most hikers and runners will appreciate the hand-holding — the topography of Thunderbolt Creek is deceptively complex, and the the south side of Paiute Pass is exceptionally steep.

Thunderbolt Creek, as seen from the Ooh La La Extra Credit route, north of Buchanan Creek. Paiute Pass is at the head of the valley and is still snow-covered.

5. Boulder Watershed

  • 33.6 miles (4.7 miles off-trail) with 11,200 feet of gain
  • +4.3 miles on gravel road without shuttle
  • Strava

The Northeast Gully is the easiest non-trail exit from the iconic Lone Eagle Cirque. But it’s the most difficult feature on the Pfiffner Traverse: it’s filled with snow through midsummer, covered in loose rock and dirt in late-summer, and always steep. Before this fall’s first snowstorm, I wanted to get on this section of the Pfiffner (again), which connects Crater Lake with Coyote Park in upper Arapaho Creek.

Loops out of Monarch Lake would have been shorter (refer to the next recommended option), but I figured that encircling the City of Boulder Watershed wouldn’t take me much longer after accounting for drive times. Plus, an end-of-season big hurrah was appealing.

Hands down, this is the most demanding loop on this page. It’s the longest and has the most vertical change; it has the most passes (five), and some of the highest (four above 12,000 feet); its off-trail sections are not straightforward; and the Northeast Gully will excite even those already comfortable on technical terrain. It could be the ultimate test piece on the Front Range.

Start or finish at Rainbow Lakes Trailhead, and ideally hitch or shuttle the five miles of gravel between it and the Sourdough Trailhead, just off the Peak to Peak Highway. The loop can be done in both directions, with a slight advantage for counterclockwise (so that you can more follow the guidebook description out of the Lone Eagle Cirque).

From Sourdough Trailhead, run/hike:

  • Over Niwot Ridge to Brainard Lake,
  • Over Pawnee Pass to Cascade Creek Trail,
  • Over Northeast Gully to upper Arapaho Creek,
  • Over Arapaho Pass to the Fourth of July Mine, and
  • Over the east ridge of Arapaho Peak, past the Arapaho Glacier, back to Rainbow Lakes.
The Lost Tribe Lakes basin, as seen from its western edge at the top of the elk trail.

6. Monarch Lake Loops

  • Min: 16.0 miles (3.1 miles off-trail) with 4,800 feet of gain
  • Max: 32.3 miles (10.4 miles off-trail) with 11,600 of gain

Monarch Lake is the western gateway to the Indian Peaks Wilderness, and sits at the confluence of four major creeks that flow off the Continental Divide: Hell Canyon, Buchanan Creek, Cascade Creek, and Arapaho Creek.

The Pfiffner Traverse connects these drainages, using Cooper Peak Pass (moderate), Paiute Pass (hard), and the Northeast Gully (very hard). This area has six loop opportunities: each includes one pass, two adjacent passes, or all three passes. Select the itinerary that best suits your fitness, interests, and off-trail skills — or stay in the area for a few days and undertake several of them.

The six specific itineraries are:

  1. Out via Hell Canyon, over Cooper Peak Pass, return via Buchanan Creek;
  2. Out via Buchanan Creek, over Paiute Pass, return via Cascade Creek;
  3. Out via Cascade Creek, over Northeast Gully, return via Arapaho Creek;
  4. Out via Hell Canyon, over Cooper Peak and Paiute Passes, return via Cascade Creek;
  5. Out via Buchanan Creek, over Paiute Pass and Northeast Gully, return via Arapaho Creek; and, finally,
  6. Out via Hell Canyon, over Cooper Peak Pass and Paiute Pass and Northeast Gully, and return via Arapaho Creek.
A carpet of avalanche lilies in upper Hell Canyon

7. Middle Boulder Creek

  • 25.1 miles (4.3 miles off-trail) with 7,400 feet of gain
  • Strava

From Boulder, Hessie Trailhead provides the quickest access into the Indian Peaks. Its popularity is proportional to its convenience, however, so the county operates a free shuttle bus on weekends to relieve parking pressure. By taking the N bus from downtown Boulder, this loop can be completed without a personal vehicle.

Go counterclockwise, so that you pass through the lowest and most heavily trafficked sections first, and reserve the wildest bit for the third quarter. The highlight is between Arapaho Pass and Rollins Pass, when this loop overlaps with the Pfiffner and mostly stays atop the Continental Divide, offering sweeping views east towards the Front Range metropolitan area (and the Kansas prairie, way beyond) and west across the Fraser Valley and over Winter Park to the jagged Gore Range, thirty-five miles distant.

Looking north over Arapaho Pass and Lake Dorothy towards Apache Peak, Lost Tribe Lakes, and the west ridge of Lone Eagle Cirque

8. James Peak Skyline

  • 17.6 miles (5.2 miles off-trail) with 6,550 feet of gain

If I’d had one more week of cooperative weather, this would’ve been the one. Years ago I completed it as an overnight backpacking trip, and was surprised that it’s not already a classic, especially given its accessibility from Denver.

Start at St. Marys Glacier Trailhead ($5/day parking fee) and run clockwise, so that you finish with the best stuff. The exception: if there is a chance of thunderstorms and if you’re getting a late start, go counterclockwise so that you can get off the Divide earlier in the day.

From the parking area, cut through Alice and join the Continental Divide Trail (CDT) where it crosses the 4WD road to Loch Lomond. Follow it to Mt. Flora, the first of five 13’ers on this route. Downtown Denver is visible forty miles east-southeast, and two popular Front Range fourteeners — Grays and Torreys — are twelve miles to the south-southwest.

Run/hike north atop the geographic Continental Divide, usually off-trail but sometimes with the help of a use trail, to James Peak, the final high point. The route is generally a mix of tundra and scree, and includes one fun Class 2 scramble across the notch between Bancroft and James.

From the summit of James Peak, descend south on the CDT until it crosses the James Peak Wilderness Area boundary. There, travel eastward off-trail until funneling into the St. Marys Glacier. Return to your car on overused tourist trails.

James Peak, one of five 13’ers on the route. Immediately to the north it gets easier for a while, with pleasant ridge-walking atop the Divide, overlooking deep cirques to the east.

Leave a comment!

  • Have questions about any of these loops?
  • If you’ve done one, what was your experience?
Posted in , on May 19, 2020


  1. Mark Turner on October 26, 2019 at 9:43 pm

    I’ve only done a couple of those trails, this looks like a great list for exploring the area. What gear would you suggest for solo efforts like this? Do you just take water/food/clothing that you commonly need, or do you add emergency gear in case you twist an ankle without anyone around?

    I mostly only take water and an extra jacket when I mess around the flatirons trails, even on shadow or fern canyon trails there are plenty of people, so I never worry about not getting out if I blow a knee. But I’ve been in some areas of the RMNP that are pretty remote, an injury may make it difficult to get out without an overnight. Your UTMB gear list was pretty extensive – do you take something like this on your more remote trail runs?

    • Andrew Skurka on October 26, 2019 at 9:51 pm

      I carry clothing that’s appropriate for the forecast, plus some emergency/survival gear.

      The forecasts were really favorable for all of these runs, so I didn’t need to carry much. Usually a rain jacket, rain mitts, and long-sleeve are wise in Colorado, plus maybe some rain pants too. That’s not as onerous as it used to be — there are multiple rain jackets/pants combos in the 10-oz range now, so you’re probably a pound all in.

      Emergency gear: headlamp, lighter, and an inReach for the more remote runs.

      I know these trails so well that I don’t need maps, but I carry them anyway on my phone with the GaiaGPS app. I have the phone anyway so that I can take a few photos.

  2. Mark Turner on November 2, 2019 at 7:33 pm

    Thanks for the ideas for gear. I should splurge on something like the inReach sometime, that’s the thing I’m normally missing, otherwise my list is similar.

  3. Brandon Titus on May 20, 2020 at 10:06 am

    Thanks for sharing these — they all look great! I’ve been eyeing something like the Middle Boulder Creek route as a quick and easy backpacking overnight for a while.

    Triple Bypass and Boulder Watershed seem like the next two I should check out.

  4. Dan Stedman on August 21, 2020 at 12:55 pm

    When I first moved to Boulder, I used to love doing that Middle Boulder loop in a day. The walk along the divide from King Lake north is spectacular.
    I preferred to do it clockwise. That way if I was going to be benighted due to starting late and overestimating myself (I was 25, it happened a lot), I could bail either at Devil’s Thumb Pass, or down 4th of July road from Arapaho pass, instead of going back by Diamond Lake and over Jasper Ridge.
    If you take the bus up in the morning, I would also add that you should take a bike and ride down the canyon to Boulder!

Leave a Comment