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ForestMatters, LLC

Warm River Loop

Caribou-Targhee National Forest, Idaho · 4 min read

Distance
5 mi
Elevation Gain
300 ft
Difficulty
easy
Route Type
Loop
Best Season
May through October
Dog Friendly
Yes
Difficulty Score
2 / 10
Trailhead Amenities
Restroom
Vault Toilet
Parking
Free
Cell Service
No Signal
Water
Nearby
Camping
Developed
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Conditions, regulations, and fees change frequently. Verify with the local ranger district before your trip. Full disclaimer

At a Glance

  • Warm spring-fed river that stays above 50 degrees Fahrenheit year-round
  • Excellent rainbow and brown trout fishing
  • Lush riparian forest with good birding for warblers and water-associated species
  • One of the most accessible loops in the Island Park area

Overview

Warm River is one of the more unusual streams in Idaho's mountain west: a spring-fed river that runs clear and relatively warm year-round, fed by the geothermal hydrology of the Island Park caldera. The water temperature, typically in the 50-55 degree range even in winter, creates conditions that support thriving rainbow and brown trout populations and a lush riparian environment unusual for this elevation and latitude.

The Warm River Loop follows the river on one bank and returns through lightly forested terrain on the other side, covering 5 miles with just 300 feet of gain. It is one of the most accessible full-circuit trails in Caribou-Targhee National Forest, suitable for hikers who want a genuine forest experience without committing to a long or demanding day.

The birding along the Warm River corridor is notably good. The combination of warm water, riparian vegetation, and the transition to upland forest creates an edge habitat that concentrates bird species. Early morning in late May and June is the most productive time for birdsong, with warblers, flycatchers, and water-associated species like American dippers common along the stream.

The route is also a good option for visitors who have come primarily for the Mesa Falls area and want a second short activity in the same day. The trailhead is along the same scenic byway, roughly 10 miles from the falls.

The Route

Miles 0 to 2.5: River Trail

The loop begins at the Warm River Campground trailhead and descends gently to the river's edge. For the next 2.5 miles, the trail follows the south bank of the Warm River through dense riparian vegetation: willows, alders, and tall cottonwoods that form a canopy over the path in midsummer. The river is visible through the trees throughout this section, and several short spurs drop to the bank for fishing access.

The gradient is essentially flat along the river. Two log bridges provide crossings over side channels of the river. The sound of moving water accompanies the entire river section.

Miles 2.5 to 5.0: Forest Return

At the lower end of the loop, the trail crosses the main river on a maintained bridge and climbs gently through lodgepole pine forest on the north bank. This section has less dramatic character than the river trail but passes through pleasant open forest. The return climb is gradual, gaining most of the 300-foot elevation back to the campground over 2.5 miles.

The forest section offers a different quality of light and a quieter, more upland character. Deer are common on this side of the loop, particularly in early morning and late afternoon.

When to Visit

May through October. The Warm River's consistent temperature means it is fishable earlier and later in the season than snowmelt-fed streams. The trail itself is typically accessible from mid-May and can be hiked through October or even later in dry years.

Summer mornings offer the best birding and the coolest temperatures for walking. Fall color in the cottonwoods and aspens peaks in late September and early October.

What to Bring

Many visitors to this easy loop carry:

  • 1-2 liters of water; the route is short but the forest can be warm in midsummer
  • A water filter if planning to drink from the river (still recommended even from spring-fed sources)
  • Binoculars for birdwatching
  • An Idaho fishing license if planning to fish
  • Insect repellent in early summer when the riparian corridor can have mosquitoes

Individual needs vary. The short distance and gentle terrain mean minimal preparation is required.

Practical Details

No hiking permit is required as of 2026. The Warm River Campground trailhead has a vault toilet. Dogs are permitted on leash. Cell service is absent from the area. The campground operates with fees (as of 2026) but day-use trailhead parking is available without a campsite reservation.

Leave No Trace principles include staying on designated trails, particularly in the riparian areas where bank erosion is a concern, and packing out all waste.

Getting There

From Ashton, drive east on Highway 47 (Mesa Falls Scenic Byway) approximately 15 miles to the signed Warm River Campground turnoff. From Island Park, drive south on Highway 20 to Highway 47 and head west approximately 5 miles to the campground. The trailhead is in the campground parking area, accessible by all vehicles.

Trailhead Parking

Warm River Campground Trailhead, approximately 15 miles east of Ashton on Highway 47. Paved road. Parking shared with campground. No day-use fee.

← Back to Caribou-Targhee National Forest

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Frequently Asked Questions

Why is it called Warm River?
The Warm River is spring-fed and maintains a water temperature above 50 degrees Fahrenheit throughout the year, warm compared to the snowmelt-fed streams typical of this elevation. The springs originate from the Island Park caldera system and produce a consistent, clear flow that supports excellent trout habitat.
Is this trail good for fishing?
Yes. The Warm River is a noted trout fishery with rainbow and brown trout. An Idaho fishing license is required. The river is open to fishing in season per Idaho Fish and Game regulations. The consistent temperature and clear water make it a good fly fishing destination.
Is the Warm River Loop good for beginners?
Yes. The 5-mile distance and 300-foot elevation gain make this one of the most accessible full-loop trails in the area. The terrain is gentle and the well-maintained path follows the river and returns through forest without any difficult sections.
Can I combine this loop with a visit to Mesa Falls?
Yes. Warm River Campground is along the Mesa Falls Scenic Byway, roughly 10 miles from the Upper Mesa Falls Scenic Area. Combining both sites makes a good full-day trip in the Ashton/Island Park area.
Is there camping at Warm River?
Warm River Campground has 23 sites with standard facilities. Fees apply (as of 2026). The campground is adjacent to the trailhead and makes a convenient base for the loop and nearby attractions.