Boulder River, Montana

Vanessa Locampo
Last Updated: February 27th, 2023

A tributary of the Yellowstone River, the Boulder River extends for 60 miles through the picturesque countryside of south central Montana. Rising from the Absaroka Range in the Custer Gallatin National Forest, the river flows north to meet the Yellowstone.

It’s particularly well-known amongst locals for its fly fishing opportunities.

What is the History of the Boulder River?

history of the boulder river

The Boulder River originates in the Absaroka Range, which stretches for around 150 miles across the Montana-Wyoming boundary. As the range is also the origin of the Yellowstone River, it was initially called the Yellowstone Range.

Captain W.A. Jones of the Corps of Engineers renamed the mountains the Sierra Shoshone in 1873, however, the United States Geological Survey rejected this around 1883 and gave the range the official title of Absaroka after the native Absaroka people of the area.

Some of the destinations along the river were named by the Lewis and Clark Expedition, including the town of Big Timber near the confluence with the Yellowstone River. This was initially named by William Clark, who was inspired by large cottonwood trees that lined the creek.

Big Timber served as a major wool-shipping depot in the late 1800s, owing to its location on the Northern Pacific Railroad Line and the many sheep in the area.

Around 30 miles downstream of the river’s origins, it passes through Natural Bridge Falls, where a stone arch once crossed over the water. The natural bridge collapsed in 1988.

The Geography of the Boulder River

geography of the boulder river

The Absaroka Range is the birthplace of the Boulder River, which begins at the confluence of South Fork Boulder River and Basin Creek. It flows through mostly public land for the first 30 miles, and the waters in this stretch are known to be too rough for drift boats. 

Downstream, the river travels through a stunning waterfall known as the Natural Bridge Monument, cutting through a limestone riverbed. This is also the site of Natural Bridge State Park.

The Upper Boulder generally refers to the section of river upstream of Natural Bridge Falls, while the Lower Boulder runs from this area to the Yellowstone River. 

The river’s three forks, West Boulder River, Main Boulder, and East Boulder River, all join near McLeod and then flow towards the small town of Big Timber, where they meet the Yellowstone River.

The Boulder River runs for a total of 60 miles and splits the Absaroka Mountain Range and the Beartooth Range. There are several boulders that line the riverbed, and they are easily viewable through the clear water.

Activities and Attractions Along the Boulder River

Giving visitors the chance to enjoy a natural experience in the great outdoors, the Boulder River is one of Montana’s most treasured gems. Plus, the Boulder River Valley around it is dotted with highlights to explore!

Fly Fish the Boulder River

fly fish the boulder river

The Boulder River is so beloved amongst Montana locals because it’s a premium location for one of the state’s favorite pastimes: fly fishing.

The river is particularly rich in rainbow and brown trout. In the lower section of the river, the fish can grow up to 24 inches in length.

Despite a large amount of fish in the river, it’s not usually overcrowded with anglers. This is due to the fact that its location is considered “out of the way” for most out-of-towners, and the river is also in close proximity to other more well-known rivers, such as the Yellowstone.

Fishing on the river is available at all times of the year except the winter season from December to March when the water typically freezes over.

Access to certain stretches of the river can be difficult, particularly as it flows closer to Big Timber, as it is surrounded by mostly private land. But there are still numerous access points that allow anglers to fish the river through state access points and public bridges.

Camp by the Boulder River

camp by the boulder river

Thanks to the picturesque scenery of the Boulder River Valley, camping around the river is another popular activity. There are plenty of campgrounds scattered along the river banks. Typically, they are equipped with tent pads, fire grates, picnic tables, and restrooms.

Several campgrounds along the river provide access to fishing and hiking trails. Among the most popular campsites are Spring Creek Campground and Trout Ranch, which is located near Big Timber along the Lower Boulder River. The campground boasts 57 RV sites, eight grass tent sites, and four pet-friendly cabins.

East Boulder Campground and West Boulder Campground are both found near McLeod and the Custer Gallatin National Forest. The camps are open year round and run on a first-come-first-served basis rather than through a reservation system.

In addition to these, there are multiple other fantastic campsites to explore along the river. As much of the land around Lower Boulder is privately owned, it’s preferable to camp in a designated campsite rather than risk setting up on private land. 

Bask in the Beauty of Natural Bridge Falls

natural bridge falls

Though Montana is home to a selection of breathtaking waterfalls, Natural Bridge Falls is a unique phenomenon and must-see highlight along the Boulder River. It’s located 26 miles south of Big Timber.

The Upper Boulder vanishes under a limestone shelf and shoots out of a hole in the cliff. Then the water at the pool at the base of the waterfall disappears underground again before reappearing down the valley. In late spring and early summer, the water tumbles over the cliff of limestone rather than traveling through the rock.

Walking trails wrap around the waterfall, providing sublime views. Eagle-eyed visitors may spot fossilized seashells in the sedimentary rock which have been there for millions of years.

There are picnic tables nestled beneath the trees within easy walking distance from the parking lot, where visitors can stop and take a breather. Visitors can also learn about the history of the falls from interpretive signs located along the trail.

Natural Bridge Falls is open for the entire year, but the best time to go is between May and July when the water is at its highest level.

Enjoy Kayaking and White-Water Rafting

enjoy kayaking

With stretches of fast rapids and rough water breaks, the Boulder River is a popular destination for white-water rafting and kayaking for experienced travelers.

The Upper Boulder tends to provide the best conditions for white-water rafting. Meanwhile, the Lower Boulder has sections that are appropriate for all levels of kayakers.

Several tour companies run rafting trips on the Yellowstone River nearby, which boasts similar rapids and fast-paced waterflow.

For those who are lacking in experience or confidence, opting for a guided tour may be the best place to start your Montana white-water rafting adventure. 

Visit the Crazy Mountain Museum in Big Timber

crazy mountain museum in big timber

One of the most exciting museums in the region is the Crazy Mountain Museum in Big Timber, a short distance from the Boulder River.

The museum exhibits the fascinating history of Sweetgrass County in the form of models, displays, and even several historic outbuildings.

On the grounds, visitors will find a tipi, a one-room schoolhouse, a replica of a Norwegian stabbur, and several sheep wagons.

The museum covers the entire history of the county, from the Ice Age to modern times. One of the major highlights is a model town of Big Timber in the year 1907.

The Crazy Mountain Museum is open from Memorial Day through Labor Day. It operates every day during that time, opening at 10 a.m. on all days except Sunday when it opens at 1 p.m. It closes at 4:30 p.m. every day.

Explore Custer Gallatin National Forest

custer gallatin national forest

One of the most ecologically diverse landscapes in Montana, the Custer Gallatin National Forest is the birthplace of the Boulder River.

It extends across seven ranger districts and encompasses a total of 3.1 million acres which serve as a gateway to Yellowstone National Park and the Absaroka-Beartooth Wilderness.

The forest is home to a network of trails winding through the dense pockets of trees and flora. There are also serene lakes, including Pine Creek Lake and Hebgen Lake, that are waiting to be explored.

Plus, visitors are almost guaranteed to spot wildlife, with grizzly bears, gray wolves, and bald eagles all residing within the forest.

Whether you enjoy hiking, climbing, cycling, or bird-watching, there’s no shortage of outdoor recreation in the forest.

In the winter months, visitors can also enjoy skiing and other winter sports in the designated ski areas, which include Lone Mountain Ranch and Crosscut Mountain Sports Center.

Interesting Facts About The Boulder River

interesting facts

  • The Boulder River has also been called Rivers a Cross, Rivers across, and Rivers-across.
  • There is another Boulder River in Montana, which begins in Jefferson County, northwest of the town of Boulder, and eventually joins the Jefferson River near Cardwell.
  • To differentiate the Boulder River of the Yellowstone from the one that joins with the Jefferson River, this one is sometimes called East Boulder River or Boulder Fork of the Yellowstone.
  • Two movies have been filmed in the Boulder River Valley: A River Runs Through It and The Horse Whisperer.


How deep is The Boulder River? 2.75 feet (.8 meters) near Big Timber

How long is The Boulder River? 60 miles (96 km)

How wide is The Boulder River? 45-60 feet (13 – 18 meters)

Where does The Boulder River start? Custer Gallatin National Forest, the Absaroka Range, Park County

Where does The Boulder River end? Yellowstone River, near the town of Big Timber, Sweet Grass County

Which way does The Boulder River flow? Northwest


The Boulder River, Montana, is a prime destination for anglers, enthusiastic white-water rafters, and those who simply want to camp in some of the state’s most beautiful scenery.

If you’ve ever visited the Boulder River, let us know about your experience in the comments below!


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About The Author

Vanessa Locampo

Vanessa is an Australian-based freelance writer and editor with a BA in Creative Writing. She’s passionate about creating travel content that inspires her readers to take a leap of faith and power through their bucket lists. When she’s not writing (with her border collie asleep at her feet), she’s devouring books, exploring the world, or planning her next trip.

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