Cooney Reservoir State Park.  

This park is a popular recreation area located about 40 minutes south of Billings.  Boating is popular, with launching ramps and boat trailer parking.  There is a swimming beach, hiking and biking trails.  There are 82 campsites, 19 with electric.  Campers may stay up to 14 days in a month.  Summer is a busy time and reservations are recommended.  In the nearby town of Absaroka (pronounced ab sor key), you will find guides for river adventures, fishing, and whitewater rafting as well as shops and restaurants.

https://www.visitmt.com/listings/general/state-park/cooney-state-park.html

Bighorn Canyon Recreation Area

Congress established Big Horn Canyon Recreation Area in 1966 following the construction of the Yellowtail Dam, named for the famous Crow chairman Robert Yellowtail.  The dam harnessed the waters of the Big Horn River to form Bighorn Lake.  Within the canyon, are 55 miles of the lake, which extends into Wyoming another 5 miles.  You can enjoy breath-taking scenery, wildlife viewing, boating, fishing, hiking, and camping in several area campgrounds.

https://www.stateparks.com/bighorn_canyon_national_recreation_area_in_montana.html

Bighorn River

The town of Ft. Smith was established in 1866 to protect immigrants traveling on the Bozeman Trail from attacks by the Sioux Indians. The Bozeman Trail crossed the Bighorn River 400 yards from the fort.  The Bighorn river flows north through Wyoming and turns northeast as it crosses the Montana border.  It flows through Bighorn Lake and on through steep canyon walls, through the Crow Indian Reservation and converges with the Yellowstone River at Hardin, MT.

Today, Ft. Smith is a small village devoted to giving visitors the best fishing experiences on the river. Many famous people come to Ft. Smith to enjoy a guided, blue ribbon trout fishing trip down the Bighorn River.

https://en.wikipedia.org/wiki/Bighorn_River

https://www.bigskyfishing.com/rivers/southern-montana/bighorn-river/

https://www.visitmt.com/places-to-go/cities-and-towns/fort-smith.html