Question: What is the largest river by volume in Montana that also double Downtown Missoula's rough boundaries to its south?

Context paragraphs:
Big Sky Documentary Film Festival: Big Sky Documentary Film Festival is an annual non-fiction film festival held in Missoula, Montana each February.  The event showcases documentary films from around the world.  The festival first began in 2003 as a seven-day event.  It is now a ten-day event.  The Big Sky Documentary Film Festival is the largest cinema event in Montana.  The festival presents an average of 150 non-fiction films annually at the historic Wilma Theater, The Top Hat, The Roxy Theater, and Crystal Theater in downtown Missoula.

Missoula Symphony Orchestra: The Missoula Symphony Orchestra is an American orchestra and 501c3 non-profit organization based in Missoula, Montana.  It is affiliated with the Missoula Symphony Chorale with which they make up the Missoula Symphony Association.  The association holds five two-performance concerts a year at the University of Montana Theatre in addition to an outdoor summer concerts Downtown Missoula, two Youth Concerts, and a Family Concert.

First Interstate Center (Missoula, Montana): The First Interstate Center, or First Interstate Bank Building, is a building in the heart of Downtown Missoula, Montana.  It is located at 310 West Front Street.  It is one of many post-modern buildings recently built in Missoula.  Standing at 6 floors it is one of the tallest in Missoula, and with 118,000 sqft of space, it is the largest square footage office building in Missoula.

Missoula County Courthouse: The Missoula County Courthouse is located in Missoula, Montana, in the center of Downtown Missoula.  It is located at 200 West Broadway, Missoula Montana.  It was added to the National Register of Historic Places listings on September 1, 1976.

Millennium Building: The Millennium Building is a Building located in downtown Missoula, Montana.  At 9 floors it is the tallest building in downtown Missoula and rises to 128 feet.

Milltown, Montana: Milltown is an unincorporated community in Missoula County, Montana, United States.  Milltown is located along Interstate 90 and Montana Highway 200 5.5 mi east of downtown Missoula.  The community has a post office with ZIP code 59851.

Clark Fork River: The Clark Fork, or the Clark Fork of the Columbia River, is a river in the U.S. states of Montana and Idaho, approximately 310 mi long.  The largest river by volume in Montana, it drains an extensive region of the Rocky Mountains in western Montana and northern Idaho in the watershed of the Columbia River.  The river flows northwest through a long valley at the base of the Cabinet Mountains and empties into Lake Pend Oreille in the Idaho Panhandle.  The Pend Oreille River in Idaho, Washington, and British Columbia, Canada which drains the lake to the Columbia in Washington, is sometimes included as part of the Clark Fork, giving it a total length of 479 mi , with a drainage area of 25820 sqmi .  In its upper 20 mi in Montana near Butte, it is known as Silver Bow Creek.  Interstate 90 follows much of the upper course of the river from Butte to northwest of Missoula.  The highest point within the river's watershed is Mount Evans at 10641 ft in Deer Lodge County, Montana along the Continental Divide.

Downtown Spokane: Downtown Spokane, officially the Riverside neighborhood, is the central business district in Spokane, Washington.  Downtown Spokane's rough boundaries are I-90 to the south, Division Street to the east, Maple Street to the west, and across the Spokane River north to Boone Avenue.  Bridges (some of them pedestrian-only) span the river at nearly every block.

Christopher P. Higgins: Capt. Christopher Powers Higgins (March 16, 1830 – October 14, 1889) was an American Army captain and later businessman who with Frank Worden founded the Hellgate Trading Post and the nearby city of Missoula, Montana.  He erected one of the first lumber and flouring mills on the Clark Fork River near present Downtown Missoula as well as many of Missoula's first buildings and establishments.  He was one of the original county commissioners, member of first legislature of the Montana Territory, and incorporator of The Montana Historical Society.  Higgins Avenue and bridge as well as the Higgins block in Downtown Missoula are named after him.  He is buried in Missoula Cemetery.

Downtown Missoula: Downtown Missoula is the central business district in Missoula, Montana, and West-Central Montana.  Downtown Missoula's rough boundaries are the Clark Fork River to its south, Madison St. to its east, the old U.S 93 highway/ North Orange St. to its west, and West Alder St. to its north, though parts south of the Clark Fork River are also at times mentioned.  It is home to Ogren Park at Allegiance Field which is the venue for the Missoula Osprey.  Downtown is also home to several parks, and historic buildings such as the Wilma Building.

Supporting paragraphs: Downtown Missoula Clark Fork River
Answer: Clark Fork River