Question: The county in which Sun River Montana is located is ranked as the what most populous county in Montana?

Context paragraphs:
Upward Sun River site: The Upward Sun River site, or Xaasaa Na’, is a Late Pleistocene archaeological site associated with the Paleo-Arctic Tradition, located in the Tanana River Valley, Alaska.  Dated to around 11,500 BP, Upward Sun River is the site of the oldest human remains discovered on the American side of Beringia.  The site was first discovered in 2006.

Gibson Reservoir: Gibson Reservoir is located just south of the Bob Marshall Wilderness in Montana.  The reservoir is formed by the concrete arch Gibson Dam and backs water up between a hilly pass for about 3 miles.  The water in the dam is initially provided by spring snow runoff and carried there via the North Fork Sun River and the South Fork Sun River right after the two join.  The reservoir is mainly used for irrigation control.

Gibson Dam: Gibson Dam is a concrete arch dam on the Sun River, a tributary of the Missouri River, about 60 mi west of Great Falls, Montana in the United States.  Located on the eastern edge of the Rocky Mountains, the dam was built by the U.S. Bureau of Reclamation (USBR) between 1926 and 1929 as part of the Sun River Project to develop about 93000 acre of irrigated land in the Sun River Valley.

Cadotte Pass: Cadotte Pass, known in the mid to late 1800s as Cadotte's Pass, is a pass in the Rocky Mountains located on the Continental Divide in the U.S. state of Montana.  Pierre Cadotte, a white settler at Fort Benton, Montana, explored the pass in 1851.  Prior to his exploration Tribal people utilized the pass while migrating to the buffalo hunting plains around the Sun River.  Isaac Stevens, Territorial Governor of the Montana Territory, named the pass after Cadotte in 1853.  The pass is 6073 ft above sea level.

Fairfield Sun Times: Fairfield Sun Times is a weekly newspaper serving Fairfield, Montana and surrounding towns Choteau, Augusta, Vaughn, Power, Simms, Ft. Shaw and Sun River.  Its circulation area encompasses parts of Teton County, Cascade County and Lewis and Clark County.

Robert Vaughn (Montana): Robert Vaughn (born Robert Vaughan; June 5, 1836 — March 23, 1918) was a Welsh immigrant to the United States and an important rancher, farmer, and businessman in the U.S. state of Montana before and after the early years of its statehood.  He homesteaded the Vaughn ranch in the Sun River valley in Montana, building a sandstone mansion as his home there.  The town of Vaughn, Montana, is named in his honor, and helped co-found the city of Great Falls, Montana.  He built the Arvon Block, a hotel and stable in Great Falls, one of the city's earliest buildings; the ranch and the hotel are both listed on the National Register of Historic Places.

Petroleum County, Montana: Petroleum County is a county located in the U.S. state of Montana.  As of the 2010 census, the population was 494, making it the least populous county in Montana and the seventh-least populous in the United States.  Its county seat is Winnett.  The Montana Legislature approved the formation of the county by separating it from Fergus County; Petroleum County was thus created on February 25, 1925, as the last of Montana's 56 counties.

J. C. Adams Stone Barn: The J. C. Adams Stone Barn is a historic Romanesque Revival barn constructed of stone and wood located about 1 mi northeast of the town of Sun River, Montana, in the United States.  It is just south of U.S. Route 89.  It is the only Romanesque Revival stone barn in the United States located west of the Mississippi River.  Chere Jiusto, Christine Brown, and Tom Ferris of the Montana Historical Society have described the Adams Stone Barn as "one of Montana's most-beloved and best-known landmark barns."  The structure was added to the National Register of Historic Places on January 12, 1979.

Sun River, Montana: Sun River is a census-designated place (CDP) in Cascade County, Montana, United States.  The population was 124 at the 2010 census.  It is part of the Great Falls, Montana Metropolitan Statistical Area.

Cascade County, Montana: Cascade County ("cascade" means "waterfall" in French) is a county located in the U.S. state of Montana.  As of the 2010 census, the population was 81,327, making it the fifth-most populous county in Montana.  Its county seat is Great Falls.

Supporting paragraphs: Sun River, Montana Cascade County, Montana
Answer: fifth-most