Question: What river begins in Lewis and Clark County outside of Missoula, Montana?
Context: Lewis and Clark Pass, el.  6424 ft is a mountain pass on the continental divide in Montana.  It lies at the head of the drainages of the west flowing Blackfoot River and the east flowing Dearborn River.  The pass is in the Helena National Forest in Lewis and Clark County.  The Continental Divide Trail traverses north and south through the pass.  At the time of the Lewis and Clark Expedition the pass was a much-used pathway where the native people living in what today is Montana crossed over the continental divide.  The pass was crossed by Meriwether Lewis on July 7, 1806, on the return leg of the Lewis and Clark Expedition with a party of nine men and his dog Seaman.  The pass was named for the expedition's two leaders–Lewis and William Clark.  Lewis and Clark Pass is the only roadless pass on the entire Lewis and Clark National Historic Trail.  It has gone from being one of the most used continental divide passes prior to the pioneer era to one of the least visited passes today.  It can be accessed by a 1.5 mi trail.  Visitors will encounter the pass much as Lewis did in 1806.  The furrows left by the countless dog and horse travois that crossed the pass are still visible (though fading) and this is one of the places along the expedition's route that visitors may still encounter a grizzly bear.  On a clear day, like Meriwether Lewis in 1806 one can see Square Butte in Cascade County, Montana, 40 mi to the northeast. Lewis and Clark County is a county located in the U.S. state of Montana.  As of the 2010 census, the population was 63,395.  Its county seat is Helena, the state capital.  The numerical designation for Lewis and Clark County (used in the issuance of the state's license plates) is 5.  The county was established in 1865 as Edgerton County, and was renamed "Lewis and Clark County" two years later.  The present name was given in honor of explorers Lewis and Clark. Missoula is a city in the U.S. state of Montana and is the county seat of Missoula County.  It is located along the Clark Fork River near its confluences with the Bitterroot and Blackfoot Rivers in western Montana and at the convergence of five mountain ranges, thus is often described as the "hub of five valleys".  In 2016 , the United States Census Bureau estimated the city's population at 72,364 and the population of the Missoula Metropolitan Area at 116,130.  In the 1990s, Missoula overtook GreatFalls as Montana’s second‑largest city, behind Billings.  Missoula is home to the University of Montana, a public research university. 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. The Blackfoot River, sometimes called the Big Blackfoot River to distinguish it from the Little Blackfoot River, is a snow-fed and spring-fed river in western Montana.  The Blackfoot River begins in Lewis and Clark County at the Continental Divide, 10 miles (16 km) northeast of the town of Lincoln (4536 feet, 1382 m).  The river's headwaters are between Rogers Pass (5610 ft, 1710 m) to the north and Stemple Pass (6376 ft, 1943 m) to the south.  It flows westward through the town of Milltown and enters the Clark Fork River approximately five miles (8 km) east of the city of Missoula (3210 ft, 978 m). The Bitterroot River is a northward flowing 84 mi river running through the Bitterroot Valley, from the confluence of its West and East forks near Conner in southern Ravalli County to its confluence with the Clark Fork River near Missoula in Missoula County, in western Montana.  The Clark Fork River is tributary to the Columbia River and ultimately, the Pacific Ocean.  The Bitterroot River is a Blue Ribbon trout fishery with a healthy population of native westslope cutthroat trout and bull trout.  It is the third most fly fished river in Montana behind the Madison and Big Horn Rivers. There are a number of small foothills known as the North Hills, in Montana.  One group is el.  3556 ft , is northwest of Missoula, Montana in Missoula County, Montana.  Another is due north of Bozeman, Montana in Gallatin County, Montana, and yet another is due north of Helena, Montana in Lewis and Clark County, Montana.  All of these foothills are generally considered part of larger ranges. The Link River is a short river connecting Upper Klamath Lake to Lake Ewauna in the city of Klamath Falls in the U.S. state of Oregon.  Draining a basin of 3810 mi2 , the river begins at the southern end of Klamath Lake and flows a short distance to the Link River Dam and continues 1.5 mi to the head of Lake Ewauna.  The "falls" from which Klamath Falls derives its name, and which in reality are best described as rapids rather than falls, are visible a short distance below the dam, though the water flow is generally insufficient to provide water flow over the rocks.  The Klamath River begins at the narrow southern end of Lake Ewauna and flows 253 mi from there to the Pacific Ocean. 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. 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.
Answer: Blackfoot River