Question: What is the largest river by volume in Montana that also double Downtown Missoula's rough boundaries to its south?
Sentence (from "Clark Fork River"): 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 .
Relevant evidence: No