Having two Boulder Rivers in Montana often leads to confusion, with some descriptions mistakenly combine the two. The Boulder River in Southwest Montana begins in Jefferson County, northwest of the town of Boulder. The river flows through a canyon along Interstate 15, at the town, it loops to the south, leisurely meandering through the Boulder Valley until it joins the Jefferson near Cardwell. In late summer, irrigation needs reduce the river's already meager water still further, making fishing and floating difficult.