Big Spring Creek, one of the largest spring-fed streams in Montana, originates 9 miles southeast of Lewistown, near the state's Big Springs Trout Hatchery. It runs northwesterly 30 miles, mainly between the Big Snowy and Judith mountains, and enters the Judith River west of Brooks, Montana. Enclosing the large spring at the head of the creek, giant willows and cottonwoods shade a park and wildlife viewing area. From the spring, cold, high-quality water flows over green mats of water plants and down between birches, willow, hawthorne, native shrubs and grasses. The creek continues its first 20 miles alongside hay and grazing fields and US 191, passing through Lewistown. Above the town, Big Spring Creek averages 38 feet wide, 18 inches deep, below to the mouth, 45 feet wide, 24 inches deep. Water quality degrades going downstream, due to erosion and pollution. In its last 10 miles, from the mouth of Cottonwood Creek to the Judith, the channel migrates. Its silty bottomland supports dense stands of cottonwoods, willows, birches, and hawthorne. Considered by anglers to be the most important trout stream in central Montana, Big Spring Creek attracts large numbers of wildlife, including waterfowl and furbearers. Agriculture, municipal and recreation uses, hunting, and homesite development depend on or are associated with Big Spring Creek.