Big Spring Creek is a tributary of the Judith River in Fergus County, Montana near Lewistown, Montana. The creek originates from a first magnitude artesian spring approximately 6 miles (9.7 km) south of Lewistown and flows north, northwest for 30 miles (48 km) to its confluence with the Judith River. The spring is one of the largest in the world flowing at approximately 50,000 to 64,000 US gallons per minute out of the Madison-Limestone formation in the foothills of the Big Snowy Mountains. The creek flows through and under the town of Lewistown. For three blocks spanning Main Street, the creek runs underneath the town in a man-made channel that was created as the town was built over the creek. The spring provides Lewistown's water supply, which requires no treatment for use by consumers.