Giant Springs, near Great Falls on the east bank of the Missouri River, is a first-magnitude spring and the defining feature of Giant Springs State Park. Fed by snowmelt from the Little Belt Mountains carried underground through the Madison Limestone, it is an opening in the vast Madison aquifer, and its water has spent about 3,000 years below ground. Discharging about 242 cubic feet per second, or roughly 150 million gallons a day, it is one of the nation’s largest freshwater springs. The Blackfeet relied on it in winter, Lewis and Clark described it in 1805, and today it remains a scenic landmark, water source, trout hatchery supply, and the headwaters of the Roe River.