Lewis and Clark came upon and described the Giant Springs, purportedly the largest freshwater spring in the world. The spring now also feeds a fish-breeding facility nearby. It burbles out of the 250-million-year-old Madison Formation, a large water-bearing formation that provides a lot of groundwater throughout the northern West. The springs send out more than 100,000 gallons a minute into the 201-foot-long Roe River, credited as one of the two shortest rivers in the world. The entire park covers 218 acres and has about 2 miles of trails.
The Great Falls that gave Lewis and Clark so much trouble have been dammed, but you can see a few remnants of their former glory from overlooks. In the spring, especially, you can see the power of the river flowing through the spillways at Rainbow Dam, spewing mist hundreds of feet into the air, creating the rainbows in the sunshine that so entranced the explorers. Lewis called Rainbow Falls "one of the most beautiful objects of nature." The Montana Department of Fish, Wildlife, and Parks also operates a fish hatchery and visitor center nearby (Mon-Fri 8am-5pm), where visitors can purchase hunting and fishing licenses.