Geologically young when compared with ancient rivers such as the Nile or the Amazon, the Minnesota River is only about 12,000 years old. It occupies a channel cut by Glacial River Warren when it drained Glacial Lake Agassiz at the end of the last glacial age, as glaciers covering Minnesota, in some cases to a depth of over a mile, melted away. Starting at the South Dakota border at Browns Valley, the Minnesota River is the state's second largest river and joins the Mississippi at the foot of Historic Fort Snelling after traveling some 355 miles. By geological definition, the Minnesota is an underfit river, occupying the channel of an earlier river. While the ancient valley is up to two hundred and fifty feet deep and in places 5 miles wide, the Minnesota is a shallow, meandering river that during flood stages swells beyond its banks because of runoff from its extensive drainage basin. Along its banks are rocks more than 3.8 billion years old, ruins of former homes, villages, military posts, and sanitoriums, Indian villages and crossing sites that predate European settlement of the region, and the course once traveled by hundreds of steamboats until their use ceased with the coming of the railroads.