The longest river in the USA is the Missouri River (it is a tributary of the Mississippi River and is 2,540 miles long), but the biggest in terms of water volume is the deeper Mississippi River.
The longest undammed river in the contiguous USA is the Yellowstone River (it is 692 miles long).
Rivers provide drinking water, irrigation water, transportation, electrical power, drainage, food, and recreation. Rivers also erode land and carry it downstream to the sea. This kind of erosion can even form canyons, like the Grand Canyon, and waterfalls, like Niagara Falls.