The correct answer is the Mississippi River. This river along with the Missouri River form the largest river system in America and rank among the longest in the World. Mississippi flows from Lake Itasca in the North to the Gulf of Mexico in the South and even though it is shorter by length than Missouri, the Mississippi River carries a larger volume of water than all the other rivers in America, making it the most important of all. It runs through 10 states and was used to define borders between them.