The Mississippi River is known as the Middle Mississippi from the Upper Mississippi River's confluence with the Missouri River at St. Louis, Missouri, for 190 miles (310 km) to its confluence with the Ohio River at Cairo, Illinois. The Middle Mississippi is relatively free-flowing.
As Hopper notes, eight major U.S. airlines offer free cancellation for tickets purchased at least seven days in advance: Delta, United, US Airways, JetBlue, Southwest, Spirit, Frontier and American (in American 's case, you'll hold your ticket instead of purchasing it).
According to the Washington Post, not canceling a flight ticket and simply not showing up for it can sometimes be cheaper than canceling it and paying the change fee, especially if you only need to change one leg of the trip, or if you used miles or some sort of credit to pay for the ticket in the first place.