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).
1 On domestic AnadoluJet flights (except for promotion classes), It is not allowed to cancel/refund less than 12 hours to the flight. If there is more than 12 hours, passenger can cancel/refund the ticket with 60 TRY penalty fee.  Cancelation/refund requests made after the first flight cannot be processed on our website.