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).
If you do not plan to travel on your flight, you must contact Southwest Airlines or Southwest Vacations by phone at least ten (10) minutes prior to scheduled departure. Otherwise, all funds on the unused portion of the reservation will be lost, and the remaining itinerary will be cancelled by the airline.