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 the customer wishes to cancel their flights, the ticket can remain in 'OPEN' status for use by the customer at a later date, at which point the appropriate change fee would be applied. A second option would be to exchange the value of the ticket less the change/cancellation fee to an MCO.