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).
Cancel your original ticket as far in advance of your travel date as possible. Many airlines have restrictions as to how soon before a flight, often 24 hours, your are able to return a ticket for refund or voucher. If you simply don't show up on the day of your flight, your ticket probably will lose all value.