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).
Members of Spirit's Free Spirit loyalty program have 90 days after accrual to redeem their miles, otherwise it's bye-bye, miles. Card-carrying members, who pay an annual rate of $59 for a Spirit Airlines World MasterCard, are exempt from the three-month expiration rule. Drinks and snacks will cost you, too.