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).
Expedia, for example, states that passengers may have the option to cancel for a refund but doesn't guarantee it. But if you're booking flights at the recommended 54 days before your trip, you're most likely in luck. The first flights you book don't have to be the ones you take. ...just take a price break instead. CORRECTION: A previous version of this post stated that airlines must either hold your flight for 24 hours after booking OR allow you to cancel for a full refund. In fact, they must do both.