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).
JetBlue gives their customers a 24-hour grace period to cancel their reservations if the flight was purchased more than a week in advance. There is one caveat: you can't first change your flight and then decide to cancel within that time period. You can only choose to change or cancel your reservation, but not both.