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).
Spirit Airlines, Inc. provides low-fare airline services. As of June 30, 2015, it operated approximately 360 daily flights to 57 destinations in the United States, Caribbean, and Latin America.