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).
I believe it's near impossible to cancel a flight on Ryanair. I think the only way to do it is ring their 0871 number (check SAYNOTO0870.COM-Non-Geographical Alternative Telephone Numbers for a geographic number to save you the 10p a minute). Depending on the fare will depend if you get any refund on that part.