There are many places to go rafting within a few hours of St. George here is a link to Utah river rafting. Here is a list of the closest places along with some for a long weekend trip...

Western Canyon (Moab, Utah) A wide varity of trips available, from a few hours to several days.
Upper Grand Canyon - Reservations can be made in several locations.
Lower Grand Canyon - These are multi-day trips going through turbulent waters to calm waters... just plain fun & excitement!
Green River (Moab, Utah) Many trips available, including through Canyonlands National Park
Others - Locations that are more than a few hours away.

Brian Habel
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