It is possible to get tornadoes to hit over water but it is rare, especially since the Catawba is not a large river. When a tornado moves over water or forms over water it is then termed a waterspout. One reason why we don't see many tornadoes over rivers is usually due to the bluffs which surround alot of rivers. These hills that get carved out by rivers over millions of years can act as a buffer to keep the rotating winds of a tornado at bay. So I would say you are safer (statistically) living near the river but tornadoes can and do happen from time to time.

~Meteorologist Keith Monday~