Most riverboats tend to have smaller cabins, and I dont know of any that have single staterooms.
However - they may have reduced single supplements.
I highly recommend river cruises - especially for single cruisers because the people onboard are very friendly - you will have no problem meeting new people and striking up conversations.
I am the editor, but I also speculate, ask questions and play devil's advocate. I reserve the right to change my mind.