Knoxville rivers to swim in- how warm are they?

I'm reading a book by Cormac Mc Carthy where there's a lot of swimming in rivers in summertime and it seems to be lovely and warm. Is this true? I live in England where river water is always freezing so unless you're a kiddie with no sense atall, you have to be mad to swim in our rivers.Is flowing river water really warm in summer? I just can't imagine it.