Prison Island once held the massive Tree of Riven and in fact the remainder of its trunk forms the base of the prison in which Katran (Catherine) is held captive. Gehn originally had the tree cut down for making his books, once he had used up all of the tree he then started decimating the trees on Jungle Island, evidence of this can be found at the cart station that takes you to Boiler (or Book Assembly) Island.
Prison Island has drifted a long way from the other islands of Riven and is only reachable via Gehn’s 233rd Age so you will need to crack the code on the domes (any of which will take you to the 233rd Age) then get past Gehn in order to reach the island.
There is an elevator at the base of the prison which will take you up to where Katran is being held but you will still need a code to unlock the elevator gate in order to release her.