Queen Zara created this river five hundred years before The Hidden Stairs and the Magic Carpet, the first book in The Secrets of Droon. She made it, along with a stone ship that sails on its surface, so her youngest son, Lord Sparr, could escape Emperor Ko's clutches and live with his older brother Urik instead. The river does not succeed in this primary purpose, as Sparr never reaches that river before Zara dies.
Seven years before the events in The Riddle of Zorfendorf Castle begin, Galen Longbeard constructs Zorfendorf Castle to (belatedly) hide the Fifth River from others who might abuse it, including Emperor Ko.
In The Riddle of Zorfendorf Castle, Ko attempts to reach the Fifth River by invading Zorfendorf Castle, but Eric, Julie, Neal, and Princess Keeah prevent the beastly emperor from succeeding. Young Sparr and Gethwing manage to escape to the Upper World, though.
Lord Sparr accesses the Fifth River again by entering it through Zorfendorf Castle in Special Edition 8. The Final Quest. He uses the stone ship on the river to sail to the Upper World, locate Demither, and find the Wand of Urik. Eric, Julie, Neal, and Keeah come with him and help him achieve these goals.
Eric accesses the Fifth River for the final time later in The Final Quest by using the Passages to quickly reach it. Again, he hops on the stone ship and sails it across the river to the Upper World. The stone ship explodes at the end of the river and sends Eric back in time and towards Urik. This is arguably when the Fifth River finally succeeds in its purpose, and since the ship annihilated itself in a burst of stone and water, it is unknown whether the Fifth River currently exists.