Following the mysterious woman he has been called by, but losing sight of her, Tancred reaches a mysterious place. The whole episode is a novelty of Gerusalemme Conquistata, it did not appear in Gerusalemme Liberata (Jerusalem Delivered).
Giunse dove perpetue e rapide onde
Con larga vena uscian da vivo sasso,
E facean cinque fonti ampie e profonde
Da l'imo al sommo, o pur da l'alto al basso.
Spargea due rivi il primo: e l'un s'asconde
Nel suo principio ritorcendo il passo;
L'altro queto scendea con l'acque chiare
Sin ch'egli si morìa nel Morto Mare.
He came where perpetual, swift waves
Gushed abundantly from living rock
Making five wide, deep springs from top
To bottom or the other way round.
The first spread two rivers: the one hid
In its own origin by turning back,
The other flowed down with clear waters
Until it finally died in the Dead Sea.
The five rivers seen by Tancred are clearly a symbol -- but, of what? It would be interesting to have a look at the footnotes in some edition of the poem, but unfortunately no edition is currently available (the only book available, the one we are using, namely this, is a copy of the manuscript; it provides notes about the text itself, not about its meaning).
From a mere literary viewpoint, both in the Middle Ages and in the Renaissance, prodigious rivers were a 'must-find' in eastern and/or tropical Edenic lands. See a wonderful modern re-enactment of this in EA Poe's Gordon Pym.
According to an online source, the rivers here represent the five ways of human perception, based on Scholasticism, but why should Tasso add a whole new section, in this context, to deal with such a specialized subject? Since he was interested in the God/universe relationship (see his long poem Il Mondo Creato, on which he worked at the same time as the Conquistata), the rivers are likelier to symbolize the main dynamics of creation.
In this case, the first river could mean the two basic "forces" that shape the universe: God, who opens to new beings while resting in himself; and -- with a modern term -- entropy.
Suggestions from readers will be welcome.