How does Venters find the openings to the canyons? How does he find Oldring's den?

Venters finds the openings to the canyons and Oldring's den by trailing the rustlers and exploring the area. Venters trails the rustlers to the waterfall, but then reaches a dead end. When he lays in wait, he sees burros moving through the water and through the waterfall, and he knows he has found the mouth of Oldring's den.