Like many mountain men, Denis Julien left no memoirs, so little is known of him.  He was apparently of French descent and probably worked for one of the Robidioux brothers, trappers out of Taos, New Mexico.  Like Ashley, he left inscriptions in canyons, so his travel can be traced to some extent.  In 1831, he was near Whiterocks on the Uinta River below Vernal, and in 1836 he was downriver in Labyrinth and Cataract Canyons near the confluence of the Colorado and Green.  By 1838 he was back up the Green in Whirlpool Canyon.

It is not clear whether Julien used boats to travel the river.  However, at least one of his inscriptions shows a boat with an apparent mast, suggesting he may have indeed floated the canyons in search of pelts.