From the moment of starting Fred had been on the alert for a chance to make a break for liberty, but none occurred.

The Sioux was too vigilant to tempt him.
The long immunity from harm had given the lad a certain self-assurance.
As yet he had formed no suspicion of the real purpose of the Sioux, but, somehow or other, he believed his own death was not likely to be attempted for a number of hours to come.
"Well, Motoza, here we are! What's the next step ?" The Indian raised one of the hands grasping a Winchester and pointed toward the canyon.
"Go dere--jump on rock!" "My gracious! I can't do that!" "Den me kill!" As if angered beyond restraint, he made a leap toward the startled youth, who recoiled a step, and, esteeming a death by drowning preferable to the one which threatened him, made haste to reply: "All right; I'll try it." But it was necessary that the miscreant should give some instructions to his prisoner.

These were simple.

He was to leap upon the rock nearest to shore, and then, by the same means, ascend the canyon until ordered to stop.

From what has already been related concerning Jack Dudley's experience, it will be seen that the task was difficult and dangerous.
There was no choice, however.