Manly's account appears entirely truthful.

He tells of canyons, rapids, etc., till near the mouth of Uinta River they met the Ute chief Walker (Wakar) who explained by signs that the fury of the river below was worse than above, and all but two gave up.

These two, McMahon and Field, stopped with the Utes, intending to continue.

The others went to Salt Lake.
Wakar (whom McMahon calls "the generous old chief") repeated his warnings.

Field lost courage, and finally McMahon also abandoned the desire.