Bison range freely throughout the park so you might encounter them on any of the trails. Keep at least 50 yards from them to avoid being charged (by the bison or the park police).
This trail runs through the valley of the North Prong of the Little Red River and is 3-4 feet wide its entire length. It crosses the "river" half a dozen times and the trail runs directly through the river bed for a couple hundred feet in a couple of places. The "river" was dry when I runed, but you wouldn't want to run it if there was any chance of a flash flood.
This trail is extremely sandy. The short little ascents/descents down into the river bed are particularly sandy. At about 2.0 miles, you'll leave the riverbed and after a short climb, you'll crest a hill and an awesome view of the red rock canyons to the west opens up. The path also becomes less sandy at this point, so you can make better progress.