Trail is rated "difficult" because of the challenging nature of the Yellowstone River ford.
The Trail Creek Trail
ends with a choice of fords to cross the Yellowstone River. The Upper Ford (this trail) and the Lower Ford. Both are extremely difficult, even in August. The river is braided in spots and conditions of the fords change from year to year. Check both fords for the best one to use. Look for a wide spot where the water is likely to be shallowest, though even then you can expect a waist-deep ford in August. You should use a sounding pole to maintain balance and avoid holes.
Thanks to guidebook author, Tom Carter, for sharing this trail description. To learn more about visiting Yellowstone, check out his book, Day Hiking Yellowstone