When Clark was leaving his canoe camp on the Yellowstone, he assigned Pryor to take the remaining horses downriver to the mouth of the Bighorn River where they would cross the Yellowstone. Clark and the rest of the party were in the canoes. Just above the entrance to “Clark’s Fork or Bighorn River” they ran into a difficult riffle.
Clark got confused between these two rivers. He was actually near the mouth of the Clark’s Fork River while the Bighorn River was almost 90 miles farther downriver. Sometime later he figured out his errors then went back in his journal entries and made several corrections. These corrections could have been made as late as after the Expedition returned and Clark was working with Biddle to get the Journals published.
He apparently wanted his historical record to be as accurate as he could make it.
Oh, Pryor finally crossed the Yellowstone with his horses some 33 miles farther downriver from Clark's Fork, in the townsite of Billings.