On July 11, 1806, Lewis descended the lower Sun ("Medicine") River and arrived back at their old camp at the upper end of the portage route around the Great Falls of the Missouri. It was a splendid day.
The morning was fair and the plains looked beautifull....The air was pleasant and a vast assemblage of little birds which croud to the groves on the river sung most enchantingly....Proceeded with the party across the plain to the white bear Islands...through a level beautifull and extensive high plain covered with immence hirds of buffaloe. It is now the season at which the buffaloe begin to coppelate and the bulls keep a tremendious roaring we could hear them for many miles and there are such numbers of them that there is one continual roar. our horses had not been acquainted with the buffaloe they appeared much allarmed at their appearance and bellowing.√ñThe missouri bottoms on both sides of the river were crouded with buffaloe I sincerely belief that there were not less than 10 thousand buffaloe within a circle of 2 miles arround that place.
Meanwhile, on July 24, over on the Yellowstone River, Clark and his contingent were to have their own experience with horses and bison. Having built canoes to carry his men and their baggage down to the Missouri, he sent Sergeant Pryor and privates Shannon, Hall, and Windsor, with their remaining 20 horses—a total of 29 had already been stolen, possibly by Crow Indians—on ahead to the Mandan villages. Within hours of their departure, the detail encountered a problem:
Sergeant Pryor informed me√ñthat in passing every gangue of buffalow Several of which he had met with, the loos horses as Soon as they Saw the Buffalow would imediately pursue them and run around them. All those that Speed suffient would head the buffalow and those of less Speed would pursue on as fast as they Could. He at length found that the only practiacable method would be for one of them to proceed on and when ever they Saw a gang of Buffalow to Scear them off before the horses got up. This disposition in the horses is no doubt owing to their being frequently exercised in chasing different animals by their former owners the Indians as it is their Custom to chase every Speces of wild animals with horses, for which purpose they train all their horses.
Two days later, Indians stole the rest of the horses!