I was fortunate to come across a couple of groups of bison crossing the Yellowstone River just upstream of the Lower Falls this past June. The groups both seemed to be in a hurry to get to the river, running from the trees nearby. As they reached the river they hesitated just for a moment, then got in and started swimming. Bison are not terribly buoyant, so only a part of their head stuck up above water. As they crossed they were breathing and grunting heavily, laboring against the current, aiming upstream. One mother was herding her calf as they swam. Upon reaching the other side they shook off gallons of water and continued on into Hayden Valley.