These are dangerous creatures. They have little fear and they don't like to be bothered. There's a story of how unpredicatable they are:
Yellowstone National Park (about 1985)
A buffalo was lying out in a field a short distance from the road. Many cars stopped. Many got out to take pictures. Over the hour(s) that the bull lay 'sleeping' in the field, people stood around, at a distance, and a the crowd waxed and waned. One visitor walked up towards the beast and took a picture. He then had his wife take his picture with the bull in the background. Next, he's walking closer to have his picture taken again. Then, he walks up to within an arms reach to have his picture taken standing next to the buffalo lying on the ground.
Meanwhile, the bull had been keeping track of everybody. When this may stood next to him, he calmly raised his head with a twitch towards the man, and sunk his horn into the soft flesh of the guys belly. The bull layed his head back down and continued to rest quietly.
Did the guy survive? How did they get him away from the bull which was still resting next to the gored body? The story dosen't say. But trust, I've seen people amble up close. When the bull is standing, he'll charge. If you run away, they won't waste time chasing. If you don't, you're a threat and they will kill.
STAY BACK, ENJOY TAKING PICTURES FROM AFAR. If you want pictures closer than you lense will allow, buy them in the gift shop. Be safe, not sorry.
We visited in June but South Dakota is known for extremes in temperatures. In June you can highs in the 50s to highs over 100F. When it gets hot it doesn't seem to be a dry heat, it's the wet sticky kind. Drink plenty of water and try to find ways to stay cool during the summer months.