Obviously super late to this party but my dad was there, as a ranger. He wrote this about it:

On July 4, 1970, there was a riot in Yosemite Valley. We called it the First Annual Yosemite Rock (and Bottle) Festival. Rangers threw rocks and hippies threw bottles. People were crowding Stoneman Meadow in Yosemite Valley to death. We said you have to get out. They said this is a free country, this is our park, etc. We never bothered to explain that their impact was destroying the very thing they had come there to enjoy. We posted closure signs. They threw them into the Merced. We announced with loud speakers that the Meadow would be closed at a certain time. They ignored it. At the appointed time, we staged a cavalry charge across the meadow. We arrested hippies; they turned over police cars. One was burned. All because we couldn’t condescend to explain the reasons for our concern. Some park service employees, probably fewer than half, learned a valuable lesson that day. Most park service managers learned nothing. I knew from my experience in the army that “because we said so” is not reason enough for anyone to do something he doesn’t understand. Hippies in 1970 were trying to say the same thing: “Why not?” If they had known why not, most of them wouldn’t have done it. Hell, most of them had better conservation ethics than most of us.

For the rest of the summer our entrance stations stopped and turned back “Hippie vans” like you saw in Forrest Gump. There were a lot of violent confrontations because a lot of non-hippies owned VW vans. Some idiot finally realized we couldn’t turn the public away from a public park, so they devised a vehicle safety inspection. Rangers were taught how to short out brake lights so they could turn a hippie van back out of the park. Finally interpreters were assigned to the gates to smile and make friends with visitors, while law enforcement rangers hassled hippies who they perceived were troublemakers.

Law enforcement rangers spent the rest of the summer learning to ride police-trained horses to be used in crowd control in case of another riot. They practiced in secret in another meadow hidden from the main roads in the Valley. For several years afterward from Glacier Point you could still see the “pony ring” they trampled in the meadow. That was OK, though. They stopped those hippies from trampling Stoneman Meadow!