Looks undisturbed. Does the field across the water get mowed? How is it there?

I'm not sure if "how is it there" means what is it like over there or how did the open field come to exist, but I can't answer either question with authority. There is an outlet with a bridge over it at the east end of the dam, and all the property beyond that is private. (Actually, the reservoir is privately owned, too, by the Pacific Gas and Electric Company, but they allow visitors to walk around their property without supervision-- most likely because it would be too costly to keep them out.) In any case, I've never visited that field, but I suspect that it is not mowed.

Whether it was deliberately turned into an open field, or is one of the natural meadows that exist throughout the forests and woodlands of the region, I couldn't say. It might be grazed, most likely by horses (it being rather small for cattle, and, being unfenced, sheep would wander off into the woods.) Many people in the area keep horses. I've even seen them tethered in vacant lots here in town, cropping the spring grass. Again, because the land looks to be unfenced, it probably gets cropped by the abundant local population of deer, as well.

In any case, if it isn't deliberately kept open, the wood will eventually move back in, and wildfires will create other meadows in other places. They will last a few years or a few decades, and then be recolonized by the woodlands in their turn. The landscape of the Sierra is remarkably dynamic and, even left on its own, would still change considerably over the years.