Yes, of course most of the parks you can't practically close, but you can officially close them - close the road gates, shut down the services, etc. In some cases, that's not a giant obstacle to using the park (a lot of the parks here on the east coast can be walked into from not too far away), but someplace like Yosemite it would be a different story. And some of them they can't fully close anyway because there are communities within the park - can't deny residents of the park access to their own homes. I'm sure they'll work something out - probably have to have the gates manned to turn away non-residents. If you want to see some amazing parkland in that part of the country, try Capitol Reef - it is cris-crossed with public roads. The only thing would be the visitors centers would be shut down.