As stated below, Reservations are required @ Colorado State Parks. This Park fills up quickly so yes, they are recommended

Reservations are actually required for camping at all Colorado State Parks. Best time to get them is 6 months in advanced when they release online.

We stayed at Chatfield State Park CG in 2019. We had reservations and I do recommend having them.

Yes, absolutely when school is out! There might be spots available at any time, but they won't be good ones.