No camping at
Hamilton Pool Preserve, since it's a nature preserve. "Nature Preserve" seems to be the official wording (at least for non-federal lands) for areas that are open to the public but do not permit development or overnight camping, due to potential impact upon the land & critters therein. (Another Texas "nature preserve" that come to mind are
Friedrich Wilderness Park in San Antonio.)
I have a
Hamilton Pool slideshow (21 photos) from a mid-November 2000 hike at Hamilton Pool. We had no swimmers and, indeed, had the park entirely to ourselves. Probably a combination of the temperatures of mid-November and recent rain -- Austinites may have assumed to park was closed (it does close in bad weather, due to flooding/danger of flooding) at not ventured out that day.