For the Little Walnut Creek Park, I saw this post on instagram:
We are pleased to announce the City of Austin is hosting a pop up event alongside @camachoactivitycenter @austinridgeriders at Little Walnut Creek Park. This is a celebration of what is likely going to the very first professionally built and fully sanctioned MTB trail system/park in Austin, Tx. This is not a work day. Bring your bike or some hiking boots. We will be there on Saturday, May 21st at 11:00 a.m. Location is 51st street park entrance across from Senate Hills.
https://www.instagram.com/p/CdG__O9FjQk/