Those of us who grew up in Austin can remember making spur-of-the-moment jaunts to Hamilton Pool Preserve, where the cool, blue-green waters offered instant relief on 100-degree days.

But as you probably know, Hamilton Pool — a natural pool created thousands of years ago after the dome of an underground river collapsed due to erosion — has experienced excessive crowding of late and several years ago switched to a reservation system.

Right now, reservations are being accepted through the end of the year. The earliest available openings are weekday slots during the last two weeks of August. September offers ample weekday openings but fully booked weekends. October offers weekend and weekday availability. Last-minute cancellations sometimes occur; check website regularly for additional openings.

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Reservations are required at Hamilton Pool between March 1 and Oct. 31 and on weekends and official Travis County holidays during November and December. You can pick a morning or afternoon reservation (9 a.m. to 1 p.m. or 2 p.m. to 6 p.m.); each reservation includes one vehicle with no more than eight people.

Want to take a dip at Hamilton Pool? Here are some other things you need to know.

Bring cash. Cost for a reservation including service fee is $11 and is paid online when you make your reservation. Make sure you bring cash to the park, too, because an additional $15 cash entrance fee is required on the day you visit.

Wear tennis shoes. It’s advisable to wear tennis shoes because a steep and rocky quarter-mile hike is required to access the pool.

Swimming is not guaranteed. Sometimes visitors are not allowed to swim due to high bacteria levels. Call the public information line for the current status of the preserve: 512-264-2740. Life vests are available but no lifeguard is on duty. No pets are allowed either.