There is a Great Place to Swim in Upper Bidwell Park in Chico, California. The Lava from Mount Lassen Flowed down there and Crystallized into Black Rocks.

I swam thru the Lava Tubes... They are Now Closed because a Student Drowned... But in 1973, they were safe... Lava Rocks cool and harden Outside first and the inside Lava keeps on flowing... creating a Lave Tube... and there is one underwater in Chico. 

There is a Giant WPA Pool at the Legendary One Mile Recreation Area on Chico Creek. It's Open All Night... Most Parks, like Oak Meadow, are Closed at Night. What were they thinking? I remember Butte Creek was a Blast to go Tubing on. A Wild Ride in the Spring with Snow Melt. There is a Big Tubing Event on the Sacramento River as well, but I never went. 

Just for Your Information... There is a Great Swimming Hole on the Big Sur River just Upstream of The Pfeiffer Campground... and a Hot Spring twelve (12) miles in at Sykes Camp. I have very few regrets in life but, Once someone invited me to go to the Esalon Hot Springs because it was free of charge after Midnight... and I declined... OOPS! You can see it in the Movie "Celebration at Big Sur" with Joni Mitchell, CSN and Joan Baez... That's an Epic movie as Joni Plays the Piano and sings her brand new song "Woodstock." When I left Los Gatos in 1973 I went to Chico State and then returned in the Late 1990s... 

Bear Hole in Chico, California - Upper Bidwell Park