What Is This?

Seriously, I don't know what this is. I just know that my "pattern picture" alarm bell went off when I saw it. This is a shallow pool in South Arroyo just off the river. The mud is covered with an inch or two or water, and looks like it will be drying up soon. We saw tadpoles in the water, so I'm wondering if these are tadpole holes -- they burrowed into the mud to enhance their chances of survival. But why are the holes so evenly spaced? In my 35 years here, I have never seen this before.