Sigmoria planca.
Photo: 07/01/18, 8 pm.

I found a couple of these millipedes under our garbage carts. I moved one of them to my truck bed that had collected some rain water, and was really surprised to see the millipede run into the water where it tried to hide. They are probably Sigmoria planca. The genus has 15 species in the US.  

bug guide (these photos):