What insect/crustacean is this?

In the summer when i go to the American river in Norcal, i always see these weird creatures under the water that look as if a hermit crab and a beetle bred. The body is black and it has six legs and a beetle-like head. the "shell" is made of sand and small pebbles that somehow stuck to the body, but my older brother pulled the "shell" off of one, so i guess it's not completely permanent. the "unshelled" body is kinda gross and grub-like. they're all over the place under rocks and stuff along the riverbed. they live underwater. i've always wondered what these things were, can someone who knows tell me?