Can someone identify this creature?
This is a Burrowing Mayfly nymph (order Ephemeroptera), more than likely of the family Ephemeridae.  The Bugguide site has very few photos of nymphs, but you'll find a near miss here (the species shown is not Californian, but the related Ephemera simulans Walker, 1853 is; see here for other possibilities [you'll want to look for nymph photos of the species listed from "USA: SW"]).