While they surely occur elsewhere, I have better luck in places with some elevation. Avoid the beaten paths, they do. Stones with lichen on them in their arid habitats is an indication that the elusive moisture they need is beneath, and yield the best results for me.I just spent several days in California, and took every opportunity to flip rocks, both within the city and out in the desert, and didn't find a single Polymorpha. Your are lucky that one came to find you. Is it doing ok?