single crayfish moving slowly across the silty bottom of a pool in South Fork Wolf Creek; it eventually excavated a space under an underwater root by the creek bank and disappeared; note the white 'signal flag' patch on the claw; native to the Pacific Northwest, this is an invasive species in California & the Sierra Nevada; it has outcompeted native crayfish and reduced benthic populations, particularly snails; once in a stream it is very difficult to eradicate.