Pacifastacus fortis (known as the Shasta crayfish or placid crayfish) is an endangered crayfish species endemic to Shasta County, California, where it is found only in isolated spots on the Pit River and Fall River Mills. The exact subpopulations for the Shasta crayfish were discovered in 2004 through a genetic study that determined three different genetic clusters: Crystal Lake, the Big Lake group (which includes Big Lake Springs, JeShe, Lava, and Spring Creeks), and Thousand Springs.