Pacifastacus fortis (known as the Shasta crayfish or placid crayfish) is an endangered crayfish species endemic to Shasta County, California, where it is found and first described in 1914, only in isolated spots along the Pit River and Fall River Mills. It is estimated that there are a total of roughly 4000 of the species still alive today. 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.