Hayfork, California. Hayfork is a census-designated place (CDP) in Trinity County, California, United States. The population was 2,368 at the 2010 census, up from 2,315 at the 2000 census. Hayfork was originally named Nor'el pom, after the Native American tribe that lived in the area. The community was so named on account of hay fields near the South Fork Trinity River.