There are 21 US Post Offices in Siskiyou County serving a total of 45,919 residents equating to 2,187 residents per post office. It's estimated that approximately 73,292 packages are handled across the 21 county post offices annually. Siskiyou County /\u02c8s\u026askju\u02d0/ SISS-kew is a county located in the northernmost part of the U.S. state of California. As of the 2010 census, the population was 44,900. Its county seat is Yreka. Siskiyou County is located in the Shasta Cascade region along the Oregon border. Because of its outdoor recreation opportunities and Gold Rush era history, it is an important tourist destination within the state."