The eight islands are split among the jurisdictions of three separate California counties: Santa Barbara County (four), Ventura County (two), and Los Angeles County (two). The islands are divided into two groups—the Northern Channel Islands and the Southern Channel Islands. The four Northern Islands used to be a single landmass known as Santa Rosae. The archipelago extends for 160 miles (257.51 kilometers) between San Miguel Island in the north and San Clemente Island in the south. Together, the islands’ land area totals 221,331 acres (89,569 ha), or about 346 square miles (900 km2).
Five of the islands (San Miguel, Santa Rosa, Santa Cruz, Anacapa, and Santa Barbara) were made into the Channel Islands National Park in 1980. The Channel Islands National Marine Sanctuary encompasses the waters six nautical miles (11 kilometers) off Anacapa, Santa Cruz, San Miguel and Santa Barbara islands.
Santa Catalina Island is the only one of the eight islands with a significant permanent civilian settlement—the resort city of Avalon, California, and the unincorporated town of Two Harbors.
Natural seepage of oil occurs at several places in the Santa Barbara Channel. Tar balls or pieces of tar in small numbers are found in the kelp and on the beaches. Native Americans used naturally occurring tar, bitumen, for a variety of purposes which include roofing, waterproofing, paving and some ceremonial purposes.
The Channel Islands at low elevations are virtually frost-free and constitute one of the few such areas in the 48 contiguous US states. It snows only rarely, on higher mountain peaks.