The Mekong Delta area of Vietnam technically extends from Go Cong Province all the way down to the Ca Mau Peninsula, the area drained by the Mekong River as it reaches the South China Sea. Occasionally the Vam Co, Saigon, and Dong Nai rivers are also included in the region known as the Mekong Delta, giving it a total of more than 26,000 square miles. The Mekong Delta area is known for its complex system of rivers and canals as well as its rice cultivation. Formed by silt deposits, the soil is exceptionally fertile, and at the mouth of the Mekong River the sediment extends the shoreline by an average of 250 feet a year. The Mekong Delta area is inhabited mainly by ethnic Vietnamese, with large concentrations of Khmers in the southwestern areas. The Mekong Delta was included in the IV Corps Tactical Zone during the Vietnam War.