Geographically, the centre of the province is the coastal plain of the Tapi river, mostly grassland interspersed with rubber tree and coconut plantations. In the west are limestone the mountains of the Phuket range which are mostly covered with forest. The Khao Sok National Park is located in these mountains. To the east the hills of the Nakhon Si Thammarat (or Bantat) mountain range start to rise, protected in the Tai Rom Yen National Park. Many islands in the Gulf of Thailand belong to the district, including the tourist islands Ko Samui, Ko Pha-ngan and Ko Tao, as well as the Ko Ang Thong marine national park. The main rivers of Surat Thani province are the Tapi River and the Phum Duang River, which join at the town Tha Kham shortly before they flow into Bandon Bay. The delta of these rivers, locally known as Nai Bang (ในบาง), is located directly north of the city of Surat Thani. It consists of several channels with small islands mostly covered by mangroves and orchards.