The Tapi (or Tapee) river is the longest river in southern Thailand. The river originates at Khao Luang mountain in Nakhon Si Thammarat Province, and empties into the Gulf of Thailand at Bandon Bay near the town of Bandon. It has a length of 230 km. The river drains an area of 5460 km2 and in 1997 had a yearly discharge of 135.4 m3/s or 4.3 km3 per year. The Phum Duang River (or Khiri Rat River), which drains another 6125 km2 west of the Tapi watershed, joins the estuary 15 km west of Surat Thani in Phunphin district. The river was named on 29 July 1915, after the river Tapi in Surat, India, shortly after the town of Surat Thani was named after the town Surat in Gujarat, India. The island of Ko Lamphu (เกาะลำพู) is in the Tapi River, about 9 km from its mouth, near Surat Thani town center. In 1975, an area of 29.6 km2 of swamp land on the east bank of the river in the Khian Sa District was declared the Nong Thung Thong non-hunting area.