The Fly at 1,050 kilometres (650 mi), is the second longest river in Papua New Guinea, after the Sepik. The Fly is the largest river in Oceania, the largest in the world without a single dam in its catchment, and overall ranks as the 25th-largest river in the world by volume of discharge. It rises in the Victor Emanuel Range arm of the Star Mountains, and crosses the south-western lowlands before flowing into the Gulf of Papua in a large delta.
The Fly flows mostly through the Western Province, though for a small stretch it forms the boundary between PNG and the Indonesia province of Papua. This section protrudes slightly to the west of the 141°E longitude line. To compensate for this slight gain in territory for PNG, the border south of the Fly River is slightly east of the 141°E longitude line. As part of this deal, Indonesia has the right to use the Fly River to its mouth for navigation.
The principal tributaries of the Fly are the Strickland and the Ok Tedi.
Close to its mouth, the flow of the Fly River encounters a tidal bore, where an incoming high tide pushes water upstream until the changing of the tide. The range of this tidal bore is still undocumented.
The estuary of the Fly River is 56 km wide at its entrance, but only 11 km wide abreast Kiwai Island, which may be considered as being the river mouth. Above this island, the river gradually contracts to a width of 1.6 km or less.
The river delta is studded with low and swampy islands covered with mangrove and nipa palm, with villages and cultivated areas on these islands. The land on both sides of the estuary is of the same character. The islands in the estuary are flat and covered with thick, fertile alluvial soil. The largest islands are Kiwai Island, Purutu Island, Wabuda Island, Aibinio Island, Mibu Island, and Domori Island. Kiwai, Wabuda and Domori are inhabited.
A list of the river delta islands is:
The inhabitants of the Fly River delta engage in agriculture and hunting. Coconut palm, breadfruit, plantain, sago palm, and sugar cane are grown.
The Fly was first discovered by Europeans in 1842 when Francis Blackwood commanding the corvette HMS Fly, surveryed the western coast of the Gulf of Papua. The river was named after his ship and he proclaimed that it would be possible for a small steam powered boat to travel up the mighty river.
In 1876, Italian explorer, Luigi D’Albertis, was the first person to successfully attempt this when he travelled 900 km into the interior of New Guinea, in his steamer, Neva. It was the furthest any European explorer had ever been into the island.
Both the Strickland and the Ok Tedi Rivers have been the source of environmental controversy due to tailings waste from the Porgera Mine and the Ok Tedi Mine, respectively. In 2008, Dr Ian Campbell, a former advisor to Ok Tedi Mining Limited, claimed that company data suggest significant portions of the Fly River floodplain are at a high risk from acid mine drainage.