The Kapuas River (or Kapoeas River) is a river in the Indonesian part of Borneo island, at the geographic center of Maritime Southeast Asia. At in length, it is the longest river in the island of Borneo and the longest river of Indonesia and one of the world's longest island rivers. It originates in the Müller mountain range at the center of the island and flows west into the South China Sea creating an extended marshy delta. The delta is located west-southwest of Pontianak, the capital of the West Kalimantan province. This Kapuas River should be distinguished from another Kapuas River, which starts on the other side of the same mountain range in central Borneo, but flows to the south, merging with the Barito River and discharging into the Java Sea.
Geography and hydrology
The river is long and up to wide at its delta; with a total area of , the river basin covers more than 67% of West Kalimantan. The average annual rainfall in the catchment area is 3,666 mm. The average runoff is around 2,339 mm. The discharge rate varies through the year, averaging around at the delta and upstream, at the confluence of the Tawang River. The discharge peaks during the rainy seasons in April and November, during which the water level may rise by overnight, overflowing river banks and flooding the nearby areas.
The river originates near the center of Borneo, south from the Indonesian-Malaysian border, in the joint between the western slope of the Müller Mountain Range, which runs through the island center, and the southern slope of the Upper Kapuas Range (), which is located more to the west. For about it flows through a mountainous terrain and then descends to a marshy plain. There, the elevation decreases by only over from Putussibau to the river delta. About from the source, near the northern shore of the river, lies a system of Kapuas Lakes which are connected to the river by numerous channels. These lakes are Bekuan (area 1,268 hectares), Belida (600 ha), Genali (2,000 ha), Keleka Tangai (756 ha), Luar (5,208 ha), Pengembung (1,548 ha), Sambor (673 ha), Sekawi (672 ha), Sentarum (2,324 ha), Sependan (604 ha), Seriang (1,412) Sumbai (800 ha), Sumpa (664) and Tekenang (1,564 ha). When the monthly precipitation exceeds about , the river overflows its banks, diverting much of its waters to the lakes at a rate of up to , and forming a single volume of water with them. This outflow prevents massive flooding of the lower reaches of the river; it also promotes fish migration from the river to the lakes for spawning, but drives birds away from the lakes.
The river discharges into the South China Sea creating a marshy delta, which spreads both inland and into the sea, with the silt deposits extending up to from the Borneo coast. The delta is located west-southwest of Pontianak, the capital of West Kalimantan province, which lies at the equator. The delta has five arms, of which the northernmost one is the widest, and is therefore called the Big Kapuas (). The largest tributary is the Melawi River, which occurs to the left near the city of Sintang, about 465 km from the mouth. Other major tributaries are the Landak, Kubu, Punggur and Sekayam rivers.
Climate
The climate is warm and very humid, with the average annual precipitation ranging from year to year between , and the number of rainy days between 120 and 309; the largest precipitation was observed in 1976 (120 rainy days) and the wettest in 1988, with only 184 rainy days. The temperature is rather stable with a typical minimum of and maximum of through the whole year.
Flora and fauna
In the upper and middle reaches, the river flows through dense tropical forests; the rich flora and fauna are the subject of international research. Discoveries of new species are frequent, such as of the Kapuas mud snake (Enhydris gyii), which was discovered in 2003–2005 by German and American herpetologists. This species is remarkable in that it can spontaneously change its skin color, similar to the chameleon.
Otters and crocodiles are common in the Kapuas River, but frogs are nearly absent. Agile gibbons (Hylobates agilis), Müller's Bornean gibbons (Hylobates muelleri), Prevost's squirrels (Callosciurus prevostii) and treeshrews inhabit the trees above the river.
There are two national parks on the river banks, Betung Kerihun with an area of 8,000 km2 and Danau Sentarum (area 1,320 km2), the latter includes the Kapuas Lakes.