Kapuas River - Geography and Hydrology

Geography and Hydrology

The river is 1,143 km (710 mi) long and up to 700 m (2,300 ft) wide at its delta; with a total area of 98,749 km², the river basin covers more than 67% of West Kalimantan. The discharge rate varies through the year, averaging around 6,000–7,000 m³/s at the delta and 2,000 m³/s 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 10–12 m 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 of the western slope of the Müller Mountain Range, which runs through the island center, and the southern slope of the Upper Kapuas Range (Indonesian: Kapuas Hulu) which is located more to the west. For about 165 km it flows through a mountainous terrain and then descends to a marshy plain. There, the elevation decreases by only 50 meters (160 ft) over 900 km (560 mi) from Putussibau to the river delta. About 350 km 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 300 mm, the river overflows its banks, diverting much of its waters to the lakes at a rate of up to 1,000 m³/s, and forming a single water mass 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 50–60 km from the island. 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 (Indonesian: Kapuas Besar). 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.