A 1945 map showing the two Kapuas rivers of Borneo (Kapoeas on the map)
TheKapuas River (Old Indonesian spelling:Kapoeas River) is a river in theIndonesian part ofBorneo island, at the geographic center ofMaritime Southeast Asia. At 1,143 km (710 mi) in length, it is the longest river on the island of Borneo, the longest river in Indonesia[5][6] and one of the world's longest island rivers.[7] It originates in theMüller mountain range at the center of the island and flows west into theSouth China Sea creating an extended marshy delta. The delta is located west-southwest ofPontianak, the capital of theWest Kalimantan province.[8] This Kapuas River should be distinguished from anotherKapuas, which starts on the other side of the same mountain range in central Borneo but flows to the south, merging with theBarito and discharging into theJava Sea.
The river is 1,143 km (710 mi) long and 99,000 km2 (38,000 mi2)[2] theriver basin covers more than 67% of West Kalimantan.[6][9] The river originates near the center of Borneo, south of the Indonesian-Malaysian border, in the joint between the western slope of theMüller Mountain Range, which runs through the island center, and the southern slope of theUpper Kapuas Range (Indonesian:Kapuas Hulu), which is located more to the west. For about 165 km (103 mi) it flows through a mountainous terrain and then descends to a marshy plain.[10] There, the elevation decreases by only 50 m (0.031 mi) over 902 km (560 mi) fromPutussibau to the river delta.[11]
About 350 km (220 mi) from the source, near the northern shore of the river, lies a system ofKapuas Lakes which are connected to the river by numerous channels. These lakes areBekuan (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) andTekenang (1,564 ha).[12] When the monthly precipitation exceeds about 300 mm (12 in), the river overflows its banks, diverting much of its waters to the lakes at a rate of up to 1,000 m3/s (35,000 cu ft/s), 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.[13]
The river discharges into theSouth China Sea creating a marshydelta, which spreads both inland and into the sea, with thesilt deposits extending up to 50–60 km (31–37 mi) from the Borneo coast.[11] The delta is located west-southwest ofPontianak, the capital ofWest Kalimantan province, which lies at theequator.[14] The delta has five arms, of which the northernmost is the widest, and is therefore called the Big Kapuas (Indonesian:Kapuas Besar).
The average annualrainfall in the catchment area is 3,666 mm. The averagerunoff is around 2,339 mm.[15] Thedischarge rate varies through the year, averaging around 5,600–7,800 m3/s (200,000–280,000 cu ft/s)[16][17] at the delta and 2,000 m3/s (71,000 cu ft/s) upstream, at the confluence of theTawang. The discharge peaks during the rainy seasons in April and November, during which the water level may rise by 10–12 m (33–39 ft) overnight, overflowing river banks and flooding the nearby areas.[6][10][13]
The largest tributary is theMelawi, which occurs to the left nearSintang, about 465 km from the mouth. Other major tributaries are theLandak,Sekayam,Ketungau,Tawang,Bunut andEmbaloh rivers.[21]
The climate is warm and very humid, with the average annualprecipitation ranging from year to year between 2,863 and 5,517 mm (Köppen Climate-classificationAf), 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.[9] The temperature is rather stable with a typical minimum of 24 °C (75 °F) and a maximum of 32 °C (90 °F) throughout the whole year.[22]
In the upper and middle reaches, the river flows through densetropical forests; the rich flora and fauna are the subject of international research.[9] Discoveries of new species are frequent, such as theKapuas mud snake (Enhydris gyii), which was discovered in 2003–2005 by German and Americanherpetologists. This species is remarkable because it can spontaneously change its skin color, similar to thechameleon.[23][24]
Part of the high species diversity in the Kapuas is related to the many different habitats in the river basin. In the headwaters arefast-flowing highland streams, typically dominated by smallloaches, and small —often acidic (blackwater)— forest streams andpeat swamps with species such as the tinySundadanio rasboras,Sphaerichthys gouramis, andmacropodusine gouramis.[28] The main river also includes several habitats, ranging from the nearshore to open waters. In the deepest sections, no light exists and in one species,Lepidocephalus spectrum, this has resulted in a complete reduction of both eyes and pigmentation (similar tocavefish).[30]
FemaleHemirhamphodon pogonognathus, about 4 cm (1.6 in) long
Owing to the warm climate and abundance of food, most fishes breed all through the year with only a few species likefire eel (Mastacembelus erythrotaenia) having certainreproductive periods. The number of individuals per species is relatively low.[clarification needed] The large variety of species may be explained by the fact that some 6,000 years ago, the Kapuas River, as a tributary of theSunda River, was connected to other tributary rivers ofSouth Sumatra,Java, and theMalay Peninsula. Apart from fish, there are numerouscrabs,prawns (such as the giant freshwater prawnMacrobrachium rosenbergii),[31]water striders, and otheraquatic insects.[32] The rich flora and fauna result in very complexfood chains, with fishes consuming foods ranging from fruits toother fish. For example,Hemirhamphodon pogonognathus feeds exclusively on terrestrial insects. Abundant fruits and seeds enter the river after falling from large trees that bend over its waters.[33] The feeding habits of the fish in the Kapuas River are distributed as follows: 54% areomnivores; 36% are carnivorous and eat other fish (14%), insects (5%), and mixed small forest animals (17%). The remaining 10% are herbivorous, with 4%specialising in algae.[25]
The Kapuas River is the major waterway connecting the center of the island with its western coast. The large river width and depth (up to 27 meters[7]) support intensive cargo and passenger shipping over most of the river length. Ships with adraft of up to 3 meters can navigate up toSintang, 465 km from the mouth, and those with a draft of up to 2 meters can reach the town ofPutussibau (902 km from the mouth). Logging and rafting of timber occur all along the river. Fishing is also common, especially at the Kapuas Lakes and near the river delta.[10][34] TheTayan Bridge which opened in 2016, crosses over the river and is the longest bridge in Kalimantan.[35] Another major bridge over the area is the Kapuas I Bridge. In 2022, the local government decided to build a newer bridge to accommodate increased traffic in commuters and goods, located to be in parallel with the existing bridge.[36]
^abcdRoberts, T.R. (1989). "The freshwater fishes of Western Borneo (Kalimantan Barat, Indonesia)".Memoirs of the California Academy of Sciences.14:1–210.
^Deein, Gridsada; Tangjitjaroen, Weerapongse; Page, Lawrence M. (2014). "A revision of the spirit loaches, genusLepidocephalus (Cypriniformes, Cobitidae)".Zootaxa.3779 (3):341–352.doi:10.11646/zootaxa.3779.3.2.PMID24871728.
