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Rivers: generally healthy, but not immune to risk

Unlike Borneo’s tropical rainforest ecosystems, most of the island’s rivers remain in relatively good condition. However, localized impacts from goldmine pollution are affecting the quality of water bodies in some places.
© © WWF / Simon Rawles
The Mahakam River, East Kalimantan - a key site for WWF Heart of Borneo work
© © WWF / Simon Rawles
Life in Borneo's waters
Soil erosion and subsequent heavy sedimentation of rivers is very obvious now in many parts of Borneo. This can be a noticeable problem where logging activities take place, and where forest is converted for alternative land use, such as industrial plantations.

Such incidents have come at a cost. There is evidence that some populations of freshwater fish have declined over the last few decades in many Borneo rivers, such as in the Barito. Lakes which are bearing the brunt of logging and conversion of forests and new settlements.

However, new species of freshwater fish continue to be discovered.

The major waterways of Borneo

Borneo’s major rivers are a vivid example of how dynamic the island’s ecosystems can be.

Some of the major waterways that stand out in the Borneo landscape include the Kapuas river (in Kalimantan, Indonesian Borneo), which is 1,143 km,only slightly shorter than the Rhine. The Kapuas flows to the west coast, draining some 2/3 of West Kalimantan province - a watershed of 100,000 km2 (an area slightly larger than Hungary).

Other major rivers in Kalimantan include the Barito (900 km), which flows south, and the Mahakam (775 km), which empties into the Makassar Strait to the east of Borneo.

In Borneo’s inland basins and in the lowlands, several major rivers form extensive lake systems. The Mahakam, Barito, Kapuas and Baram rivers form oxbow and seasonal lakes in their lowland reaches, some of which are of economic importance for fisheries.

The journeys of Borneo’s rivers

Like sponges, Borneo’s tropical rainforests regulate the quantity of water that drains into streams in the uplands. These merge with others to form larger waterways, which change water volume depending on the seasons. During the rainy season, water levels can change very fast. For example, much of the Kapuas can rise 10-12 metres overnight during heavy rainfall.
© WWF-Indonesia / West Kalimantan
Fishermen in Danau Sentarum National Park area, West Kalimantan, Indonesia
© WWF-Indonesia / West Kalimantan
© Rob Buiter / WWF
Danau (Lake) Sentarum National Park, West Kalimantan (Borneo), Indonesia. October 2004Danau (Lake) Sentarum is one the most unique wetlands in Asia, marked by outstanding biodiversity. This area was declared as a National Park in 1999, covering 132,000 hectares. The park is located in the Kapuas Hulu District, which declared itself a Conservation District in 2003.
© Rob Buiter / WWF
From trickles to roaring torrents
From the fast and clear headwaters to the wide-bodied and slow-flowing rivers of the lowlands, the ecology of Borneo’s rivers varies enormously during their course.
  • Montane streams

High up in the Heart of Borneo’s montane forests, waterways mostly consist of cold-water torrents. There are few plants onthe river banks and wildlife is mostly aquatic insects.

In these streams, evolution has endowed invertebrates with useful adaptations such as flattened bodies and the presence of hookers, to avoid being swept away by the strong current. One fish genus in particular has evolved a handy ‘tool’ - the sucker fish (Gastromyzon species) has suckers to attach itself to the bottom of fast-flowing waters.
  • Upland streams

Between 100 metres and 1,000 metres, waterways become cool-water torrents. Some plants have evolved to be streamside specialists. Aroids (familyAraceae) such as Piptospatha species appear on the sides of smaller tributaries while Saraca species, small trees of the legume family, grow along larger streams.

Compared to montane streams, upland streams havemore activity. Fish found here are often strong swimmers, with well-adapted streamlined bodies. There may be 20 species each of dragonflies, mayflies and stoneflies.
  • Lowland rivers

Follow the course of a river below 100 metres and a noticeable ‘wall’ of tall trees begins to form on either side. Underwater, sedimentation often causes the water to become turbid. This favours fish that have adapted barbells - such as catfish - to identify prey in low visibility conditions.
© Jonne Seijdel / WWF-Netherlands
The Kapuas river captured from above out of an ultralight plane.
© Jonne Seijdel / WWF-Netherlands
© Jonne Seijdel / WWF-Netherlands
Dragonfly, Teluk Aur, West Kalimantan, Indonesia. Close-up of a dragonfly sitting on a green leaf. Black head with white, orange, black tail.
© Jonne Seijdel / WWF-Netherlands
Borneo’s freshwater wildlife
For Borneo’s freshwater wildlife, life is shaped by many factors (e.g. temperature, water flow, chemicals and competition with other species) that restrict their distribution in a very precise way. As a result, wildlife of river and lake ecosystems is heavily compartmentalized, with each species occupying a well-defined place in an ecosystem.

Seemingly insignificant factors can affect species distribution. For example, the location of freshwater prawns and crayfish can be determined by how big the sediment particles are.

More obvious factors also define the ecological make-up of a particular freshwater habitat. Plant and invertebrate food supply, shade provided by overhanging vegetation and the presence or absence of riffles (shallower, faster moving sections of a stream) and pools are factors that are all considered when a species seeks a home.

In particular, top predators such as the false gharial (Tomistoma schlegelii) and the estuarine crocodile (Crocodylus porosus), river turtles, monitor lizards and otters shape the ecology of other species lower in the food web.

The endangered otter civet (Cynogale bennettii) is a very competent fisher in forest rivers. Webbed feet, and valve like nostrils and ears allow it to swim freely in search of fish, molluscs and crayfish.
© Alain Compost / WWF
Young False gharial (or gavial), Tomistoma schlegelii. Found in freshwater lakes, rivers & swamps of Indonesia (Sumatra, Kalimantan, Java, possibly Sulawesi), Malaysia (Malay Peninsula, Borneo), and possibly Vietnam. A poorly-studied species. Few surveys done, but those completed reveal very low populations in fragmented habitat. Enforcement of protected areas in limited. Habitat destruction (cultivation, dams & flood mitigation), drowning in fishing nets, overfishing of food resources and, to a limited extent, the skin trade threaten the species.
© Alain Compost / WWF
© WWF-Indonesia/Jimmy Syahirsyah
Spiny terrapin (Heosemys spinosa)
© WWF-Indonesia/Jimmy Syahirsyah