Река Раджанг в Сараваке, Малайзия

River in Sarawak, Malaysia

Malaysia's largest and tallest (160m) hydro electric project, the Bakun Hydro Electric Dam, is located on the Balui River, a tributary of the Rajang. Other important tributaries include the Katibas River, Ngemah River, Iran River, Pila River, Balleh River, Bangkit River and the Kanowit River. Upriver areas, such as the areas surrounding the headwaters, are called "Ulu" (which means "upriver") or "Hulu" in Malay. "Hulu Rajang" is the upriver section of the Rajang river. Places along the river that are too small to be called towns are named "Nanga" in the Iban language, which is the language used in the areas where the tributaries join the main river. Examples of Nangas are: Nanga Ngemah, Nanga Dap, and Nanga Poi. In Iban, "Nanga" means "longhouse" and is often followed by the name of the river that runs by it. There are certain communities that use "Rumah" (meaning "house" in Malay), often followed by the name of the headman (thus it can change in time). "Long", meaning "confluence", is used by the Orang Ulu (upriver people) to name places located at the confluence between the smaller tributaries and the major river, just as Malay uses the name "Kuala" (meaning "river delta"). Chinese immigrants may have been reminded of another "Swan River" back in China.

The Rajang is the longest river system in Malaysia, with its source in the Nieuwenhuis mountains, where it forms the political boundary between Malaysia and Indonesia. There is more than of rainfall each year in these mountains, which forms the headwaters of Rajang. The river flows from northeast to southwest. The Punan Bah people reside at the headwaters of the Rajang. One of the Rajang tributaries, the Balui River, north of the Hose Mountains, flows into the lands of the Kayan people and Kenyah people. At the western end of a great bend in the river is the Belaga District, and further downstream are the Pelagus Rapids where the Iban people dominate. The rapids are located 180 mile (290 km) from the Rajang river mouth. As the Balleh River joins the Balui river from south of the Hose Mountains, At this intersection point, about 160 mile (257 km) from the river mouth, the river begins to flow from east to west, near the town of Kapit. Kapit is the last stop for express boats coming from Sibu, and is inhabited by the Chinese, Iban, and Kayan people. Kapit is the biggest town in the upper Rajang river. Further downriver are smaller towns such as Song, which is located at the mouth of the Katibas River. At about 120 mile (193 km) from the sea, there are owned by both Chinese and Iban people. The lands are divided into parcels measuring several acres, where smallholders clear the land for agricultural activities. In the midst of these Mixed Zone Lands, 105 mile (169 km) miles from the sea, is the town of Kanowit; the river here is 0.5 mile (1 km) wide. The biggest town on the banks of the river is Sibu, some 65 mile (105 km) upriver from the mouth of the Rajang. It can be reached by ocean going vessels, and is the political, economic, cultural, and education centre of the central region of Sarawak. Other towns located further downriver are Sarikei (24 mile (39 km) from Rajang town) and Bintangor (11 mile (18 km) from Sarikei). These two towns have been mainly settled by the Chinese. Tanjung Manis District (16 mile (26 km) from the river's mouth) is located near the Rajang delta where the Malay and Melanau people dominate. Bruit Island is situated inside the Rajang delta, with a lighthouse at Sirik Point.

The population of the Rajang basin was very low in the 19th century during the Bruneian Empire. The people living along the river's banks traded with Malays from Brunei. At that time, the Melanau, Kanowit and Rajang ethnic groups lived downriver; Bhuket, Punan Bah, Lugat, Sihan and Kejaman ethnic groups lived at the middle section of the river, with Penan and Seping tribes living in the upriver area. After the 19th century, the Kayan and Kenyah migrated to the Balui river from the present day Indonesian Kalimantan. Meanwhile, the Iban people migrated from West Kalimantan to the lower Rajang valley, moving their agricultural base. Inter ethnic conflicts followed the introduction of new tribes into the Rajang basin, and some ethnic groups fled to the Balingian and Tatau rivers. In the late 19th century, the government of Charles Brooke began constructing forts along the Rajang in an attempt to achieve political stability in the Rajang basin. The Chinese then started to move inland and settle near the forts and trade various forest products with the natives. In 1884, Fort Vyner was constructed, and Belaga soon became the political and economic centre of the district. Subsequently, government offices, schools, a bank, a police station, a mosque, and churches were built around the fort. On 25 March 1973, during the period of the Communist insurgency, the Sarawak government set up the "Rajang Special Security Area", and Rajang Security Command (RASCOM) was formed as a result of co operation of civil, military, and police command headquarters, to clamp down on communist activities in the Rajang delta. Construction of the Bakun Dam on the upper Balui River, from 1997, resulted in the Sungai Asap resettlement scheme.

A total of 122 species of bird was recorded by the 2004 study, with 21 species along the mouth of the river, 96 species in the Hose Mountains, and 88 species at Lanjak Entimau in the interior of Sarawak. The only endemic species of bird found in the Rajang basin was the Dusky munia, found abundantly in paddy fields. Other species of bird commonly found along the Rajang basin are the Pacific swallow, Little spiderhunter, and Asian glossy starling. Twenty species of birds (mainly eagles, egret, kingfishers, woodpeckers) are protected, while seven species of birds (mainly hornbill and pheasant) are totally protected under the Sarawak Wildlife Protection Ordinance 1998. Empurau (Tor tambroides) and Semah (Tor douronensis) fish are considered popular fishes among the locals. the Empurau is still found breeding near the upper tributaries of Rajang such as the Balleh River, and also near the Bakun Dam.

The major economic activity in the upper reaches of the Rajang River is logging. Among the types of wood that are extracted for exports are Ramin, plywood, Kapur, Meranti, Keruing, and Belian. Rice is the dominant crop along the middle section of the Rajang basin. Trading of Engkabang fruits ("illipe nuts") were commonly traded in the 1960s and 1970s, with the trade centred on Sibu. The fruits are also a food for fish such as empurau that populate the river. Other agricultural activities in the Rajang basin include the cultivation of rubber, pepper and vegetables, and poultry domestication.

The Rajang Port Authority (RPA) is the main regulatory body for coordinating the trade activities of all five river ports along the Rajang River. RPA serves to provide maintenance, adequate and efficient port services for all users of the ports. It is located at the Sibu port operation centre Sibu and Sungai Merah can be assessed via a shorter route through the Paloh river where the distances from the mouth of the river can be shortened to 82 km and 92 km respectively. although its total cargo throughput went down from 5.0 million to 3.2 million tonnes from the previous year. The five river ports along the Rajang river (arranged by their distances from the river mouth) are: but were terminated because of logistical and operational difficulties. During 2017's "Visit Sibu Year", longhouse visits along the Rajang River were promoted as an ecotourism project. Crocodiles, which live by the river banks, became the mascot for the event.

The town of Sibu can be assessed by oceangoing vessels for 80 miles (130 km), while an additional 100 miles (160 km) of the river can be assessed by shallow draft craft. The remaining parts of the river, leading into the Sarawak interior, can only be assessed by small canoes.

Logging operations are rampant on the upstream of the Rajang river. A major study conducted along the Rajang river from May to December 2004 found that the main channel of the Rajang river was turbid except for smaller tributaries. The water transparency was about 4 cm to 5 cm at that time. Based on Taxonomic richness index, the stretch of upstream river between Kanowit and Belaga river is recovering from logging 15 to 20 years ago with the exception of Penaan river because the local community uses toxic chemicals to capture fish. The stretch of river downstream from Lebaan to Selalang river is considered unclean.

Floods in the Rajang delta near the coast are usually caused by the rising tide while floods 120 km inland near Sibu is caused by heavy downpour with inadequate drainage caused by sediment accumulation in the river bed; as a result of heavy development of the river upstream. In September 2020, heavy rain in the upper tributaries of the Rajang caused flash floods in the low lying areas of Kapit, Song, Kanowit and Sibu.

On 6 October 2010, heavy rain have caused a massive landslide that brought logs and debris down into the Balleh river, tributaries above Kapit, and then into the Rajang river. It was estimated that the volume of the logs and debris had exceeded 300,000m in length. The logjam also affected the Igan river. The log debris, stretching 50 km, reached Sibu on 8 October, causing transportation difficulties for the people living alongside the river and killed many fish. On 2 November 2012, Global Witness claimed that the logging operations of Interglobal Empire, a subsidiary of the WTK group of companies, owners of the logging concession T/3476, were responsible for the landslide into the Melatai River which subsequently blocked the Rajang River. Similar logjam occurred in early 2017 at the upper reaches of the Balui. In August 2021, logjam blocked the Rejang River, shutting down the water treatment plants in Sibu. In March 2024, logjam happened near the town of Song, Kapit Division.