Rajang River Delta, Sarawak, East Malaysia

Located on the western coast of the island of Borneo, the Rajang River is Malaysia’s longest waterway, extending 563 km from the coast adjacent to the South China Sea to the interior of the island. The river drains the central Massif, where Mount Kinabalu (Gunung Kinabalu) is the highest mountain on the island, (elevation of 13,435 ft; 4,095 m), ranking it the third-highest peak of an island on the planet. When the river reaches the coastal zone, the river spreads out over a wider area, bringing its sediment load into the ocean. The sediment load, comprised of sand and silt, is and deposited in a deltoid shape influenced by strong tides.

Photograph courtesy of Dr. K. Bartram - Distributary "T" Channel The course of the Rajang River, though, course is altered by deep-seated faults and diurnal (twice a day) tides that range in amplitude up to 6 meters in displacement. Fault lines in the subsurface control the forking of the river’s course. In most other coastal deltas of the planet, today, rivers that debouch into the ocean fork in a “Y” shape, with each half of the fork distributing half of the water volume and sediment load (for example, the Mahakam Delta on the other side of the island). In the case of the Rajang River, deep seated faults that run parallel to the coastline actually direct the river to fork in “T” shapes. Once the river’s course has breached the “T,” each fork continues towards the ocean until it encounters the next fault where, again, the river bifurcates in a “T” configuration.

Tidal activity along the South China Sea varies from the southwest to the northeast coastline. Tides, here, are classified as either mesotidal (tidal range of 2 to 4 m amplitude) or macrotidal (greater than 4 m amplitude). The lowest of low tides are referred to as occurring during the Neap tidal cycle; the highest of high tides occur during the King tides. During King tidal cycles,  the tidal bore extends more than 120 km inland to the town of Kanowit. When the tidal bore reaches its peak at turnaround, the Rajang River actually stops flowing inland and the river waters become calm. Velocity increases rapidly when the waters regain their velocity and are directed seaward.

Photograph courtesy of Dr. K. Bartram - Rajang Tidal ChannelTidal waters that surge into the coastal plain modify the course of each tidal channel, resulting in a serpentine (snake-like) configuration. These channels remain open to daily tidal incursion, unless blocked by fallen shoreline vegetation, with depths equal to those of adjacent main river channels. Those depths can reach more than 10 meters. Due to the diurnal nature and strength (velocity) of the waters pushed into the coastal zone and pulled back into the South China Sea, the river mouths are modified into a funnel-shaped geometry.

Photograph courtesy of Dr. K. Bartram - Lassa tea-colored riverToday, two types of sediment load characterize the river channels. Rivers and tidal channels that drained the peat forests had been tea-colored, appearing as a light, clear, brownish-to-blackish hue. The color was the result of the presence of a high proportion of dissolved carbon and tannic acids. Until the early 1960s, the main distributary (river) channels (from southwest to northeast: Rajang, Belawai, Paloh, Lassa, and Igan) also ran clear for much of the year, without a significant suspension-sediment load. Sediment was transported in the channels during monsoonal rains, but the volume of sediment relative to water was low. Since deforestation of the peat forests and their replacement by palm oil plantations, tea-colored waters no longer flow in the delta. The image to the left, taken in 1993, shows the interface between silt-laden and silt-free (tea colored) waters near the junction of the Lassa tidal channel with the distributary channel east of Daro.

Since deforestation began in the 1970s and 1980s, mainly in the interior of the island, sediment load increased dramatically. Today, the Rajang River complex is a light yellowish-brown color, with its hue changing in response to increasing and decreasing sediment load throughout the year. The high sediment load in these rivers is easily observable on satellite images ranging back to 1984 (see GoogleEarth historical imagery). The image above was taken in October 1992 during a King Tide event where the height of the river exceeded the height of the river’s levee, flooding the area around the stilted Iban longhouse. The frequency of flooding as a result of King Tides has increased, significantly, as deforestation and replanting of the delta has occurred, and peatlands continue to subside. 

NASA’s Earth Observatory highlighted the Rajang River Delta in one of their 2017 monthly posts. You can read about their coverage here.