In Kapuas Hulu Regency, West Kalimantan, lies a wetland area with a unique natural phenomenon. There sits Lake Sentarum, a lake whose appearance changes completely throughout the year. It is located far upstream along the Kapuas River, about 700 kilometers from the river mouth.
During the rainy season, the lake area is filled with water, but during the dry season, the water recedes until the landscape transforms into a vast grassland.
Lake Sentarum is not merely a single lake, but a giant basin that holds a collection of about 20 seasonal lakes. Natural freshwater swamp forests and peat swamps grow all around these lakes.
The landscape that combines lakes and flooded swamp forests is what the people of upstream Kapuas call lebak lebung.
The entire wetland area spans approximately 132,000 hectares. It serves as a vital habitat for hundreds of freshwater fish species, including high-value species such as the Asian Arowana, Clown Loach (Botia), Asian Redtail Catfish (Toman), and Featherback (Belida).
Due to its unique ecosystem and biodiversity, Lake Sentarum was designated as a Ramsar site in 1994. A few years later, the Indonesian government officially designated the area as a National Park in 1999.
The Fluctuating Water Mechanism of the Kapuas River
The transformation of Lake Sentarum occurs because of its direct connection to the main channel of the Kapuas River. The lake basin acts as a natural water reservoir when the water flow of West Kalimantan’s main river increases.
During the rainy season, high rainfall triggers a dramatic surge in the water volume of the Kapuas River. The overflow that the main channel cannot hold is pushed into the lake basin through connecting tributaries, such as the Tawang River.
Such a condition reverses when rainfall decreases during the dry season. The declining water level of the Kapuas River gradually pulls water back from the lake area. The process steadily drains the water in Lake Sentarum, which leaves behind a drying land.
Landscape Changes During the Dry Season
As the water empties, the landscape around the lake changes drastically. Areas that were submerged under tens of meters of water during the rainy season slowly turn into open land that can be walked upon and crossed.
The sight is clearly visible from Tekenang Hill, a hill located right in the middle of the lake basin. During the rainy season, the hill looks like a small island surrounded by water. However, when the lake dries up, Tekenang Hill stands prominently amidst a vast expanse of green grassland.
The drying lake bed is quickly covered by wild grass. Swamp trees such as putat (Barringtonia) and tengkawang (Shorea) that were previously submerged reappear above the hardened, cracked muddy soil exposed to the sunlight.
The Emergence of Small River Networks
Even as the lake dries up, the wetland area does not dry out uniformly due to the varying depths of its bed.
As the water recedes, the remaining water collects in deeper pockets. Locally, the pockets are known as lebak. This is where freshwater fish gather in the remaining water to survive throughout the dry season.
The dry season is also the time for wild honey harvesters to go to work. When the swamp trees begin to bloom, they attach tikung—wooden boards used as artificial nesting sites—to tree branches to attract giant honey bees.
Besides the lebak, the receding water also reveals a network of small rivers at the bottom of the lake. The natural channels cut across the green grassland, and serve as the final path for the remaining water to flow back into the main channel of the Kapuas River.
