The Tidal Lake in Semangko Fault

Andang Bachtiar, former Chairman of the Indonesian Geologists Association (Ikatan Ahli Geologi Indonesia / IAGI), was amazed when he was in around Tarusan Kamang Lake. Understandably, the lake is located in the Kamang Mudiak village, Agam regency, West Sumatra, it may subside for a year, then ​​38 hectares area becoming a stretch of grassland.

Tarusan Kamang Lake. (Picture from: http://www.antarasumbar.com/)

At one time, the water suddenly gushed freely from limestone crevices. The meadow was drowned and a lake which is where the locals could get fish and shrimp with fishing pole and nets. For several months, even up to two years, the lake was not receding.

Imran Malin Mudo (50 years) a resident of Babukik, Kamang Mudiak, explains, since he lived on the banks, the lake can survive pooled maximum of two years. "It can't be expected when the water disappeared," he said. According to the story the old people in the village, the lake could be in a dry condition maximum of 3.5 years. "It happened when the Japanese invaded Indonesia," he said.

Andang Bachtiar. (Picture from:  
http://www.wartablora.com/)

The area of ​​the Tarusan Kamang Lake is not too large. Using an engined-rubber boat, it only takes about 15 minutes to surround it. In some parts of the lake has a depth of less than 2 meters. On the other can reach 10 meters.

For locals, the ebb and flow of water in the Tarusan Kamang Lake is a natural phenomenon ordinary. However, this rare occurrence in the lake that makes the geologists interested in doing research on February 23, 2013. "This is a very interesting phenomenon. Many karst lakes in other parts of Indonesia, but only Tarusan Kamang Lake that have a direct connection with the underground river," said Andang Bachtiar, who is currently the Chairman of the Advisory Board of IAGI. With Nofrins Napilus and Kurnia Chahk, Andang conduct preliminary research on Tarusan Kamang Lake. Also accompany with Indra Catri, the Regent of Agam.

From initial observations, Andang concluded that Tarusan Kamang Lake, as well as karst lakes, is a separate-strain  tectonic lake associated with the Sumatra fault. The fault named Semangko, this great fault divide Sumatra Island. Thus, the lake is located in the Semangko fault zone (Jump to Semangko fault related article).

Tarusan Kamang Lake, when full of water and dry. (Picture from: http://izzawa.wordpress.com/)

The karst lakes, also called doline is the lake that located in the rocky limestone. This type of lake formed by erosion or dissolution of limestone. Former erosion formed basin, which then filled with water, and forming the lake.

"It is certain karst lake because it is below the old age limestone hills, which is 300-400 million years old, older than the age limestone in Java," said Andang. In addition, he found that the lake plains, the grassy green is an alluvial fan that is characteristic of the morphology of the normal active faults.

Northwest-southeast fault, which is part of the Sumatra fault, paired with the east-west fault, which limits the southern part of the lake. Evidence of the presence of an east-west fault this is the direction of fracturing in areas of limestone that is in the lake and alluvial fan that has smaller dimensions.

"This reinforces the notion that Tarusan Kamang Lake, in addition to having properties karst lake, the lake has a component-separating tectonic rift," said Andang. He also took samples of lake sediments, to investigate the age of the lake.

The phenomenon of the dried and filled of the lake's water follow the relating vessel law. This vessel connecting underground rivers, which are at a depth of cliffs and limestone ridges to the east and south of the lake, with low or basin formed from fragments. When the underground water level receded to a level lower than the bottom of the lake, the lake dried up. If the river water level rises, the lake was filled.

In a further observation, Andang was climb as high as 300 meters up the hill, goes into the cave in the karst, and saw the hole water ingress. According to him, at the height of the 100 meters, there is a water hole. In it appears there was an underground river.

When the waters receded, the lake will be dry. If the river water level rises, the lake will be filled. So that needs to be investigated is what the factors that controled the rise and fall of the river water. "There needs to be mapping the caves and underground river underneath it," said Andang.

Underground rivers and Tarusan Kamang lake considered essential for the conservation of water. "So this automated monitoring tool. If the lake rises, it is the absorption of water up to the back of the Bukit Barisan goes well. If the mountain was denuded, will no longer have water up and down," said Andang.

Relationship of Tarusan Kamang Lake with an underground river beneath it. (Picture from: KORAN TEMPO 4160)

Because of the region's unspoiled and full of lush trees, the caves under the hills are still active. In the western part of the lake, right at the foot of the hill, still visible seven aisle and out of the water into the underground river.

Although the lake could dry up at any time, there are still some types of fish and shrimp that inhabit the lake. Fish are considered native of Tarusan Kamang Lake is pantau fish and white bada fish. Pantau fish shape like the tin heads fish. While white bada fish resembling a small fish in the Maninjau Lake. However, both the body color of family Cyprinidae fish are more transparent.

"Both of these fish can adapt to its environment. Therefore, with more transparent body color," said Hafrijal Syandri, a fisheries expert from the Bung Hatta University, Padang. "When the water receded, they were in the hallway of water in the basement and living microorganisms, worms or insects in the cave." While the shrimp are able to live in the waters of the cave as well as eating detritus or micro-organism.

To breed, when to spawn or reproduce, the fish began to fully take advantage of the lake to spawn. This type of fish does not need a substrate to attach eggs and live on the lake to raise a child. "After the fry have more big sizes while the lake dried up, these fish back into the halls of the river," said Hafrijal.

The number of fish and shrimp in the Tarusan Kamang Lake were enough. These animals are commonly captured by citizens and served as side dishes for daily meals. There are also another fishs that entered the lake, such as tilapia and carp. Therefore, at low tide there pools are not deliberately formed and used to preserve fish. *** [FEBRIANTI | KORAN TEMPO 4160]

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