Post 01/02/2026

Lake Toba can be likened to a bowl on a table: a large bowl with a chip on its eastern side. Because of that chip, Toba is only half filled with water, spilling out to the east. The water that spills out becomes the Siguragura Waterfall and flows down into the Asahan River.

In the 1970s, Japan built a 600-MW hydroelectric power plant at Siguragura. All of the electricity produced was used exclusively to power a Japanese-owned aluminum smelter in Kuala Tanjung, near the mouth of the Asahan River. Not a single watt was allocated to communities around Lake Toba.

From the outset, Japan had a strong interest in maintaining the water volume of Lake Toba. They were deeply concerned that deforestation or sedimentation might reduce the lake’s water level. They feared that a drop in water volume would deprive the power plant of sufficient water and disrupt the operation of the aluminum smelter.

As a result, Japan carried out studies to determine the best way to preserve Lake Toba’s water volume. This research was conducted by the consulting firm Nippon Koei.

The study identified a highland mountainous area, about 1,600 meters above sea level, west of Lake Toba in Dairi. This area is the headwaters of several small rivers. These rivers do not flow into Lake Toba but instead run in the opposite direction, toward Karo and Aceh Singkil, eventually emptying into the southern sea.

Those rivers were dammed. Water from the dams was forced to flow into an artificial river approximately 11 kilometers long. Around nine to eleven other small rivers were also compelled to merge into this artificial channel.

Naturally, the terrain of Dairi causes rivers to flow westward, away from Lake Toba. However, the artificial river was engineered to slope eastward, toward Toba. This artificial river is invisible because it takes the form of a large tunnel carved beneath rocky mountains. When the tunnel reaches the Silalahi area, a vertical shaft about 100 meters deep was drilled downward through the rock.

The water collected bit by bit from the eleven small rivers amounts to ten cubic meters per second. All of it is dropped through the 100-meter shaft, creating an artificial waterfall inside the mountain. This waterfall is then captured by two turbines to generate electricity at the Renun Hydroelectric Power Plant. After driving the turbines, the water is discharged into Lake Toba, replenishing the lake.