A hydraulic dam is an obstacle built through a river or a lake to divert water or retain it.

In the case of the Engolasters dam, it serves to retain the water derived from the rivers, create a vertical drop and take advantage of its the energy thanks to the associated infrastructure.
In the other lakes the dam serves to save the water that comes from the spring thaw and the summer and autumn rains. Before the winter begins and the rivers freeze, the dammed water is returned to the river to increase its flow and to use its energy in Engolasters where it’s collected.

The dams in the high mountain lakes have a very important role in the production of electricity in Andorra. Its reservoir capacity can generate up to 6.5 million kWh of electricity per year which would represent 1.1% of the total consumption of the country.

The dams are built to raise the natural level of the lake and to increase its reservoir capacity within its possibilities. These depends on the basin of the lake and the precipitations of the area.

In addition you have to build an emptying tunnel to empty the lake and thus take advantage of the water reservoir. This tunnel is made from the lowest point of the lake and to an area located under the dam where a valve is placed that acts like a tap to open or close the watercourse. The valve is usually closed at the beginning of the spring to save the water from the snow melt and from the rains of the summer period. This fills the lake and, if necessary, the excess of water pours by the dam's discharge channel. At the beginning of December, before the frosts, the valve is opened to empty the water reservoir progressively and increase the flow of the river so that the water is used to produce electricity in the hydroelectric plant of Encamp, located below the Engolasters lake, where water from the rivers has been derived.

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