Partially constructed river in the upper reach
This river runs into the lake and is harnessed for the production of electiricty, but not entirely. There are still many stream sections in the upper part of the river. There are two hydroelectric powerplants on this river with a reservoir, that is used for regulation, between them.
The river flows on the terms of the power plant
The upper hydropower plant is a run-of-river type plant. Its efficiency depends on the amount of water that runs in the river. The natural flow rate of the river varies and is low during the dry season. When this happens, the discharge flow through the turbines is reduced and the water is dammed in the upper channel. This can also be a manoeuvre that is practised in regular intervals: water is directed through the turbines most intensely in the daytime, when the price of electricity is highest.
During night-time the water flow through the turbines is reduced because the production of electricity is less profitable. This way the flow of the water in the river is short-term-regulated depending on the time of the day. The power plant is designed to work most efficiently at certain levels of water flow. If the water flow is much lower than the optimal flow for the plant machinery, the turbines are stopped completely and the power plant is operated with interrupted flow.
With the short-term operational adjustments, water level above and below the power plant oscillates. The continuous changes in the water level erode the river banks and deplete the vegetation. The channel downstream of the plant sometimes has very little water or even dries out.
The reservoir changes the river bank and the flow conditions
Steady and sufficient water supply for the lower hydropower plant is guaranteed by the reservoir that is dammed to regulate the water flow. The original river with its streams and riparian areas disappeared under the increasing water levels of the reservoir. The surrounding forests and swamps have also disappeared underwater. Just after the reservoir formation, the dead vegetation and peat contaminated the water making it more acidic and with higher concentrations of pollutants, such as mercury, compared to a natural lake.
Naturally, during winter there is little flow in the water system as water falls as snow or concentrates as ice. In springtime when the snow melts the water can flow in the river again. In contrast, the flow of water for power production and the regulation rhythm of the powerplant are reversed. The water level in the reservoir is increased before winter, because that is when water is the most valuable for the power production.
In winter, the demand for electricity and the electricity prices are at their highest, but the flow rates of the waterways are at their lowest. In autumn months the water level in the reservoir is increased to the upper regulatory limit, that is higher than the natural upper water level. This water level will drop from February till April until it reached the lower regulatory limit. At this level, the oxygen concentration in the water may become limited, due to the small volume of water. In summertime, the water level is raised again to suit people’s recreational uses.
Flow regulation limits the intensity of spring flooding. Natural river and riparian ecosystems that are adapted to seasonal flooding suffer from regulation and protective measures against floods. In contrast, the agricultural cultivation along the riparian zones, benefit from flood control.
The natural river channel remains mostly dry
The amount of water in the reservoirs is controlled by the dams – the powerplant dam and the regulatory dam. By controlling the water discharge through the dams the water is directed to the upper channel of the power plant. The channel is excavated in a naturally steep part of the river, to allow the largest possible fall height. Water level in the sub-channel is clearly lower than the water level in the upper channel.
The natural river channel is mostly dry, apart from when the amount of water in the reservoirs threaten to reach a level that is too high. Then the water is discharged through the locks in the regulation dam and temporarily floods the natural river channel.
Powerplants prevent fish and other organisms from migrating up and down the main river channel. Fish cannot pass the power plants or reach the upstream reproduction areas. A natural tributary, inhabited by a local trout population, runs into the reservoir.