13. Lakes and rivers

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13.1 The water cycle
13.2 How are lakes formed?
13.3 Lake and river systems
13.4 Groundwater
13.5 The uses of lakes and rivers
13.6 Gallery: The uses of lakes and rivers
13.7 Threats to water quality
13.8 Sewage treatment

13.1 The water cycle

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The water cycle is a term that describes how water moves cyclically between the seas, the atmosphere and the surface of planet Eearth. Seas and oceans are central to this cycle, as most of the planet's atmospheric water both evaporates from and rains into them.

Atmospheric water vapor is also distributed to air masses that are located above the ground. From these air masses, the water will rain down to the surface of the Earth in the form of either rain or snow. After raining down, the majority of this water will return to the planet's oceans through rivers.

Some water is absorbed by the soil. Some of this water evaporates back into the atmosphere either directly or through plants, wheareas some of the water sinks down to the ground, forming reserves of groundwater.

13.2 How are lakes formed?

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Finland contains approximately 190 000 lakes or ponds.

This relatively high number of lakes is caused by the temperate and wet northern climate, as well as by the qualities of the Finnish soil and bedrock.

Unified bedrock and dense soils prevent water from sinking down into the ground. As a result, most of the water will remain in depressions and valleys above the ground.

Lakes can form in fissure valleys, as is the case with Lake Päijänne, or in depressions, such as the case of Inarinjärvi.

The oldest Finnish lakes have formed in craters that have been created by meteorites that have crashed down on Earth. Such crater lakes include Lappajärvi and Sääksjärvi. The Ice Age also caused the formation of various pools and depressions in which lakes have subsequently formed.

The youngest Finnish lakes include the lakes that have formed as a result of post-glacial rebound in shore areas, as well as artificial lakes that have been built for electricity production, such as the Northern Finnish lakes Lokka and Porttipahta.

Most Finnish lakes are shallow, contain only little water and have gentle banks. The water of most Finnish lakes has been colored by soil types such as clay or humus.

The average depth of Finnish lakes is only seven meters. The deepest Finnish lakes are Päijänne (95 m) and Inarinjärvi (92 m).

13.3 Lake and river systems

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Lake and river systems comprise of large lakes and the rivers that connect them. The water of a lake and river system flows into the sea via a specific main river. Lake and river systems have their own catchment areas.

Watersheds are located on the borders of lake and river systems. They divide the flow of rainwater in different directions. They are often flat and boggy areas that are located at high elevations.

The main watersheds of Finland are called Maanselkä and Suomenselkä. The Maanselkä watershed divides Finnish rainwater between the catchment areas of the Baltic Sea and the Arctic Ocean. The Suomenselkä watershed, on the other hand, divides Finnish rainwater between the cathcment areas of the Gulf of Bothnia and the Gulf of Finland.

The lake and river systems of Finland have been named according to their main rivers:

the Paatsjoki water system (flows into the Arctic Ocean),
the Kemijoki water system (flows into the Gulf of Bothnia),
the Kokemäenjoki water system (flows into the Gulf of Bothnia),
the Oulujoki water system (flows into the Gulf of Bothnia),
the Vuoksi water system (flows into the Gulf of Finland via Lake Ladoga) and
the Kymijoki water system (flows into the Gulf of Finland).

The main rivers of lake and river systems are called outlets. They are larger than other rivers of the system. The longest Finnish river is Kemijoki, which is 550 km long and flows into the Gulf of Bothnia. The largest Finnish lake is Saimaa, which is a part of the Vuoksi water system. The Saimaa channel (over 43 km long) connects Lake Saimaa to the Gulf of Finland.

13.4 Groundwater

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Groundwater is a type of water located under the planet's surface. It is formed on top of dense rock or soil layers inside the ground.

Groundwater is often of a higher quality than surface water, because it is naturally filtered as it passes through soil and rock. The Finnish groundwater supply is five times larger than the amount of water found on the nation's surface. However, only 10 % of this water supply is currently used.

Groundwater is mostly used as water for drinking and domestic use. The largest and best quality groundwater supplies are found in eskers that comprise of sand and gravel as well as in gravel ridges such as the Salpausselkä ridges. The groundwater in these areas is so pure that it can be drunk without filtering.

The quality of groundwater is damaged by sand and gravel collection from eskers and ridges, as well as by the excessive use of agricultural fertilizers and road salt.

13.5 The uses of lakes and rivers

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The lake and river systems of Finland have been used by humans since the time when the nation was first inhabited. The first patterns of inhabitation moved inland through water routes, creating settlements on the shores of lakes and rivers. Later, water routes were primarily used to transport wood and lumber.

Water power was used to produce energy to fulfil the growing need that emerged after the World Wars. During the 1950s and the 1960s, over 90 % of the total Finnish electricity production depended on water, whereas nowadays it covers only 10–20 % of the current energy usage.

Water from lakes and rivers also has a significant role as water for drinking and domestic use. For example, the water of Lake Päijänne (pictured) is filtered and used to cover some of the water demand of the Greater Helsinki region.

In addition to their economic and domestic uses, the rivers and lakes of Finland also provide many kinds of recreational possibilities.

13.6 Gallery: The uses of lakes and rivers

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: Rapids are important tourist destinations. Kitkajoki in Kuusamo.
: Rivers can be dammed for electricity production. These dams prevent river fish from reaching their spawning locations upriver.
: Rivers have been important transport routes for millennia. Turku, Aura.
: So-called fish ladders can be built near dams. These ladders help fish to reach their spawning locations upriver. Halinen, Turku.

13.7 Threats to water quality

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Human activity threatens the water quality of Finnish lakes and rivers.

The use of agricultural fertilizers causes nutrients such as phosporous and nitrogen to accumulate in the Finnish lakes, causing eutrophication. If and when these fertilizers end up in the lakes, they cause plants to grow and reproduce rapidly. Finally, this leads to a rise in the number of decomposers in the lakes ecosystem. This will cause lakes to lose their oxygen supplies, making them unsuitable for supporting life.

Eutrophication changes the organism community of a lake.

13.8 Sewage treatment

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Humans produce large amounts of waste water. For example, Finns produce over 200 liters worth of waste water per capita each year. Waste water is full of nutrients and is one of the most important causes of eutrophication. This is why sewage treatment is important.

Waste water is treated in sewage treatment facilities. These facilities receive water from houses and buildings via a sewer system and filter the water through various mechanical, chemical and biological processes. After being processed, the water can be safely let into the lake and river system.