Synonyms

Major lakes in Sweden

Definition

Sweden’s great lakes. The four largest lakes in Sweden with regard to surface area are, in order, Vänern, Vättern, Mälaren, and Hjälmaren. Vänern is the third largest lake in Europe after Ladoga and Onega in Russia, and Vättern is the 11th largest lake. On a global ranking, Vänern is the 33rd and Vättern the 78th largest lake in the world (Herdendorf, 1982).

Introduction

Sweden has close to 100,000 lakes, where a lake is defined as a water body with a surface area greater than 0.01 km2 (Lindkvist and Danielsson, 1987). Approximately 9% of the area of Sweden constitutes lakes, which corresponds to a total lake surface area of about 40,000 km2. In total, 22 lakes have a surface area of more than 100 km2, 358 lakes an area between 10 and 100 km2, and 3,990 an area between 1 and 10 km2. The ten largest lakes with regard to surface area, together with their volume and mean and maximum depths, are given in Table 1 (the exact numbers can vary somewhat since many lakes are regulated). The deepest lake in Sweden is Hornavan with a maximum depth of 221 m and a surface area of 250 km2. This lake, which is located in the very north of Sweden where all the deepest lakes are found, is the third largest with regard to volume, but only the ninth largest if the surface area is considered.

Sweden’s Great Lakes, Table 1 The ten largest lakes in Sweden according to surface area together with lake volume and mean and maximum water depths (* indicates lack of data; mainly after Lindkvist and Danielsson, (1987))

Besides being important parts of the ecosystem, the lakes in Sweden are often utilized in a variety of human activities (Willen, 2001). Lakes are used for water supply both in municipal and industrial applications, as well as in irrigation occasionally. Their recreational value is substantial, supporting the local economy through the tourism they generate and sustain. Historically, early settlements developed at the major lakes because they provided opportunities for fishing and transportation, which in turn promoted trade and economic growth. As the urban areas rapidly developed in the nineteenth and early twentieth century, the lakes functioned as receiving waters for a wide range of pollution discharge, mostly untreated wastewater. In the 1950s, the pollution levels in many lakes were becoming a problem and treatment plants were introduced to remove pollutants in the wastewater before release to the lakes (Persson et al., 1989). The continuous expansion of treatment plants with increasing sophistication, where more and more of the pollutants are being removed, has to a large degree restored the initial water quality of the lakes.

Many lakes in Sweden have been formed in depressions associated with fault lines or uneven deposition of material in connection with the melting of the ice cap that covered Scandinavia during the latest glaciation. Thus, tectonic movement together with effects related to glaciation has determined morphometric properties of the lakes that to a large degree control different lake processes such as circulation, stratification, through-flow, and lake mixing and residence time. During the melting of the ice cap (deglaciation), large amount of water was released, simultaneously, as sediment transported with the water was deposited downstream the ice margin. Subsequently, after the ice cap melted, the land mass that was depressed by the ice experienced a rebound (an uplift) creating a vertical shift of the land relative to the sea level. This rebound is still going on in Sweden today and it has been of great significance for the development of the lakes (Bengtsson, 1984).

The water circulation in the lakes, and the associated mixing, is affected by different hydrodynamic processes, including water inflow, convective currents from temperature differences, wind, and seiching (Rubin and Atkinson, 2001). Water inflow tends to have only local effects with little implications for the basin-scale motion. In contrast, convective currents resulting from seasonal changes in the heat transfer between the lake surface and the atmosphere initiate motion on the scale of the lake. Most Swedish lakes that are not shallow are subject to thermal stratification in the summer followed by complete mixing in the fall as the temperature drops in the atmosphere. Also, in the winter, lakes may become ice-covered, especially in Northern Sweden, again leading to stratification with the occurrence of complete mixing during the ice breakup in the spring (Bengtsson and Svensson, 1996).

The wind induces large-scale circulation as it blows over the water surface, creating shear stresses that initiate water motion in the downwind direction. Simultaneously, surface waves are generated that may grow to substantial heights, if the lake area is large. In Sweden, large waves are typically found in Vänern and Vättern, where fetch lengths (i.e., the distance the wind blows over to build up waves) are the greatest. Besides inducing a drift at the water surface, the wind causes a tilt in the mean water level with increasing deviation from the still water level in the downwind direction. In a stratified lake, this displacement of the water surface causes an associated tilt in the thermocline, several orders of magnitude greater. Seiching may be induced, at the water surface or the thermocline, if the lake water surface is inclined and the wind stops blowing.

The four largest lakes in Sweden are Vänern, Vättern, Mälaren, and Hjälmaren (see Figure 1). Together they have a surface area corresponding to almost 25% of the total lake surface area in Sweden. The four lakes are more or less distributed around a straight line from Stockholm on the east coast of Sweden to Gothenburg on the west coast. During the deglaciation, a connection existed along this line between the Baltic Sea and the North Sea during several periods (Björck, 1995). The Baltic Sea discharged its water to the North Sea through this connection, which appeared about 9,000–11,000 years ago, and it had a great influence on the properties of these lakes.

Sweden’s Great Lakes, Figure 1
figure 1842figure 1842

Map over the location of the four largest lakes in Sweden.

In the following, the main characteristics of the four largest lakes are briefly presented with focus on the morphometry and hydrology, as well as the major processes governing the circulation and stratification in the lakes. Table 2 summarizes key hydrological data for the four lakes according to Kvarnäs (2001).

Sweden’s Great Lakes, Table 2 Key hydrological data for Sweden’s great lakes (mainly after Kvarnäs, 2001)

Lake Vänern

Vänern is the largest lake in Sweden with a surface area of 5,648 km2, a mean depth of 27 m, and a total volume of 153 km3. The inflow to Vänern originates from the largest catchment in Sweden encompassing almost 47,000 km2, that is, more than 10% of the area of Sweden, although some part of the catchment is located in Norway. The outflow occurs through the Göta River providing the largest mean flow rate (about 500 m3/s) for any river in Sweden. Based on the lake volume and the through-flow, the lake residence time has been calculated to be approximately 10 years (Kvarnäs, 2001). Vänern consists of two subbasins that are separated by a shallow area containing a multitude of islands, which markedly influences the dynamics of the water circulation in the lake. The lake was regulated in the middle of the 1930s for hydropower purposes.

Direct precipitation on the lake corresponds to about 25% of the inflow from the catchment, and the river Klarälven contributes with 35% of the inflow. The Vänern catchment is located in a region where snow is common during the winter, leading to a marked peak in the runoff during the spring when the accumulated snow is melting. However, all the major rivers discharging into the lake are regulated, implying that the flow peak is attenuated in time.

Vänern is large enough to be influenced by the rotation of the earth. Thus, wind-induced currents may be affected by the Coriolis force, leading to a geostrophic circulation in the counterclockwise direction (Lindell and Welch, 1992). If the lake is stratified, this circulation also causes the thermocline to deform from its horizontal level. Geostrophic circulation has not been observed in any of the other large Swedish lakes, probably because of size, geometry, and shoreline configuration.

Thermal stratification occurs annually in Vänern, primarily in the summer. In August, the depth to the thermocline in the lake is about 15 m (mean depth 27 m). During some years, Vänern becomes completely ice-covered, but typically it is only in the coastal areas where ice develops. In the spring, a special type of horizontal stratification in the shallow areas known as a thermal bar may occur (Malm et al., 1998). Water in the shallow parts of the lake is heated up more quickly than the deeper parts and a vertical barrier (bar) between 4-deg water in the deeper parts and warmer water in the shallow parts emerges. This barrier moves offshore as the heating continues in the shallow parts. The thermal bar is typically present during a month (Kvarnäs, 2001). When the lake becomes completely ice-covered, slow-moving currents may still develop, although the effects of wind are absent (Bengtsson, 1996).

Lake Vättern

Vättern is the second largest lake in Sweden with a surface area of 1,912 km2, a mean depth of 41 m, and a total volume of 78 km3. The catchment supplying water to Vättern is 6,359 km2, implying that the ratio between the catchment area and the lake surface area is 3.3, which is considerably smaller than for the other large lakes. Thus, the annual volume of direct precipitation on the lake surface is almost equal to the inflow. Vättern has an elongated shape with only one basin and relatively few islands, and the lake is oriented in the SSW-NNE direction with a maximum length of about 130 km. The lake was regulated in the late 1930s.

The annual mean flow through the lake is small with respect to the lake volume, resulting in a residence time that is rather long (58 years) compared to other Swedish lakes. Thus, seasonal or annual variations in the inflow to Vättern have limited effects on the water quality, since it takes considerable time before the inflow can contribute a runoff volume comparable to the lake volume. The main inflow is through the Huskvarna River and an adjacent smaller lake system, whereas the outflow occurs via Motala River to the Baltic Sea.

Seiching has been observed in Vättern and the period of the lowest mode is estimated to be about 180 min with maximum amplitude of 13 cm (Norrman, 1964). Subsequent studies have shown the presence of higher-order modes with periods of 98 and 81 min (Björk and Lundberg, 1990). Internal seiching in the thermocline also occurs where vertical movement of 15 m is possible at the ends of the lake (Kvarnäs, 2001). Strong winds may generate short-period surface waves of significant heights because of the relatively long fetch in the longitudinal direction of the lake. Theoretically, the significant wave height has been estimated to be over 2.0 m for wind speeds at 21 m/s (Norrman, 1964). The waves are often steep, that is, the wave height is large with respect to the wavelength, which could cause problems for boats.

Similar to Vänern, Vättern is seldom ice-covered during the winter, except in the coastal areas, and vertical stratification with a distinct thermocline primarily develops during the summer. In August, the depth to the thermocline is about 16 m (mean depth 40 m). The epilimnion (upper layer) temperature in Vättern is the lowest among the four largest lakes (just below 12°), since deeper lakes have larger heat storage capacity, implying a slower heating up (and cooling down).

Lake Mälaren

Mälaren is the third largest lake in Sweden with a surface area of 1,140 km2, a mean depth of 13 m, and a total volume of 14 km3. The Mälaren catchment is almost 20 times larger than the lake surface area, which is the largest ratio among the great lakes of Sweden (e.g., the corresponding ratio for Vänern is 8.3). Mälaren has a complicated shoreline configuration since it was created through a number of faults generating fissures in the north–south direction. Thus, it is possible to identify five subbasins that have emerged as a result of tectonic movement and glacial processes (Kvarnäs, 2001), and the lake contains a large number of islands. Mälaren was first regulated in the beginning of the 1940s, and an important aim of the regulations has been to avoid saltwater intrusion from the Baltic Sea at low lake levels.

Having the largest catchment with respect to the lake surface area, direct precipitation on the lake surface has only a minor effect on Mälaren. The precipitation is about 10% of the inflow from the catchment. Mälaren is almost ice-covered every year and because of the thermal stratification that develops during the winter, oxygen deficit occurs in some parts of the lake. The stratification in the summer results in a thermocline that is located at 12 m water depth in August. Mälaren was the first Swedish major lake where water quality sampling was carried out in a systematic and comprehensive manner. This sampling was performed in the mid-1960s. After some years, water quality studies were also initiated in Hjälmaren and Vättern, followed by Vänern in the early 1970s (Willen, 2001).

Lake Hjälmaren

Hjälmaren is the fourth largest lake in Sweden with a surface area of 484 km2, a mean depth of 6 m, and a total volume of 3 km3. The ratio between the catchment area and the lake surface area is 8.4, which is similar to Vänern. The lake has five subbasins with a number of fault zones extending in the east–west direction, with the northern part of the lake having a multitude of islands (Kvarnäs, 2001). The water that exits Hjälmaren enters into Mälaren trough Eskilstuna River, providing about 35% of the input flow to Mälaren. Hjälmaren was regulated at the end of the nineteenth century with the purpose of generating more agricultural areas. This led to a general decrease in the mean water level of about 1.3 m.

Hjälmaren is also ice-covered during most winters, whereas the limited water depth typically prevents development of a thermal stratification, even during the summer.

Cross-references

Baltic Sea Basin, Since the Latest Deglaciation

Basin-Scale Internal Waves

Circulation Processes in Lakes

Europe, Lakes Review

Geneva Lake

Hydrodynamics of Very Shallow Lakes

Ice Covered Lakes

Internal Seiches

Ladoga Lake and Onego Lake (Lakes Ladozhskoye and Onezhskoye)

Mixing in Lakes

Stratification in Lakes

Surface Seiches

Swedish Glacial Lakes: Estimation of the Number of Lakes of Different Sizes

Thermal Bar

Wind Waves