Watercourses connect the dry land and the sea, and apart from being habitats for a large number of plants and animals, watercourses act as dispersion channels in the landscape. This means that watercourses have a key role in relation to other habitat types.
There are about 65,000 km of watercourses in Denmark. The majority of these (about 75 %) are small streams and ditches less than 2.5 meters wide at their bottom.
The Watercourse Act aims at ensuring that watercourses can be used to drain water with consideration for the natural and environmental quality of the watercourse according to other legislation. A total of 28,000 km of watercourses are also covered by the Nature Protection Act, which protects against changes in the condition of watercourses except for routine maintenance. The regulations on watercourses in the Act apply for open watercourses, as well as watercourses led through pipes and drains.
The Watercourse Act does not contain a definition of an open watercourse. The Act presumes a formation in the terrain which would normally be perceived as "a watercourse". As a minimum there should be a clearly defined depression in the terrain which constantly, or more or less regularly, carries water. The channels which arise after heavy downpours or thawing do not meet this requirement, nor do roadside ditches, boundary ditches, depressions etc. that do not regularly carry water. See the comments on the Watercourse Act, Tolstrup and Barfod, Watercourse and Water Supply Act, Juristforbundets Forlag 1975 (in Danish).
The provision that the watercourse must constantly or more or less regularly carry water means firstly that water is led from one place to another, and therefore it is not still, and secondly that it carries water for most of the year. That watercourses dry out in dry periods, usually during the summer, does not necessarily imply that these are not watercourses, however a requirement is that the watercourses must clearly be water-bearing for the main part of the year. It is not possible to determine a fixed number of days with water flow per year in order to qualify as a watercourse, as precipitation can vary considerably from year to year.
The municipalities are the watercourses authority. According to Statutory Order on classification and registration of watercourses, the watercourses authority should register the rules that apply for the individual watercourses. This information is available to the public.
The majority of watercourses have been impacted by human activities such as water abstraction, discharges of waste water, straightening for drainage, pipe-laying and barrages. These impacts mean that many watercourses are not in optimal environmental condition. In order to ensure the ability of watercourses to drain water away, maintenance is carried out in almost all watercourses, including vegetation cutting to aid drainage of the riparian areas along watercourses.
In recent years the condition of nature and the environment in many watercourses has been improved considerably through more gentle vegetation cutting and restoration of watercourses. This means that watercourses which were previously straightened can develop more naturally again. Some watercourses have been returned to their old meandering course, and watercourses led through pipes have been opened to improve the living conditions of their plant and animal life.
Environmental targets for watercourses
The environmental quality of watercourses has been stipulated according to the provisions in the Danish Environmental Targets etc. Act Specific targets for watercourses are stated in the water management plans. Targets have been set specifically for about 22,000 km of Danish watercourses. The majority of these watercourses have the environmental target "good ecological status".
Denmark has about 120,000 lakes larger than 100m2. These correspond to 1.4 % of the land area. By far the majority are small lakes, and only about 2,700 (or just over 2 %) are larger than 10,000 m2 (1 hectare). In addition, there are about 75,000 ponds under 100 m2.
The lakes in Denmark are spread all over the country with most of them are in the hilly landscape in northern Zealand and central and east Jutland.
Even the largest Danish lakes are very small compared to the other Nordic countries. Most Danish lakes are shallow, like Arresø, with its greatest depth of 5.9 metres. However, there are also lakes with depths of more than 30 metres, like Lake Furesø north of Copenhagen, Lake Hald near Viborg and Lake Ravn near Ry (both in Jutland). Lake Furesø is the deepest lake at 35 metres.
The five largest lakes in Denmark:
1. Lake Arresø in northern Zealand: 39.9 km2
2. Lake Esrum in northern Zealand: 17.2 km2
3. Stadil Fjord in west Jutland: 17.1 km2
4. Lake Mossø in central Jutland: 16.9 km2
5. Lake Tissø in the western part of Zealand: 12.5 km2
Different types of lakes
Most lakes are rich in limestone, but on sandy soil in west Jutland in particular there are also low-lime lakes. In forests and in old peat bogs, the water can be brownish due to humus. Along the west coast of Jutland are many large and shallow saline lakes (e.g. Stadil Fjord), otherwise the majority of lakes are freshwater lakes. The physical and chemical differences between the types of lakes are very important to the composition of the plant and animal life in the lakes.