NERI Technical report no. 531. Restoration of Skjern River. Summary of environmental monitoring results 1999-2003. Andersen, J.M. 2005. 96 pp.
English summary
Project decision
In 1987 the Danish Parliament decided to restore the lower reaches of Skjern River and its valley. The objectives were to restore the nutrient retention capacity of the river and its valley, to restore an internationally valuable wetland, to promote fishery and to increase the recreational and touristic values of the project area.
Skjern River is the largest Danish river. It drains 2,490 km2 of cultivated, sandy plains in western Jutland and discharges into the lagoon Ringkøbing Fjord at the North Sea coast.
A working group and an advisory board were established with representatives from local and national authorities and NGOs. Their tasks were to assess the different restoration possibilities and to act as advisory fora for the Danish Government.
A project proposal and an Environmental Impact Assessment (EIA) were published in 1997. This was the background for a public hearing about the project. Ideas and proposals from this public hearing phase were included in the Parliament Act on Restoration of Skjern River (Act no. 493, 1 July 1997).
The objective of the act is to restore the lower 19 km of the Skjern River and its river valley of about 22 km2. This river reach was channelized and deepened in the 1960s and the river valley wetlands were reclaimed for agricultural cultivation, partly by pumping the drainage water into the channelized river. From the reclamation in the 1960s until the restoration the dominating land use was barley and wheat production.
The restoration project includes re-establishment of a meandering river and the natural water levels and water level fluctuation in the river and its valley with the purpose of enhancing living conditions for plants and animals, safeguarding a high water quality in the river and in the downstream lagoon (Ringkøbing Fjord) and improving possibilities for outdoor recreation.
The Skjern River Act includes an obligation to carry out a monitoring programme in a cooperation between the Ministry of Environment and the regional environmental authority, Ringkjøbing County. The objective of this monitoring programme is to evaluate the consequences of the project on the environmental and ecological qualities of the river system and the river valley within the project area.
This report describes the main results of these monitoring activities and the conclusions on the effects of the restoration. Monitoring was undertaken by the National Environmental Research Institute, Ringkjøbing County, Danish Institute for Fisheries Research and the Danish Forest and Nature Agency.
Construction work and costs
The construction work started in June 1999 and was largely finalized by autumn 2002. The main activities were excavation of the new river course, removal of existing dikes from the land reclamation in the 1960s and the filling of the old channelized river reaches. An old pumping station and a weir established in connection with the river channelization were also removed. The activities also comprised construction of bridges, paths and car park. Whenever possible, one of the original river banks formed one of the banks of the restored river.
In total, 2.7 million m3 soil was moved. The total costs were DKK 283 million, corresponding to about EURO 35 million This is of the same order of magnitude as the costs of the land reclamation in the 1960s. The restoration project was granted EU LIFE support of DKK 25 million.
General changes of the project area
40 km of restored river has been established. The length of the main river has increased from 19 km to
26 km. As a general rule, no river maintenance (weed cutting) will be carried out in the restored river. The old landscape with the cultivated fields has disappeared, and the valley has changed into an open valley with a meandering river and with the lower parts of the valley permanently flooded as a shallow lake (Hestholm Lake).
For the state-owned areas a nature management plan has been prepared. An extensive grazing with cattle or sheep has been established or the meadows are used as hay fields to avoid unwanted growth of bushes, trees and reed. The grazing fields are fenced, but public access is ensured through gates.
Environmental effects of the restoration
Water level increase
The water levels of surface water and ground water have increased within the project area, but no changes have occurred outside. This water level increase has formed a new shallow lake (Hestholm Lake) of about 5 km2. During high water discharges (more than about 40 m3/s) as a result of heavy rainfall or snowmelt in the catchment, further flooding of the river valley occurs.
Nutrient transport and retention
One of the aims of the project was to trap some of the nutrients (nitrogen and phosphorus) transported through Skjern River and thus reduce the nutrient load of Ringkøbing Fjord. Such a reduction will occur when a valley is flooded with water from the river, because some of the phosphorus from the river water is deposited on the flooded areas and some of the nitrate in the river water is transformed into atmospheric nitrogen in the flooded area.
Attempts were made to measure this reduction in nutrient transport through Skjern River by monitoring the nutrient concentrations and flow upstream and downstream the project area.
However, the difference between the transport into the project area and the transport out of the area was so small that it could not be quantified. The reduction was estimated to be less than 10% of the river transport. The overall conclusion of this monitoring programme is that the transport of nutrients through Skjern River into Ringkøbing Fjord is determined by the size of the nutrient sources in the entire catchment and is not reduced significantly as a consequence of the restoration.
Sedimentation in the river valley
Both in the permanent Hestholm Lake and in the flooded parts of the river valley, soil particles from the catchment is sedimented and nitrate is denitrified. In the flooded areas and in the parts of the lake where substantial deposits were registered, the deposited amount of material was measured and the deposited amount of phosphorus and nitrogen estimated. Based on the amounts deposited during the winter floodings in 2001/2002 the depositions in years with typical flooding are estimated at 5-10 tonnes phosphorus and 10-20 tonnes nitrogen. Furthermore, retention of nitrogen occurs because of denitrification of nitrate to atmospheric nitrogen. This denitrification is estimated at 200 tonnes nitrogen in years with typical floodings. However, there are very large differences in flooding from year to year. In 2003, no flooding occurred and consequently there was no retention in flooded areas.
A comparison of the estimated typical retentions of phosphorus and nitrogen with the total amount transported through Skjern River indicates that the retention is about 5-10% during years with typical floodings.
Morphology and habitats in the Skjern River
The river morphology was studied before and after the restoration to describe the changes in river banks, water depths, the character of the river bed (mud, sand and gravel) and in the velocity of the water flow. Three reaches within the project area and one upstream reference site were studied.
The restoration has led to less regular cross section profiles in the river with larger depth variations and a smaller slope of the riverbanks. The river width and the water depths have generally decreased and the water velocity increased. As a consequence, mud deposits in the river have been reduced and the dominating bottom substrate is sand, but also the occurrence of gravel and stones has increased.
With these physical changes the restoration has created a more diverse range of habitats for the river fauna and vegetation, both within the river itself and at the riverbanks. These physical changes are likely to lead to a biological diversity of the plant and animal life of the river and the riparian zone that is still larger than before the restoration.
Vegetation in Skjern River
The river vegetation and river morphology were monitored at the same reaches. Further studies were made to describe the occurrence of the rare species water-plantain (Luronium natans) and water-dropwort (Oenanthe fluviatilis).
The vegetation coverage was reduced after the restoration, because the plant communities had not yet colonized the shallow river bank zones in 2003. Many new species had colonized the river bed and river banks, but in 2003 only with a low coverage. Branched bur-reed (Sparganium emersum) and common waterweed (Elodea canadensis) were the most common plants in 2003.
In Europe, water-dropwort only occurs in Ireland and in western Jutland, Denmark. It was common in the project area and seems to occur more frequently as a result of the restoration.
Water-plantain is also a very rare species, which is threatened in Europe and only occurs at very few sites in Denmark. In 2003, the plant was found in a remaining part of the channelized river section and occurred sporadically in the restored Skjern River.
Invertebrates in Skjern River
The small animals (invertebrates) living at the river bottom are mainly insect larvae, but also worms, snails and leeches are common. Invertebrates were monitored in 2000 and 2003, before and after the restoration.
The general biological quality of Danish rivers is assessed from the occurrence of the river invertebrates, and a quality index, Danish Stream Fauna Index (DSFI), is calculated based on the invertebrates found. In very badly polluted streams the index value is 1, and in unpolluted streams with a diverse fauna the index value is 7.
In Skjern River the DSFI values were 7 both before the restoration (2000) and after the restoration (2003). The invertebrates have rapidly colonized the new river reaches and the fauna was already in 2003 very similar to the fauna of the river reaches upstream the project area. High densities of invertebrates were found in places with a gravel bottom or with vegetation. Because of the increased occurrence of gravel in the river after the restoration, the project has strengthened the invertebrate diversity and the occurrence of clean-water species.
Remarkable and generally rare species found are, for example, the dragonfly larvae Ophiogomphus celicia, the mayfly nymph Baetis calcaratus and the stonefly nymph Isoptena serricornis.
Smolt migration and mortality
25,000 smolts (juveniles about 2 years old) of salmon migrated towards the sea in 2003, as opposed to 5,000 in 2000. The number of smolts of trout was approx. 7,000 in 2000 and 8,500 in 2003. The production of smolt is not influenced by the restoration because the smolt production occurs in the upstream river reaches.
The mortality of the smolt during their migration through the river and the lagoon to the sea was about 50% for salmon and about 25% for trout. The main causes of this mortality is predation from pike and from birds, not only in the river, but also in the downstream lagoon, Ringkøbing Fjord.
The restoration has led to increased populations of pike, cormorants and herons in the project area. This has apparently increased the smolt mortality from about 8% to about 20%, but other causes, such as a different discharge pattern and the unestablished vegetation in the new river reach, may have influenced the smolt predation.
Spawning migration of salmon
The number of salmon returning from the sea to Skjern River to spawn has increased during the last 10 years up to about 1,200 salmon per year. The annual number of salmon seems to be closely related to the number of juvenile salmon released into the river 1-2 years before. These juvenile salmon (smolts) are produced in a fish farm from salmon caught in Skjern River during their spawning migration.
Lampreys
Larvae of river lampreys were found at the majority of the sites investigated in the Skjern River and its tributaries. The density of sea lampreys was lower compared to the river lampreys and found at 3-4 out of the 25 sites investigated. The upstream migration of the lampreys is facilitated by the removal of a weir in the eastern part of the project area (the weir at Kodbøl).
Pikes in Hestholm Lake
As part of the project a new lake (Hestholm Lake) was formed in the bottom of the river valley. The river does not flow through the lake. Approx. 75% of the smolts produced in a small tributary to the lake are eaten by pikes in the lake. The pikes in Skjern River (inkl. pikes from Hestholm Lake) eat around 5% of the smolts migrating through Skjern River.
Vegetation in the river valley
Monitoring of the vegetation in the river valley comprised an extensive surveillance of the development of the vegetation in the western part of the project area where, until 1999, the land use was cultivated fields. More intensive monitoring took place at 3 existing nature areas close to the river and 2 protected areas in order to monitor possible ecological changes here as a consequence of the restoration.
In the former cultivated fields in the western part of the project area, dramatic changes in the plant species took place between 2000 and 2003. Domestic, cultivated grasses and typical weeds in cultivated fields were replaced by species typical for natural wetlands, or even water plants. In 2003 the vegetation was still under rapid development, adapting to the new conditions. This development is expected to continue over the next years, and the remaining species that typically occur as weeds in cultivated fields will disappear. Until 2003, especially the occurence of common rush has increased, and the coverage of tufted hairgrass, reed canarygrass and common reed is expected to increase during the coming years.
In the protected areas (wetlands) Råddensig Kær and Albæk Mose, no significant change in plant communities had occurred in 2003 compared to 2000 that could be ascribed to the restoration. The minor changes registered were ascribed to natural fluctuations in the vegetation.
Otter
Protection of the European otter (Lutra lutra) has attracted much attention in Denmark during the past decades because it has been close to extinction. Therefore, monitoring of the occurrence of the otter was part of the monitoring programme.
In 1999-2000 faeces and footprint from the otter were found at 12 out of 19 sites visited. In 2003-2004 the corresponding findings were made at 18 out of 20 sites. It is concluded that there is a marked increase in the occurrence of otter in the entire project area and a permanent population. The increased population follows the general spreading of the population from northwest Jutland to the rest of the Jutland peninsula.
Amphibians
In the western part of the project area the occurrence of amphibians was monitored during the Spring in 2000 and 2003. The monitoring was primarily through listening to the croaking and observations with binoculars.
The restoration has improved both the breeding possibilities and the general survival possibilities of the amphibians common toad, natterjack toad, common frog and moor frog. The improvements are due to the creation of the many shallow ponds and flooded areas surrounded by non-cultivated land. It is expected that the populations will increase further in the years to come in line with the maturation of the new ecosystems.
Breeding birds
The impact of the restoration on the population of breeding birds was monitored by counting the birds in May-June in 2000 and 2003 before and after the restoration works in 2000-2002.
The number of species of breeding waterbirds increased from 6-8 species in 2000, just after the cultivation of the fields had been given up, to 34-36 species in 2003 when all the wetlands and the shallow lakes had been formed.
The major immediate changes in the composition of breeding waterbirds from before to after restoration of the wetland were as follows:
• No grebes were breeding before restoration, whereas 4 species were breeding after restoration with Black-necked Grebe as the most common (38-54 pairs).
• The number of dabbling ducks increased from 44 pairs (2 species) in 2000 to 301 pairs (5 species) in 2003 with mallard as the most numerous species before as well as after the restoration.
• The species Water Rail, Spotted Crake, Morhen were not recorded breeding in 2000, but bred with 7-22 pairs after the restoration. The number of Coot increased from 5 to 165-200 pairs.
• Three species of waders were recorded breeding in 2000 with Lapwing as the most numerous (74 pairs). Immediately after the restoration 125 pairs were breeding. Little Ringed Plover appeared as a new species (7-8 pairs), but it is expected to disappear because the bare sand surfaces will become more or less covered with vegetation. Other "new" breeding waders in 2003 were Avocet (85 pairs) and Ringed Plover (1-2 pairs).
• Black-headed Gull settled as a breeding bird after the river restoration and bred with 735 pairs in 2003.
The breeding bird surveys have demonstrated that the restored area has developed from a habitat of inferior significance to a nationally important wetland for a long list of breeding waterbirds. Several species of grebe and ducks found favourable breeding and feeding habitats immediately after the restoration. Further, several species preferring reed-swamps settled to breed.
Migratory birds
The restoration has turned the project area into an extremely important feeding and roosting site for especially dabbling ducks during their autumn migration. More than 105 different species of birds have been registered. The western part of the restored area, especially in and around Hestholm Lake is the most important area for migratory waterbirds.
Most numerous were Wigeon with up to 12,000 individuals as the maximum counted in 2003. Also Teal and Lapwing were numerous with up to about 5,000 of each. Other very common species with maxima above 1,000 individuals were Whooper Swan, Pink-footed Goose, Barnacle Goose, Mallard, Pintail and Golden Plover.
Conclusions and perspectives
The monitoring results from 2003 do not reflect the future ecological conditions in the restored area, but indicate in which direction the vegetation and the animal life will develop in the years to come. The main results of the restoration are:
• A 22 km2 natural and semi-natural river and river valley ecosystem has been restored. A coherent ecosystem formed, including the largest Danish river and the shallow lakes and ponds, wetlands and meadows created in the restored river valley.
• The restored area has become an important breeding area for waterfowls, an important resting area for migrating birds and consequently a popular area for bird observations.
• The restoration has improved the breeding and survival possibilities for amphibians, because of the formation of a large number of shallow ponds and bogs surrounded by uncultivated meadows.
• In the restored and meandering river more diverse habitats have been created as compared to the former channelized river. As the water quality is good, it is expected that the diverse plant and animal life of the Skjern River will further increase the diversity in years to come.
• The restoration has not led to negative impacts on rare species in the area, except for a minor increase in the mortality of migrating smolts of salmon and trout. This is mainly caused by the increasing predation from cormorants and herons.
• The number of registrations of otter in the restored area has increased, primarily because of a general increase in the number of otters in Denmark.
• The monitoring programme has not made it possible to calculate the amount of nitrogen and phosphorus retained in the restored river valley. The monitoring results indicate that the retention is small (< 10%) compared to the total transport of nutrients through the river into the coastal lagoon, Ringkøbing Fjord.
During the next years the flora and fauna in the project area will develop further towards a new ecological equilibrium corresponding to the new conditions. Both in the river and the lakes and in the wetlands and meadows, the vegetation and the vegetation types will develop according to the competitiveness of the different species.
Future development of the ecosystem depends to a wide extent of the management of the meadows and wetlands. The populations of terrestrial animals, fishes and birds depend widely on the type of vegetation, and therefore their development also depends on the management and degree of management of the restored area. A management plan has been prepared. It is a general aim to keep large parts of the restored area grazed by cattle or sheep to ensure low and open vegetation.
However, any management will favour some species but hamper others, or may lead to other unwanted impacts. Therefore, management plans should be adjusted continuously to consider such conflicting interests. Some examples from the restored area are:
• The creation of Hestholm Lake has increased the number of dabbling ducks, grebe and pike, but has also increased the mortality of salmon smolt because of increased predation from herons and cormorants.
• Grazing by cattle or sheep is a precondition for the formation of typical meadow vegetation and birds attached to this habitat, but at the same time it excludes high vegetation and thereby the animals attached to this habitat.
• Grazing near the shores of lakes, ponds and streams prevents the establishment of a natural reed belt vegetation and the grazing increases the nutrient loading to the waterbodies, but some birds attached to meadows benefit from the naked mudflats created by the grazing animals.
Future monitoring
From 2004 the environmental monitoring programme for the project area is part of the national environmental monitoring programme (NOVANA). This surveillance monitoring is more extensive compared to the 2000-2003 monitoring programme for the Skjern River project area. At selected sites the monitoring include:
• river invertebrates
• vegetation in rivers and the riparian zone
• river water discharge and concentrations and transports of pollutants
• otter
• amphibians
• birds
• protected species
• protected habitats
• salmon.
Full report (in Danish) in pdf format (3.465 kB).
