Removal of dams in the Varde river, Denmark

Introduction

The Varde river system is the largest river system flowing into the Danish section of the Wadden Sea. The river system’s total catchment area is approximately 1,100km². The main tributaries originate west of the Jutlandic ridge, from where they run to the west through moorland plains and moraine islands. The Varde’s mean water flow at its outfall in Ho Bay is 16,200 l/s, varying from a minimum of 4,200 l/s to a maximum of 60,600 l/s. The Varde is the only river system where water exchange with the Wadden Sea is not regulated by a sluice. Since there are no summer dikes along the river banks, the hydrological regime in the lower parts of the river and adjacent areas are in their natural state.

The river fish populations were in poor condition even though the Varde system was still of international importance regarding its populations of Atlantic salmon, twaite shad, sea lamprey, brook lamprey, otters and a small population of houting, all protected species under Annex II of the Habitats Directive. Water flow in the river was manipulated to a significant degree, with 90% directed towards an artificial lake and Karlsgårde hydropower station. After being used for power generation, the water is returned to the Varde approximately 24 km upstream of the outfall to the Wadden Sea. As a consequence, the natural hydrology of the river was significantly impacted, and demands for water to the hydropower station courses severely reduced flow along a 16 km stretch of the river.

Although the weir at Karlsgårde power station was equipped with fish ladders, passage conditions for migratory fish were very poor due to the dams and disturbance of the natural hydrology. Houting could not pass at all, thus preventing access to 75% of their potential spawning grounds upstream of the hydropower station.

©Jan Kamman

©Jan Kamman

©Jan Kamman

The houting project

To save the houting from extinction and to significantly improve other migratory fish populations, an ambitious project was designed to renaturalise the Varde river. This was undertaken by the Danish Nature Agency, in cooperation with Ribe County and Southern Jutland County, and supported by the European LIFE Programme.

The project’s objective was to restore hydrological continuity and the condition of the Varde river to improve the populations of houting, Atlantic salmon, twaite shad, sea lamprey, brook lamprey and otters. The follow activities were carried out:

• Redirection of all water to the Varde river and closure of the hydropower channel;
• Removal of Karlsgårde power station and dam;
• Removal of dams along 13 km of the heavily modified stretches of the Varde and remeandering of the reach into a natural watercourse approximately 18 km long;
• Reestablishment of free passage for all fish at a fish farm at Sig town (source: http://naturstyrelsen.dk).

Presence of Atlantic salmon in the Varde river (source: https://varde-sportsfiskerforening.dk)

©Jan Kamman

Results

The results were impressive. The hydrodynamics of the river were rapidly restored and migratory fish found their way back to their spawning grounds. The fish populations, including houting, have improved significantly over the last 10 years. The population of Atlantic salmon improved significantly, as demonstrated through the results of monitoring between 1987 and 2016. In 2016, 1,000 mature salmon were caught on their return to the river to spawn. Removal of the dams and renaturalisation of the river were key to achieving these results.

Based on new results from the yearly monitoring project, the Atlantic salmon population in the Varde River is estimated to have reached a stable state after a slight decrease since the peak in 2016 and 2018. To support the rehabilitation of the salmon population in the Varde River, the river is stocked with additional hatchery salmon. Stocking is done only with native salmon species which should naturally reproduce with wild salmon – ideally leading to a self-sustaining population. (Koed et al., 2019)

Atlantic salmon (Salmo salar) caught by rod fishing in the Varde river system per year, 1987-2020. Source: https://varde-sportsfiskerforening.dk (new data for 2017-2020)

Since 2016, the monitoring project also takes into account the ratio between wild salmon and hatchery salmon. Although the majority of the population still consists of hatchery salmon, the share of wild salmon did increase with 5% between 2016 and 2019. However, the fact that the salmon population in the Varde River still relies on stocked salmon fifteen years after the restoration project, indicates that there is still work to be done in the river system.

Luckily, municipalities and nature organisations are already taking on this task. In 2021, the Varde Municipality, in collaboration with Norlys, Aage V. Jensen Naturfond and Den Danske Naturfond, has started a major project in the Holme River, which is a tributary of the Varde River. This project will include restoration of suitable spawning and rearing areas for salmonids and removal of barriers on the lower 12 km of the river. On top of that, in the tributary Grindsted River, a free passage at Utoft Aquaculture will be recreated in 2021 which will give fish, including salmon, access to approximately 50 km of watercourse.

Many thanks to Finn Sivebæk, DTU Aqua, for providing us with updated information.

More information about the Salmon population in the Varde River (Link).

Source:

Koed, A, Birnie-Gauvin, K, Sivebæk, F, Aarestrup, K. From endangered to sustainable: Multi-faceted management in rivers and coasts improves Atlantic salmon (Salmo salar) populations in   Denmark. Fish Manag Ecol. 2019; 27: 64– 76. https://doi.org/10.1111/fme.12385

Newly received: video from 2009 giving a clear impression of the decommission works and the physical changes in the Varde River