Emån - Emsfors
Emån - Emsfors
Key features of the case study
In the following section, background and motives of the restoration project which led to the initiation of the project are introduced.
Drivers and Pressures
River Emån has historically been an important reproduction site for Baltic salmon and sea trout. Extensive exploitation of the river for hydropower production in the 20th century has however created numerous barriers for migrating fish in the form of hydropower dams and substantial fragmentation of the river network. Hydropower dams are moreover typically constructed in river sections were the stream gradient is high and the damming of riffles and rapids results in a loss of habitat for salmonid fish. The hydropower dam at Emsfors used to be the first barrier in the river (located ca. 4 km from the sea). There was an old fish ladder at the hydropower dam but this had limited functionality and it was estimated that only ca 50 % of spawning migrating salmonid fish would pass through the fishway. Other fish species such as whitefish (Coregonus sp.), ide (Leuciscus idus) and wels catfish (Siluris glanis) could not pass the dam.
Timber floating has been an important activity in the River Emån catchment until the 1960s. To facilitate the downwards transport of timber, narrow shallow sections of the river were deepened and boulders were removed. This has created a more homogenous bottom structure and more homogenous flow in these river sections and reduced their suitability as spawning areas and habitat for salmonid fish.
The lower reaches of River Emån are surrounded by extensive floodplains, periodically flooded wetlands, grasslands, pastures and forests. The river section is considered as one of the most important localities for wels catfish in northern Europe. Catfish migrate into the flooded riparian habitats to forage for food and to spawn. In the 20th century large scale ditching campaigns transformed many of these floodplain areas into arable land and dikes were constructed to prevent flooding. This has reduced the habitat for wels catfish. In the Emsfors region the floodplains are still relatively undisturbed and large areas surrounding the river are periodically flooded.
Global objectives
The main goal of the restoration project was to restore the Emsfors area in the lower reaches of River Emån to near natural conditions. Restoration of lateral connectivity and habitat improvement at Emsfors was expected to improve the potential for natural reproduction of Baltic salmon and sea trout in the river and would help to secure the populations long term survival. Other animal groups that were expected to benefit from the restoration included the European eel (Anguilla Anguilla), European river lamprey (Lampetra fluviatilis), asp (Aspius aspius), wels catfish, thick shelled river mussel (Unio crassus) and fresh water pearl mussel (Margaritifera margaritifera). The restoration was deemed important and in line with the conservation plans for two Natura 2000 areas in the River Emån catchment (“Emåns vattensystem i Kalmar län, SE0330160” and “Våtmarker längs Emåns nedre lopp, SE033173”) and for the long term conservation status of the habitat type “Fennoscandian natural rivers, 3210” according to the EU Habitats Directive.
Specific goals
- To restore lateral connectivity at Emsfors the Emsfors hydropower dam was to be completely removed. A fishway with a low inclination for fish species that “are less prominent swimmers” should also be constructed to create “exceptionally good conditions for fish migration” at the site.
The head of Emsfors hydropower dam was concentrated to the area 200 m upstream from the dam. Hydrological modeling, with the retention water level of the hydropower dam as a starting point, did however reveal that a removal of Emsfors hydropower dam could cause a substantial drop in water level a long distance upstream from the dam at low flows. The water level would remain relatively unchanged at high flows. There was concern that a drop in water level could damage important floodplain habitats in this area. This could also have a negative impact on the catfish population. This concern emanated in a secondary goal of the restoration project.
- Threshold areas/riffles upstream from Emsfors hydropower dam, that had been deepened to facilitate timber floating, should be restored to reduce the drop in water level upstream from the dam at low flows after dam removal and to protect the flood plain habitats in this area. The threshold areas/riffles suitability as spawning areas and habitat for salmonid fish was also expected to be improved by the restoration.
Site description
No information found.
Measures selection
The Emsfors hydropower dam was bought by the Swedish Environmental Protection Agency in 2005. The dam lids were permanently opened in 2006 and after a ruling in the Swedish Environmental Court (now replaced by the Swedish Land and Environmental Court) the dam and the power plant could be completely removed in 2010-2011. A nature like fishway (bypass) with a low inclination (1.2°) was constructed in 2010-2011 in the old intake channel for the hydropower plant.
Threshold areas/riffles damaged by timber floating located 200 – 1000 m upstream from the dam were restored in 2006. These areas were restored by boulder additions and salmonid spawning gravel additions.
Success criteria
In general, success criteria for Swedish restoration projects are poorly developed and no such criteria for these restorations could be found.
Ecological response
The area within 200 m from the old hydropower dam was transferred from a dam into a river section of alternating riffles and pools after the opening of the dam lids in 2006. Salmonid fish were rapid to colonize these new habitats. No salmon or trout could be caught by electrofishing at the site less than a month after the opening of the dam lids but one year later high numbers of salmon and trout could be caught at the site. Densities of salmonid fish have remained high in the following years (compared to other similar stretches in River Emån) with some annual variation. Salmonid spawning has also been observed on some of the gravel beds that were created in the area.
The reduction in water level in the area within 200 m of the old dam structure has damaged the flood plain habitats in this area. In total a few hundred square meters of flood plain habitats have been destroyed. However, in 2008 the plant community in this area had started to adapt to the new hydrological conditions and colonization of the old dam bottom by sedges, rushes and other hydrophytes had started to occur.
The river section more than 200 m upstream from the old dam structure remains relatively unchanged after the restoration. At high flows the water level in this area is basically the same as before the restoration, but at low flows the water level drops below the retention water level of the old hydropower dam. The restoration of threshold areas/riffles 200 -1000 m upstream from the old dam structure was not quite sufficient to completely cancel out the drop in water level at low flows caused by the opening of dam lids. Since the water level is not affected at high flows, the floodplain habitats in this area are still regularly flooded and appear to retain their ecological function. The return to a more natural flow regime will probably have a positive effect on the flood plain habitats upstream and down stream of the old dam structure in the long run, with more natural flooding, at spring and autumn high flows, increased flooding frequency, larger water fluctuations but somewhat reduced inundation times.
The effect the restoration has had on the wels catfish population in the Emsfors area has not been investigated. Some flood plain habitats were damaged by the restoration and one of these was an important known spawning site for catfish. Increased connectivity and increased genetic exchange between the catfish populations upstream and downstream of the old dam structure is probably beneficial for the catfish population in the long run. The catfish population in River Emån has increased in numbers and spread to new areas of the river in recent years.
The salmonid fish community in a threshold area/riffle that was restored by bolder additions and salmonid spawning gravel additions ca. 1 km upstream of the old dam structure has shown a weak positive response to restoration. No salmon or trout were caught by electrofishing at the site before the restoration. One year after the restoration a few young salmon but no trout could be caught at the site. Densities of salmonid fish have remained low in the following years.
Hydromorphological response
The permanent opening of the dam lids and later removal of the hydropower dam at Emsfors has restored lateral connectivity in the lower reaches of river Emån to near natural conditions. Lateral connectivity might even be improved beyond natural conditions because of the construction of a fishway with low inclination for fish species that are weak swimmers. The restoration has transformed the river section within 200 m from the old dam structure from a dam with very slow flowing water (lenthic environment) into a natural river section with high stream gradient and alternating rifles and pools (lothic environment). The river section more than 200 m upstream from the dam remains relatively unchanged, in this area the river is deep and slow flowing and surrounded by extensive flood plains. At high flows the water level is basically the same as before the restoration in this area, but at low flows the water level drops below the retention water level of the old hydropower dam. The restoration of threshold areas 200 – 1000 m upstream from the dam resulted in an increase in water level by ca. 10-20 cm upstream of all the restored threshold areas, but this was not enough to completely cancel out the drop in water level at low flows caused by the opening of dam lids. Since the water level is not affected at high flows, the floodplain habitats more than 200 m upstream from the old dam structure are still regularly flooded. After the permanent opening of the dam lids and later removal of the hydropower dam, the river channel and flood plain habitats up- and downstream from Emsfors are no longer affected by water regulation at Emsfors hydropower dam. The return to a more natural flow regime will probably have a positive effect on the flood plain habitats up- and downstream of the old dam structure in the long run, with more natural flooding at spring and autumn high flows, increased flooding frequency, larger water fluctuations but somewhat reduced inundation times. The restoration by bolder additions and salmonid spawning gravel additions of areas damaged by timber floating has increased the substrate- and flow heterogeneity in these areas.
Water quality response
The restoration was not expected to have any significant long term effect on water chemistry. Sediment PCB concentrations in Emsfors hydropower dam were high (exceeded Swedish threshold limit values for PCB in sediment samples) and Cd concentrations were elevated (below Swedish threshold limit values for Cd in sediment samples). Precautions were taken to prevent sediments from entering the water body when restoration work was carried out and only a limited increase in water turbidity could be observed downstream of the restoration. The risk of significant Cd- and PCB contamination was therefore considered low.
Monitoring before and after implementation of the project
Socio-economic aspects
Contact person within the organization
Extra background information
References
Related Measures
- Establish environmental flows / naturalise flow regimes
- Increase flood frequency and duration in riparian zones or floodplains
- Remove barrier
- Facilitate downstream migration
- Recreate gravel bar and riffles
- Shorten the length of impounded reaches
- Install fish pass/bypass/side channel for upstream migration
