Spree - Restoration and remeandering of the Müggelspree - downstream Mönchwinkel
Spree - Restoration and remeandering of the Müggelspree - downstream Mönchwinkel
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
Various hydro-engineering measures led to an extreme degradation of the River Spree until the 60s of the 20th century. In order to facilitate flood protection, generation of energy by water mills, agricultural use of the wetlands, and shipping, the river was channelized by (I) shore fortification by rip-rap or hybrid poplar plantings, (II) meander cutting, and (III) constructing a symmetrical trapezoidal cross-section. The mean discharge was artificially increased by the removal of ground water during upstream open-cast lignite mining activities. As a consequence, the river lost most of its natural variability of the flow regime, sediments, and morphology leading to subsequent degradation of the riverine biota.
In 2004, the Federal State of Brandenburg started implementing an intense restoration program in one section of the River Spree, the so called Müggelspree. The main objective of this program was to reconnect the previously cut meanders to the main channel. Since then, several old oxbows have been dredged and reconnected on both sides of the main channel. The reconnection of oxbows was accompanied by the removal of rip-raps and groynes and the replacement of the introduced hybrid poplars by native willows, oaks, or alders.
The restoration and re-meandering of the Müggelspree aimed to:
- Restore the natural hydrology, morphology, and oxygen balance of the river
- Improve water retention in the landscape
- Develop habitats to improve benthic and rheophil species
The Müggelspree is a sixth-order section of the Lower River Spree that extends from Große Tränke to the village of Neu Zittau, east of Berlin. As a result of the river straightening described above, the Müggelspree has a trapezoid channel profile with a mean slope of 0.015%, a mean water depth of 1.25m at medium discharge, and a mean channel width of 25 m (Schulz et al. 2003). Between 1998 and 2002, discharge has ranged between 2.5m3 s−1 in summer and 30m3 s−1 in early spring (Schulz et al. 2003). In the early 1990s, the Lower River Spree changed from a turbid to a macrophyte-dominated state (Köhler and Hoeg, 2000). Shifting sand covers the midstream riverbed, whereas stable sand prevails in the lateral parts of the riverbed. The latter is often colonized by macrophytes or mussels. Total nitrogen and total phosphorus concentrations range between 0.7 and 3.4mg N l−1, and 70 and 180g P l−1, respectively. (Schulz et al. 2003). Therefore, the Lower River Spree is classified as a eutrophic section. About 53% of its banks are lined by trees such as willow trees (Salix spp.), poplar trees (Populus spp.), and alder trees (Alnus glutinosa) (Schulz et al. 2003).
A large broad of measures were implemented along the Müggelspree through different projects improving 15.3 km of the river. These measures were:
- Connection of 4 historically cut meander from 2005 to 2008 (Mönchwinkel I, Mönchwinkel II, Sieverslake, Freienbrink III). At each site, the former main channel was blocked by a gravel dam to redirect all flow through the new meander; the remaining old main stem stretches serve as new flow protected habitats (new oxbows) and their depth and width variability slightly improved by alternating sand bars.
- Replacement of hybrid poplars: In areas of high ecological value, introduced trees have been replaced by native willow, oak, and alder at a length of 11.3 km along the river stretch from 2004-2007 and in 2009/2010. Initial plantings of native trees were conducted in 2005/2006 and from 2009-2012.
- Selective removal of shore fortification: For the development of natural habitat structures and to improve riverbed structure, groynes have been removed in 2009. A natural undercut, eroding bank that enables natural erosion processes was constructed in 2011.
- Restoration of oxbow lakes: 1 historically cut meander (Kirchhofen II) was restored by dredging sediment and connected at one side to the main channel in 2010/2011.
Within the next 10 years, the project “Restoration of the hydraulic capacity of the Müggelspree” will be implemented. During this project, lateral restrictions of the river bed by reed or sediment deposition will be removed; further shore fortification and groynes will be removed; and dredging of the river bed in the lower course of the river stretch will be performed. Furthermore, a detailed river development plan will be established by the beginning of 2014.
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Following the restoration and the re-meandering, the ecological response was monitored for macrophytes, benthic invertebrates, and fish.
Macrophytes The full connection of the monitored meanders led to a shift towards a type-specific macrophyte community including rheophil species within one year after implementation of the project (Grünert et al. 2007). The macrophyte community was afterwards dominated by European bur-reed (Sparganium emersum) and arrowhead (Sagittaria sagittifolia), with co-occuring various-leaved water starwort (Callitriche platycarpa) which are characteristic species representing the natural riverine plant assemblage (Hilt et al. 2007). Species representing the former oxbow macrophyte community (Potamogeton pectinatus, Myriophyllum spicatum, Ranunculus circinatus) still persist and refer to the increased hydromorphological heterogeneity following the re-meandering process. Nevertheless, establishment of Ranunculus fluitans (river water crowfoot) communities could not be observed during the monitoring. This species is also described to represent natural riverine plant assemblages (Hilt et al. 2007), however, the flow heterogeneity that emerged after restoration enables possible establishment of such communities present in the lower stretches of the river.
Invertebrates Immediately after reconnection of the old meanders to the main channel, benthic invertebrate communities were highly distinct from reference conditions in the main channel (Gelbrecht et al. 2008). This was rather an effect of the absence of rheophil species than of the presence of limnophil species. However, one year after the reconnection of the meander, no significant differences in the invertebrate community could be detected between the newly connected meander and the main channel. The high biocenotic similarity of both communities was most likely a direct result of the increased sediment heterogeneity following the re-meandering. The relative proportion of rheophil species and EPT taxa (Ephemeroptera, Plecoptera, Trichoptera) increased following the re-meandering, while the relative proportion of invasive species declined. The increase in habitat/sediment heterogeneity also promoted an increase in the heavily endangered species Unio crassus (thick shelled river mussel) and Ophiogomphus cecilia (green gomphid).
Fish Although the re-meandering was beneficial for aquatic vegetation and benthic invertebrates (Grünert et al. 2007, Gelbrecht et al. 2008), it failed for type-specific fish because some major bottlenecks were not addressed (Wolter 2010). Beside longitudinal and lateral connectivity, increasing flow velocities and maintaining river bed dynamics, suitable spawning sites are a major prerequisite for the recovery of type-specific river fish assemblages. Migration facilities and gravel spawning areas have been identified as major limiting factors for riverine fish in the River Spree (Wolter et al. 2002) and should be reinstated to improve the habitat conditions for type-specific river fish assemblages. Contrastingly, the re-meandering of the main channel resulted in a lowered river slope and reduced flow velocity, which is the opposite to that desired to reinstate the rheophilic or lithophilic fish fauna. To overcome this problem, the re-meandered sites could be substantially improved in the future by reconstructing shallow overtopped gravel bars in the narrowed stream channel instead of the dam. Narrowing the stream channel, instead of complete blocking it, should increase the flow velocity, which should keep the gravel clean from siltation and provide suitable spawning substrata for lithophilic fishes.
River ecosystems are highly dynamic and subject to continuous succession processes, hence, the overall ecological responses of re-meandering might only appear at the long-term scale. However, the restoration measures applied in this project proved also successful at a short-term for macrophytes and invertebrates.
The monitoring of the connection of two oxbows to the main channel showed significant improvement of the hydromorphological conditions. Only one year after connection to the main channel, sediment composition shifted from almost 100% particulate organic matter (POM) to stable mineral (sand) sediments. Within the next year, also sediment patches dominated by gravel established. Therefore, all types of substrates according to the main channel references emerged within only two years. Overall, the connection of the meanders to the main channel resulted in substantial increase in habitat diversity and substrate heterogeneity.
Monitoring before and after implementation of the project
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Extra background information
- Improve water retention
- Remeander water courses
- Install fish pass/bypass/side channel for upstream migration
- Remove sediments
- Remove bank fixation
- Surface water abstraction
- Hydrological regime modification
- Discharge diversions and returns
- Artificial barriers downstream from the site
- Artificial barriers upstream from the site
- Alteration of instream habitat
- Embankments, levees or dikes
- Loss of vertical connectivity
- Alteration of riparian vegetation
- Channelisation / cross section alteration