English abstract

Context: Functionally intact and biodiverse freshwater ecosystems form the backbone of a healthy society. They provide essential and unique ecosystem services: from productive services such as food supplies, to regulatory functions such as natural waste processing and cultural values such as recreation. To secure these vital functions for the future, the development of strong environmental guidelines, freshwater management plans, and river restoration works is crucial. Although the value of aquatic ecosystems is inestimable, they are currently among the most sensitive systems in the world. Habitat loss and anthropogenic changes have stabilized many processes which were once and should still be naturally morphodynamic. Worldwide, river courses and water levels are artificially controlled by structures such as pumping stations, hydroelectric power plants, dikes, weirs, dams, and locks. Such interventions have radically altered river morphology, restricting their natural flow space and severing ecological connections. These changes continue to cause the fragmentation of floodplains, altering their very character. To reverse this trend, the European Water Framework Directive (WFD) has served as the central compass for a uniform European water policy since December 2000. The WFD aims to achieve good status for all water bodies with two main objectives:

To safeguard water resources and water quality.
To mitigate the effects of floods and droughts.

For natural waters, ecological status is measured using biological indicators — such as fish, macroinvertebrates, and phytoplankton, — supported by abiotic hydromorphological and chemical parameters. For heavily modified or artificial waters, the policy instead focuses on good ecological potential, giving nature the maximum opportunities possible within the framework of human use.

Research area and objective: River restoration projects typically aim to improve ecosystem services and, ideally, restore damaged freshwater systems without compromising downstream and coastal ecosystems. River restoration is now practised worldwide, although changes in land use and other constraints mean that it is often no longer possible to restore watercourses to their original condition. Despite similar challenges, in the autumn of 2016, the local water authority (Flemish Environment Agency, VMM), was able to begin a number of relatively large habitat restoration measures in the Zwarte Beek valley in Limburg, such as re-meandering and the remediation of a number of fish migration bottlenecks. These measures aimed to restore functional habitats by creating the best possible abiotic starting point and then allowing the river system to further develop spontaneously and naturally. The restoration works were completed in spring 2017 and the measurement campaigns of the system were carried out in 2016, 2019 and 2024.

Results:

The soil texture has remained stable since 2016, consisting largely of equal parts of fine sandstone, coarse sandstone and coarse organic material. However, there are tentative indications that the proportion of silt is decreasing in favour of fine gravel and stones, which would be beneficial for various communities.
Although the average bed elevation appears to have barely been affected by the re-meandering, there does seem to be a significant change in the spatial variation in bed elevation, resulting from the natural creation of pools and riffles which have become more pronounced over time across both the depth and length of the stream.
The re-meandering has reduced the average water flow velocity, which aids both water retention and the settling of suspended matter.
Despite an increase in the spatial variability of flow velocity, typically the result of changing morphology, this cannot be directly linked to the re-meandering itself, as it was also strongly evident in the control areas.
In summary, we see increased variability in the structure of habitats, particularly in terms of bed elevation and, to a lesser extent, flow velocity.
Comparison of upstream and downstream water quality measurements before and after the restoration works indicate a small positive effect on nitrogen compounds, but no change in phosphorus compounds, suspended matter or oxygen balance was recorded. However, as these trends are characterised by high temporal variability, they may be caused by other factors than the restoration works.
After fish communities underwent an initial decline during 2016 to 2019 in their population size, biomass, diversity and biotic integrity, in 2024 a significantly positive evolution was seen, even exceeding the baseline position of 2016. In addition to a shift towards better ecological quality, more species were observed, with stream-loving species such as chub being more strongly represented and forming part of healthy populations (in terms of length distribution). Hydromorphology proved to be the most important factor in explaining the changes in the fish community structure, the most decisive element being flow velocity, followed by depth, proportion of organic material and bed elevation. Additionally, the proportion of aquatic plants and riparian vegetation also proved to be influential on the structure of the fish communities. Removing the barrier effect of weirs seems to result in a more diverse fish community, due to the upstream movement of various species such as chub, bitterling, pike and burbot. The quality of the macroinvertebrate and macrophyte (aquatic plant) communities did not seem to be affected over time.

Conclusion: The re-meandering of the Zwarte Beek appears to have had a net positive effect on both habitat quality and associated fish communities in 2024. Communities need time to adapt to the conditions created by restoration works, and monitoring over a longer period is therefore crucial. Despite the improved river structure in 2019, fish species did not shift accordingly until 2024. It is advisable to continue monitoring the ecology surrounding the restoration works to determine how the system will evolve further.

Data availability: We use multiple abiotic and biotic datasets collected as part of this study. An accompanying article is currently being prepared and these datasets will be published in it. Until then, please contact stijn.bruneel@inbo.be with any questions about the data. The “habitat description” protocol used has been added to the appendices.

Bruneel, S., et. al. (2026). 10.21436/inbor.141733221