For Free Flowing Rivers!

Author: Ildikó Varga | Date: January 10, 2025

Credit: haerdirolf / Pixabay.

River restoration from various aspects

Ensuring the longitudinal connectivity of rivers and creating free-flowing waterways have been among the goals of the Water Framework Directive, considered essential measures to achieve good ecological status. In the context of restoring freshwater ecosystems, the EU Biodiversity Strategy 2030 sets out that at least 25,000 km of rivers will be restored into free-flowing rivers by 2030 through the removal of primarily obsolete barriers and the restoration of floodplains and wetlands.

In the following article, we look at the issue of river restoration from various aspects, looking at the current situation, what tools have been used to help fish migrate and how effective they are, how large-scale interventions can be financed and what smaller interventions may be appropriate.

 

How many migration barriers are there on European rivers and why are they a problem for fish?

Rivers are a dynamic and continuously interconnected system of sections and water bodies, characterised by a diversity of habitats. The different hydromorphological types of sections are also interconnected, and species do not stop at the boundaries of each section.

European rivers have been fragmented by more than 1 million (estimated number 1,2 million) barriers based on AMBER atlas.

The conditions for fish species are significantly affected by transversal barriers on rivers, which, by blocking longitudinal passage, inhibit fish movement and migration, mainly due to the difference in water level caused by the artificial raising of water levels. In addition to reducing migration opportunities, dams also lead to changes in bank shape and velocity, fluctuations in water levels, narrowing of floodplains and thus a significant reduction in spawning habitat.

The barriers will cause a swelling in the upstream section, leading to stagnant water conditions, which will create unfavourable habitat conditions for the species that prefer the current, with their populations being significantly reduced and, in some sections, disappearing. In the downstream section, the main changes are structural alterations of the bank and the riparian zone and changes in water discharge and sediment dynamics. Dams also fundamentally change the character of the cross-section, which is reflected in a reduction in habitat diversity and species richness.

The impact of the barriers also affects the transversal permeability of the river - the connection between the river and its interconnected tributaries, oxbows and wetlands in the floodplain. This is a risk because tributaries and backwaters are important spawning and overwintering grounds for fish and shelter during floods.

According to the Dam Removal Europe Report, at least 487 barriers were removed in 15 European countries in 2023 and more than 4300 km of rivers were reconnected through removals. There are at least 10 reasons why barrier removal is a proven, effective and affordable way to help local communities, improving public safety and tackling the effects of climate change.

 

What the construction of the Iron Gates meant for a classic migrant fish, the great sturgeon?

The great sturgeon (Huso huso) is the largest sturgeon species live in the Black-Sea, Azov-Sea, Caspian-Sea and, to a lesser extent, in the Adriatic Sea. It is a native migratory fish in Hungary that swims up on River Danube to reproduce and before the river regulation once reached as far as Passau. Its distribution was gradually reduced to the lower reaches. The former large-scale migration was a major factor in the fishery of the domestic rivers.

However, in addition to overfishing, the construction of the Iron Gate hydropower dams under the Kazan Passes sealed the fate of the giant.

István Széchenyi, famous for his transport development plans, dreamt of making the Kazán Passes (Large-Kazán and Small-Kazán) and the lowland valley exit navigable. As a first step, the most dangerous rocks of the strait were blasted on the initiative of Széchenyi and Vásárhelyi, but due to the financial cost of removing the cascades only a towpath on the left bank of the river was built. Regulation was delayed until 1890 due to technical, financial and diplomatic difficulties.

Although the half-finished canal was officially inaugurated in 1896, work continued until 1899. As a result, an 80-meter wide and 3-meter-deep navigation channel was created on the Serbian side, allowing safe navigation except at the lowest water level.

The real problem, and the prevention of the migration of great sturgeon and other fish species, was the construction of the hydroelectric power dam in the early 1970s, which drastically altered the water flow, the coastline and the wider landscape.

The dam raised the Danube water level at the power station by 33 metres. In a second phase, Iron Gate II was also built 80 kilometres downstream, regulating the river's exit from the lowlands. As a result of these interventions, the cascades in the Kazan Pass have been eliminated and the river has become practically stagnant.

All is not lost! In 2010, a great sturgeon reintroduction programme was launched to save the species. Several specimens released and signed in Hungary have been recaptured near the Black Sea and in Serbia, which means that these specimens have passed through the Iron Gate hydroelectric dam. It is also important to protect habitats and to investigate how boat locks can act as fish locks.

 

Are Fish Passages Effective?

In the past, fish ladders were primarily implemented as mitigation measures for existing barriers in watercourses. However, restoration efforts now emphasize large-scale interventions, such as the removal of dams, to ensure genuine river connectivity. But how effective are fish ladders in practice?

The effectiveness of fish ladders, constructed typically in the 21st century, is evaluated using RFID technology, where fish are tagged with microchips and their passage is tracked via antennas. Studies of fish ladders across Europe, including those along Hungary's River Rába, have demonstrated that their functionality depends on proper design. Natural and nature-like, longer passages tend to operate better. Key elements for effective fish ladders include:

• Favourable slope for easier ascent,
• Balanced water levels,
• Sufficient channel width,
• Features to slow water flow, such as increased roughness (stones and rocks),
• Installation of debris deflectors,
• Regular maintenance and ensuring the conditions for upkeep.

When these criteria are met, fish passage success rates can reach 80–90%. However, the success also depends on the fish's size and swimming ability of fish species.

Despite careful design and operation, fish ladders cannot compare to the ecological continuity provided by natural rivers without dams. While they offer a good compromise under certain circumstances, the real river connectivity requires solutions like dam removal to restore the original ecological conditions.

 

How can we finance the large-Scale dam removals?

In the context of nature conservation-focused restoration initiatives, even when the goal and tools are clear, the most common question is: what funding sources are available? This is especially critical for large-scale, costly interventions. In different member states, the feasibility of implementation varies depending on political and legislative circumstances, as well as available financial mechanisms.

In addition to traditional non-profit funding, there exist market-based, profit-oriented options. In the frame of a project, Rewilding Europe and Dam Removal Europe have outlined 11 financial models. However, further evaluation and broad testing of these models are necessary.

We emphasize one of the models, the revival of floodplain management and other sustainable, nature-based economic activities in the areas reclaimed after dam removal. This approach could be viable in the Central and Eastern European region. The sale of floodplains for such purposes could cover the costs of dam removal, and the value of new economic activities may even exceed the benefits previously provided by the dam.

Among nonprofit financial options, we highlight the Open Rivers Programme, which relies on private donations. This program not only funds dam removal but also supports the identification and prioritization of obsolete dams, as well as the preparation and design of interventions. Funding for these interventions may come later from the programme or could be implemented by a third party.

Up to now, the program has supported 127 projects. Unfortunately, if we examine the map of funded projects, the Central and Eastern European region remains largely unrepresented, with no projects in Hungary and the Czech Republic having been implemented with this funding.

 

Zero Stage River Restoration

To achieve the restoration goals of the EU Biodiversity Strategy 2030 for freshwater ecosystems and free-flowing rivers, it is crucial to understand the concept of the "zero stage" and the „Stream Evolution Model”.

What does "zero stage" mean? The "zero stage" refers to the initial state of a river system, undisturbed by human activity, as described in the „Stream Evolution Model”. At this stage, the river exhibits a wide range of hydromorphological attributes and provides extensive ecosystem services. The watercourse is fully connected to its floodplain and groundwater through the hypogeic zone. This concept emphasizes the need to consider rivers in three dimensions: longitudinal (along the river's length), lateral (across the river valley), and vertical (including surface and subsurface flows). These dimensions combine to form an interconnected and integrated system.

Historical research has revealed that before significant human impact, riverbeds were much shallower and exhibited an anastomosing structure, infilling the entire floodplain. This contradicts the earlier assumption that rivers predominantly meandered. Vegetation played a crucial role in shaping these basin morphologies. It may not be surprising, that the ecosystem engineer beaver is also able to establish and maintain "zero stage."

The "zero stage" approach underscores the importance of understanding the historical evolution of river systems and calls for collaboration among various disciplines to define the ideal target conditions for each specific context. This understanding is vital for achieving sustainable and effective river restoration.

 

Every Little Action Counts: The Many Shades of River Restoration

Restoring nature means reconnecting rivers, revitalizing floodplains, and bringing life back to aquatic ecosystems. While large-scale dam removals and landscape-level interventions grab headlines, smaller, thoughtful actions also play a crucial role in restoring river health.

Here are some key ways to help rivers thrive again:

• Replacing artificial riverbanks with natural ones
• Restoring the riverbed to its original form
• Planting and maintaining riparian vegetation
• Bringing back the river’s natural meanders
• Reconnecting cut-off meanders and oxbow lakes
• Implementing sustainable riverbed management

But restoration isn’t just about action – it’s about smart planning, adapting to local conditions, and using the right methods for long-term success. Sometimes, simply stopping harmful practices can already make a big difference!

 

Rethinking Riverbank Protection

Replacing artificial riverbank protection with bioengineering techniques isn't just good for nature – it enhances ecosystem services! By switching from conventional riprap (large stones) to natural solutions like reed beds and softwood (willow) vegetation, we can:

• Remove up to 20-30x more nitrogen and phosphorus from river water
• Increase carbon sequestration up to 30x higher in natural vegetation
• Enhance self-purification and biodiversity in riparian zones

For example, removal of artificial riverbank reinforcement has been implemented in the frame of DANUBE4all on the Austrian section of the Danube (Paradeiserinsel), and it is also in progress at the Erebe Islands on the River Danube in Hungary.

 

Protects, Preserves, and Provides – The Vital Role of Riparian Vegetation

Riparian vegetation is more than just green space along watercourses, it plays a crucial ecological role interacting in three dimensions with waterbodies. Longitudinally extending along rivers as natural corridors, laterally forming a transition between aquatic and terrestrial habitats, and vertically connecting groundwater, soil, and canopy.

Why is it so valuable?

• Prevents erosion by stabilizing shorelines with its root system
• Shields riverbeds from degradation
• Reduces flood risk by slowing runoff
• Creates microhabitats for aquatic life through fallen leaves, branches, and roots
• Regulates water temperature through shading, supporting aquatic ecosystems
• Provides nutrients for both water and land organisms
• Filters pollutants, improving water quality

Preserving and restoring riparian vegetation and connection of riverine habitats is key to protecting our rivers! Healthy rivers start with healthy riparian zones!

A good example on a large scale  (River Salzach) and a small scale (Csömöri stream) riparian restoration.

 

Straight or Meandering?

If we want to achieve natural water management, retain and distribute water in the landscape, the answer is clear: the latter! Meandering (winding) watercourses provide a range of benefits:

• Flood protection – slower water flow reduces flood risks and increases water retention
• Improving water quality – pollutants and sediments settle
• Increasing biodiversity – meanders create diverse wetland habitats for wildlife
• Groundwater recharge – water stays longer in the area and infiltrates deeper soil layers.
• Erosion control – slower currents prevent erosion

And let’s not forget the breathtaking landscapes! Who wouldn’t want to admire a natural river like the Sava River in the 19th century?

 

Half-measures won’t cut it! Only fully reconnecting old riverbeds can restore natural water flow, improve hydromorphology and aquatic biodiversity.

Useful resources

• A German study about the meander reconnection method
• Hungary’s Boronka Creek good practice
• Sava River restoration plans & river restoration guidelines

 

The "Volunteer" Restorer

Nature restoration is a huge opportunity to improve habitat conditions and address environmental crises. But in this effort, we must not overlook our best volunteer helpers – beavers!

What We Can Learn from Beavers? Beavers create thriving wetlands by instinctively choosing the perfect spot to build their dams, flooding areas that would naturally hold water.

What is the result? Flooded, marshy, diverse wetland habitats. Their engineering skills are unmatched, shaping rich, dynamic ecosystems that support countless species. And no, a beaver wetland is not a stagnant swamp, rather, it is a slow flowing, seasonally dynamic, continuously evolving habitat.

What can we do? Simply, let our volunteer engineers do their work! By adapting to their presence and the water-rich landscapes they create, we can harness their natural restoration power. Even beavers themselves adapt – they sometimes tolerate the dismantling or lowering of their dams. If interventions are needed – this is the rarer case – there are effective, nature-friendly solutions.

Another incredible habitat transformation along the Kerca Stream!

 

Useful resources

• The Role of Beavers in Water Management - Recording of the 2024 Conference (in Hungarian)
• Beaver Project (Beaver Pond) of the Talpalatnyi Vadon Nature Conservation Foundation