What does ‘renaturation’ mean?
Nowadays, most waterways are heavily modified and can boast only a few sections which remain in their natural state and functions. Straightened riverbanks, groynes and riprap as well as dykes border the river landscape and ensure that large freight ships can navigate the waterways and that the population is protected from flooding.
As a result, the river ecosystem is losing its diversity – but is being optimised for human use and convenience.
To counteract the increasing anthropogenic pressures upon rivers and to reduce human intervention in the river systems, restoration works have been carried out on many rivers in Central Europe since the 1980s. This includes the increase of water quality but foremost the restoration of the typical functions of the rivers as disturbance driven ecosystems. As such the restoration of habitats; converts monotonous river landscapes into living biotopes by increasing structural diversity and leaving the river course to follow its own dynamics, thereby establishing a lively and living riverbed with fallen trees, dense vegetation and wet marshes.
How does restoration work?
Restoration measures can be implemented at scales along a waterway.
One initial method of renaturation is refurbishing of riverbanks. In doing so, bank reinforcements are levelled or removed, thus creating diverse boundary zones and backwaters. This helps to create a near-natural, living landscape.
Another component of renaturation is the transformation of straightened sections into meandering and braided sections. These measures serve to reduce the uniformity of the current, since the water is slowed down in the curves while being increased in the outside sections of bends. Diversity in the current velocity ensures that sediments are removed in one section and settle in another one rendering the river structure more diverse and thus providing more habitat for specialists. Overall the water remains in the river section for a longer time, which increases the biological purification as well as the retention of water in the landscape which is important especially under the scenarios of climate change.
Wetlands are natural flood areas in a river system where water collects when the water level rises. These areas are often lost due to the embankment of rivers, but can be recovered by relocating lateral banks or “cutting through” them, thus enabling the water to get through. In order to protect human settlements, dykes are necessary but the protection of pastry land from floods becomes less and less essential when the rivers are reverted from wastewater canals full of toxins to living water courses and floods don’t rise as high when they have sufficient space where they can spread in the upstream sections of the river, reducing the risk for the downstream settlements too.
Why do we need these measures?
A restored river doesn’t just look more diverse and lively, but also actually offers many creatures sanctuary. Restored tributaries of a river are used by many amphibians and species of fish as spawning grounds. For instance, sturgeon prefer to lay their eggs in sections of gravel fed by the current in tributaries just as many river fish species, the same holds true for the feeding or nursery areas, which are to provide a plethora of different invertebrate species as feed for the fish. This differs largely from the deep, mobile fast current raceways that rivers have been converted into. In order for initiatives for the protection of aquatic species such as the sturgeon to succeed, restoration is often urgently necessary. The sturgeon, which can be established in its historic range through stocking measures, can only continue to sustain itself in the long term when its habitat is restored.
However, it is not just the species that live in the water of the river itself which profit: birds and insects also find perfect breeding and feeding sites on in near-natural riparian strips.
These riparian strips, with their reed beds, tall forbs and trees, also have a second advantage for nature. Harmful substances from agriculture are absorbed by the roots of the plants and do not make it to the water, where they spread out across the food chain and cause damage to many organisms.
Not least of all, humans themselves benefit from the renaturation of the rivers. Initiatives such as the relocation of dykes along the Elbe or the ‘unblocking’ of the Rhine create natural flooding areas, where river water can escape when the water level rises. The same thing happens when natural sidearms are connected. Many animals and plants manage to find a suitable habitat in these natural protection measures against flooding at the same time.