Biodiversity is returning to the Alte Aare

Until recently, the river Alte Aare seemed more man-made than a natural watercourse. This river in the canton of Bern was so devoid of life that in some places it rather resembled a canal. Yet the Alte Aare is essential for the survival of the wetlands around it. Hydraulic engineers from Basler & Hofmann oversaw a wide range of revitalisation measures to maintain and promote biodiversity in and around the river. In the process, they also helped improve flood protection for the nearby residential areas. The project is the largest of its kind to date in Switzerland.


Niels Werdenberg and Diego Studer are the only people around as they wander through the tranquil riverscape of the Alte Aare in the Swiss canton of Bern. Here, near the village of Dotzigen, the river gushes cheerfully over tree trunks and stumps and around bushes, while, in the treetops above, the birds are chirping away. You would never guess that, only a few weeks ago, this whole area was a building site, echoing with the roar of excavators and chainsaws. But appearances can be deceptive. A stretch of the river and surrounding floodplain several kilometres in length has just been radically transformed. New backwaters have been dug, thousands of cubic metres of earth removed from around the banks and countless trees cleared. As Niels, the environmental specialist who supervised the construction work, explains: “Doing all that in a conservation area – that’s really rather specialised work.”

The main reason for this unusual construction project was to reduce the risk of flooding from the Alte Aare. Villages such as Dotzigen and Studen have fallen prey to flooding in the past. This risk prompted the 10 nearest municipalities – who together form the Wasserbauverband Alte Aare (Alte Aare hydraulic engineering association) – to launch a project to improve a number of dams and resurrect an old backwater of the river. But for Niels, who works as a biologist and environmental engineer at Basler & Hofmann, the best part is the way in which the flood protection measures are being combined with efforts to enhance biodiversity in the area. For the past 140 years, the floodplain around the Alte Aare has been sorely lacking in dynamism. Ever since the first “correction” of the waters of the Swiss Jura in the 1880s, when the Aare was re-routed into Lake Biel via a new channel branching off at Aarberg, only a small, regulated amount of water has been allowed to flow along the remaining stretch of the old river. As a result, the Alte Aare has gradually shrivelled into a bland, feeble canal. Apart from a few large, but very infrequent floodings, the river did not flood often enough to sustain biodiversity in the surrounding wetlands. The floodplains degraded and ceased to provide the usual habitats. No doubt about it: it was high time to breathe new life into the Alte Aare.

The construction work is turning the Alte Aare into a valuable habitat once more.

New life for the Alte Aare

At Dotzigen, the revitalisation measures are already complete. Niels is on his way to inspect the work and carry out some GPS surveying, aided by his assistant Diego, a third-year apprentice drafter specialising in engineering. The trees are still dripping after the morning rain, but now the sun starts to peek through the foliage here and there. The two men wander along the Alte Aare. First stop: two new amphibian ponds that have just been created near the river, designed to provide habitats for endangered frog and toad species. “Sadly, the European tree frog is only found in isolated locations these days. The new measures could help populations to start growing again”, hopes Niels.

Ponds like this are designed to provide new habitats for the European tree frog and other amphibians.

After surveying the ponds, Niels and Diego resume their walk up the river. The sounds beneath their feet suddenly change from thuds to crunches – the ground is now gravel rather than earth. They have arrived at one of the new pioneer areas. This large gravel bank in the forest was created to give reptiles like the grass snake, many lizard species and various types of ground beetle new places for hiding or basking in the sun. A few steps further on, Niels looks up at the tops of the trees and squints. More sun penetrates through the canopy here than elsewhere in the forest. “Orchids used to grow here”, explains Niels. “We’ve thinned out the pine trees to give the flowers a chance to grow again.”

Diego Studer (left) and Niels Werdenberg are in their element as they document the measures.

Working wonders with wood

Niels and his colleagues also tried to re-establish fish populations in the Alte Aare, with the help of large numbers of deadwood elements distributed around the river. Everywhere you look there are felled trees anchored to the banks and projecting into the water as well as tree trunk groynes and sills with water rushing around them. “These in-stream structures all make the underwater environment more dynamic and thus a better habitat for fish”, says Niels. They are needed to create parts of slower- or faster-flowing water, which results in a varied underwater landscape with areas of different depths to give the fish places for resting and hunting. “There isn’t enough of this kind of natural wooden ‘furniture’ in the rivers today”, explains Niels. “We’ve done our bit to rectify this here.” In total, around 1,000 cubic metres of deadwood were used along the six-kilometre stretch of river. “This is more than anywhere else in Switzerland”, says Niels. No trees were cut down unnecessarily to produce the wood. All the deadwood used is a by-product of the thinning-out of the forest around the Alte Aare. “The deadwood elements are very beneficial from an environmental perspective, but they only marginally affect the course of the river”, he says. Thus, no forests or cultivated landscapes suffered.

It is a different matter for the other revitalisation measures. To enhance the wetland landscape, the excavators had to remove large volumes of soil from along the Alte Aare. Because there was not enough water left in the canal to regularly flood the surrounding wetlands, in some places the level of the ground around the banks was lowered by up to a metre. This has helped save the precious wetland landscape – judged to be “of national importance”. On the lower-lying land, softwood groves, carr and marshes are now able to develop, providing habitats for numerous endangered animals and plants. The project team have also created several new backwaters for the river, which enlarge the wetlands and at the same time improve flood protection

Before and after: this newly dug backwater is beneficial primarily from an environmental point of view, but it also helps protect against flooding.

Back at the installation location, Niels assesses progress with the help of his sandbox models of the area. Next week, he will be using these to explain to a group of schoolchildren what he and his colleagues are doing here, and why. For Niels, this flagship project is far from over. He is already looking forward to the next construction phase – on the stretch of river between Dotzingen and Busswil.


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