Cet article est republié à partir de The Conversation. Lire l’article original.

Karl M. Wantzen, UNESCO Chair River Culture/Fleuves et Patrimoine, Université de Tours

The works of Franz Kafka often describe people trying to reach a goal but never arriving – all ways are blocked, any possible detours turn out to be impassable. Migratory fish and invertebrates in European rivers today are in a similarly Kafkaesque situation. New research in the journal Nature, published December 17, reveal that on average, there are at least 0.74 barriers per kilometre of European waterway. This means that an organism can hardly travel more than 1,000 meters without being stopped by a dam, weir, culvert, sluice or ramp.

The movement of flowing water is the essential nature of all running water systems, from small rivulets to large rivers. But this natural movement, and especially its pulsing nature, with flood events and droughts, is too dynamic for human beings who have sought to become “master and possessor of nature” (Descartes, Discours de la methode) since their earliest days. Building bridges to cross waterways, controlling river flow to ease navigation, mastering floods, diverting water for irrigation – all of this is the high art of engineering, and a part of human culture since prehistory.

At first, limited impacts

Until the industrial age, humans only had local or regional influence on the environment, and these impacts were relatively limited (Wantzen et al. 2016). Still, studies indicate that even in the Middle Ages, mills constructed in tributaries of the Rhine interfered with salmon attempting to reach their spawning grounds and cut their population in half (Lenders 2017, Wantzen et al. 2020). Half of “very much” may still be “enough”, but for how long?

Today, only a few very remote rivers can flow freely (Grill et al. 2019), their waters inundating and fertilising natural floodplains full of fish and with flourishing vegetation, their sediments are transported downstream, build islands and even deltas in the sea. In Europe, it is hard to find such exceptions like the Vjosa River (Schiemer et al. 2020), which flows from Greece through Albania to the Adriatic. Most of the rivers in the industrialised Global North as well as those in developing countries of the Global South have been fragmented into pieces that hardly flow any more, and in which sediments are blocked above dams, and their bed becomes incised by erosion.

The river deltas of the Ebro, Nile and Mekong are vanishing, receiving only a small percentage of the natural sediment yield from their headwaters (Kondolf et al. 2018). Fish searching for their spawning sites are blocked or become weakened by a series of fish passes that only few of them survive. And even if they arrive, their offspring migrating downstream struggle to survive the return trip.

As if there had been an invisible plan for their eradication, clandestinely executed in the past two centuries, almost all large migratory fish species worldwide have been brought to the brink of extinction (Winemiller et al. 2016). Fascinating, living fossils like the million-year old sturgeon species can still be visited in zoos, but are rarely found in nature.

Other species that depend on migratory fish, such as river mussels that use them for transporting their larvae, are in peril. One example is the freshwater pearl mussel in Europe (Prié et al. 2017. Also under threat are the guardians of the fish, the traditional fishermen (Dugan et al. 2010 whose livelihoods depend on them. Awareness of the scale and severity of this problem has arrived only slowly in the society.

Understanding the scale of the problem

International treaties such as the European Water Framework Directive (Carvalho et al. 2019), have given rise to efforts that may tackle the problem, by removing selected dams, creating detours or by releasing water according to natural flows.

The first step is to understand the dimension of the problem. How the many dams are there? Large ones, higher than 15 meters, can be seen on satellite images. In most countries, they also require construction permits, so that information can be obtained worldwide with high precision. However, smaller barriers such as low-profile dams and culverts can be (and are) built without detailed formalities, and can hardly be seen from space. Moreover, there is a dizzying confusion about expressions to name these constructions, even within a single country.

The Horizon 2020 project named AMBER, coordinated by Carlos Garcia de Leaniz of Swansea University in Wales, has gathered scientists from all over Europe to develop an atlas showing these barriers. In the framework of my work as the UNESCO Chair “Fleuves et Patrimoine – River Culture” and as coordinator of the LeStudium Consortium on Ecohydraulics and Dam Removal, I was able to contribute to the AMBER team of more than 50 scientists who jointly assembled the highly heterogeneous data into a unified databank for all of Europe.

While France has for long established a public data bank for flow obstacles, ROE, the situation in other countries is more complicated. In Italy, dozens of expressions for very similar types of barriers exist, and in Spain, less than 50% of the obstacles were known. In Germany, with its federal structure, each Bundesland (county) has a different way of recording data. However, the Länder collaborate in a joint working group, the LAWA.

An atlas showing all flow obstacles in Europe

Once in hand, all these data were brought together into a single format with a consistent terminology, so that they were applicable to the Barrier Atlas, which can be freely downloaded from the AMBER website. During the time-intensive process of gathering and processing the data, we learned how the European collaboration for sustainable environmental management can be improved – the French word millefeuille (a thousand leaves) describes the current situation well. We also have to overcome the institutional barriers at so many levels, within and between the countries.

Two of the findings for our paper were particularly shocking:

• The number of barriers in Europe is far higher than we had expected, more than 1.2 million.

• Approximately 10% of those barriers are obsolete and could be removed.

These findings raise important questions, including how to remove the most disturbing of the 120,000 obsolete barriers, what to consider during their removal, and how to slow the ever-growing damming trend in the Global South. Scientists all over the world, including our consortium, are currently working these issues, and advances are being made.

Still, what’s most needed is the political will by the society to switch from a single-sided view of rivers as a mere resource to be exploited to an acknowledgement of their role as cradles of humanity, as essential life-support systems, and as biological and cultural entities to be preserved for their own nature – as part of a “River Culture” (Wantzen et al. 2016).