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Rivers supply essential services to society, but their exploitation by human beings has constantly fragmented them. In Europe, more than a million obstacles impede the free flow of rivers and disturb the balance of these ecosystems.
Considerable and underestimated fragmentation
The fragmentation of rivers entails modifying their ecological continuity by placing obstacles along their course. This phenomenon is particularly apparent in Europe, a continent where for centuries rivers have been used to drive watermills and turbines, and supply water for forges and industries. An original study carried out in Europe in the framework of the Amber (Adaptative management of barriers in Europe) programme and published in the journal Nature last December gave the figure of more than 1.2 million barriers on the rivers of 36 European countries (i.e. an average density of 0.74 artificial barrier per kilometre).
The main indicators of barrier density are agricultural pressure, river-road crossings, and the coverage and altitude of surface waters. Most of them were built to control and divert discharges of water or to raise its level, like spillways (30%), dams (10%), locks and diversion canals. Other permit the stabilisation of river beds, such as ramps and weirs (31.5%), or to accommodate roads such as dead-ends (17.6 %) and fords (0.3 %).
Whatever their purpose, these obstacles that impede rivers have an impact on biodiversity and above all on the free circulation of fishes. Between 1970 and 2016, the population of migrating fish alone fell by 76% (see our previous article). Other impacts can be observed like the fall in water levels, increased coastal erosion and higher water temperatures in reservoirs and rivers. The degree of river fragmentation is an important indicator that informs on the level of fragmentation, artificialisation, and morphological modification of rivers, and their migratory transparency. It measures the difference between the natural slope and the sum of the artificial heads caused by the presence of obstacles. Thus, the higher the degree of fragmentation, the more degraded the habitat and fish populations. The maximum empirical degree of fragmentation to reach a good state for a body of water was set at 40%.
The difficulty of evaluation
However, the first major problem is being able to apply this indicator: the evaluation of the number of obstacles and their type. The researchers of the study performed in Europe estimate that the figure is greatly underestimated (by at least a third), since it does not include small water courses or ancient and poorly mapped structures. Above all, the data diverge considerably from country to country in terms of quality and quantity.
Evaluating the fragmentation of a river is difficult due to the ramification of hydrographic networks, the seasonal nature of hydrological regimes and the variability of the impacts of barriers in time and space. Apart from large dams, information is still very sparse regarding smaller and older dams. At present, remote detection technologies that allow mapping the emplacement of dams with precision cover only large infrastructures, which represent only 1% of existing dams. Smaller structures, though with high impact, are often not taken into account whereas the concept of river continuity is included in the European Union’s Framework Directive on Water and inventories of physical obstacles are required in hydrographic basin management plans (PGBH).
Surgical solutions
Several types of solution are possible to restore the connectivity of rivers. The first will be to draw better maps and monitor barriers on the global scale to obtain complete data and act efficiently. The second will consist in eliminating the structures by first targeting small barriers, which are easier and cheaper to remove. The good news of the study is that most of the obstacles identified are less than two metres high and abandoned, which facilitates their elimination. Nonetheless, this will not suffice if other barriers are kept, or built elsewhere on the river. Although the height of obstacles such as dams higher than 10 metres is a disadvantage for fish, the number of miscellaneous obstacles and their position on the course of rivers also play important roles. Also, any strategy to ensure the ecological restoration of a river must include parameters other than fragmentation, like climate change and the different species present.
Three researchers from Duke University in America (Cary Institute of Ecosystem Studies) published a scientific review in which they focus on the causes and consequences of fragmentation appearing in a report published in 2015. According to them, the responsibility of the river manager will be to define the best level of fragmentation to reach the objective of conserving the habitat and carry out a full analysis based on existing data. Predictive models should be used to evaluate the costs and benefits of different obstacle mitigation scenarios. Such an analysis “will require taking into account the cost and feasibility of eliminating each obstacle, its potential benefits for target species, and all its secondary effects, whether positive or negative, on other conservation objectives and on societal values”. This strategy will mainly consist of “surgical strikes”, that is to say operations on target obstacles that greatly affect species.
For its part, the European Union has set the objective of achieving natural flows over more than 25,000 kilometres of European rivers by 2030 in the framework of its strategy in favour of biodiversity.