Best practices for selecting barriers within European catchments

With over 1.2 million dams and other instream barriers, Europe has possibly the most fragmented rivers in the world, but also the opportunity to benefit enormously from barrier removal. Resources available for barrier removal, however, are limited and some form of prioritization strategy is thus required to select barriers for removal that will provide the greatest gains from restoring river connectivity in the most efficient possible way. To properly restore river function, barrier removal programs need to consider all types of artificial instream barriers that cause river fragmentation, not just those that impede fish movements.

Opportunities for barrier removal depend to a large extent on barrier typology, as this dictates not only where barriers are typically located, but also their size, age, condition, and impacts. Crucially, the extent of river fragmentation depends chiefly on the number and location of barriers, not on barrier size. However, because barrier removal costs typically increase with barrier height, acting on many small barriers may be more cost-efficient than acting on fewer larger structures.

Here we review the main strategies available to prioritize barriers for removal and mitigation, with special emphasis on removing non-ponding, low-head (<3 m) barriers, as these are the most abundant across Europe and other regions. To increase the success of barrier removal programs, we recommend that barriers considered for removal fulfill four essential conditions: (1) they would bring about a meaningful gain in connectivity; (2) are cost-effective to remove; (3) will not cause significant or lasting environmental damage, and (4) are obsolete structures.

There are dozens of prioritization methods in use. These can be broadly grouped into six main types depending on whether they are reactive or proactive, whether they are typically applied at local or larger spatial scales, and whether they employ an informal or a formal approach. These include, in increasing order of complexity: (1) opportunistic response; (2) use of local knowledge and expert opinion; (3) scoring and ranking; (4) geographic information system (GIS) scenario analysis; (5) graph theory; and (6) mathematical optimization. We review their strengths and weaknesses and provide examples of their use. Overall, mathematical optimization sets the gold standard for effective and robust barrier mitigation planning, but to be practical, it needs to factor in the constraints imposed by uncertainties and opportunities. Accordingly, a hybrid approach that considers uncertainty, the presence of natural barriers, the importance of future-proofing, and opportunities provided by local knowledge is likely to be the best overall approach to adopt.

Various studies have shown that a small proportion of barriers is typically responsible for the majority of river fragmentation. These ‘fragmentizers’ can be identified and located using the prioritization methods discussed herein and a targeted approach can produce substantial gains in connectivity by acting on a relatively small number of structures. Unfortunately, many of these ‘fragmentizers’ cannot be easily removed. Removal, therefore, is constrained by opportunities and what is practically feasible. Mapping of barrier removal projects according to the three axes of opportunities, costs, and gains can help locate the ‘low hanging fruits.’ Opportunities normally develop over time as infrastructure ages, so acting on some barriers now will likely open opportunities for acting on others later on to create a snowballing effect.

The ability to simulate benefits and costs of barrier removal and select barriers for removal is critically dependent the quality of the data at hand, particularly with respect to the number of barriers, which can be grossly underrepresented. Uncertainty caused by incomplete barrier records diminishes the effectiveness of barrier mitigation actions but these can be overcome to some extent by (1) ground truthing via river walkovers or (2) predictive modelling. Other critical sources of uncertainly include those caused by inaccurate stream networks and spatial errors regarding the exact locations of barriers. Although uncertainties can be reduced by collecting more information, it needs to be weighed against the cost of waiting. Waiting to collect more data to reduce uncertainties tied to barrier removal may lead to ‘paralysis by analysis,’ while species and ecosystems continue to decline due to stream fragmentation.

To better understand how barrier prioritization is implemented in the real world, we sent out an online questionnaire to river restoration practitioners located in Europe and North America. Results show that most organizations (~60%) have a plan to achieve free-flowing rivers. Most respondents (34%) use expert judgment, followed by consultation with stakeholders (17%) and a combination of methods (28%) to prioritize barriers for mitigation. Only 12% used specialized software or algorithms. Attributes most frequently considered by practitioners in barrier prioritization were barrier ownership and rights, results of field surveys, and the obsolescence and conservation status of barriers. The most important rational flagged by practitioners to prioritize barriers for removal was to improve fish passage.

Our study suggests that no matter what prioritization approach is ultimately adopted, decision makers need to be mindful that no priorities should be set in stone. Planning needs to be agile and flexible enough to adapt to changes and react to opportunities.