Free-flowing rivers are becoming increasingly rare. Centuries of human activity have altered channels leaving us now with the huge challenge that is reconnecting vital pathways for our migratory fish. New research published recently has shown that there is on average one barrier per mile in Europe’s rivers, choking off life in these critical arteries.


The term connectivity is used to describe how connected a river is and the main cause of connectivity loss are man‐made structures, such as dams, weirs, and culverts for road crossings. These structures act as physical barriers for migratory fish species (salmon, steelhead, brown trout, shad, etc.). Frustratingly, they are often old and no longer in use, relics of an industrial past. Removing barriers provide unprecedented opportunities for restoration. However, one of the biggest tasks for scientists and conservationists tackling this problem can be finding, documenting, and building an inventory of these barriers.

Dam in the Soča River (Slovenia) (Jan Pirnat). The Soča River, in the Slovenian Alps, like others in the Balkans, represent some of the last relatively unfragmented rivers in Europe but are now threatened by dams

Thus, a robust understanding of the number and location of barriers is essential to prioritize restoration efforts and to focus funding and mitigation works to key obstructions. The impacts of barriers can be variable, from short delays to complete blockages often depending on things like water flow conditions, swimming capacity of the fish, and timing of migration. One of the most pressing issues faced by managers today are impassable barriers that can reduce or fragment species distributions completely. Barriers like this result in diminished populations that are increasingly genetically isolated and at greater risk of significant population decline or extinction. For example, a study earlier this year found that populations of migratory freshwater fish have declined by 76% globally since 1970, with an even greater decline in Europe, a staggering 93%. The decline in many of these populations has been directly attributed to river fragmentation caused by man-made barriers. 

Big and Small barriers

We are all aware that artificial barriers, like large dams, are one of the biggest threats to rivers and can completely change a river system. What many people may not be aware of is that almost all barriers no matter how small have some effect on river life and more often than not they can have a direct effect on the migratory fish that we will love to catch. Even small barriers can prevent fish from traveling up or downstream to complete their lifecycles and can stop the natural flow of sediments which is vital for rivers. New research published recently in Nature has found that at least 85% of barriers on European rivers are smaller structures such as weirs, culverts, fords, and sluices less than 2 meters in height. These smaller barriers are often overlooked although their cumulative impact can be significant. 

National databases are in existence for large dams and hydropower structures however researchers working on rivers have noted that databases have an almost complete omission of small or unusual barriers. In the European barrier survey scientists surveyed about 2,700 km of the river network in 26 countries. The researchers recorded each barrier, location, size, and whether it was abandoned or still in use. None of the 147 surveyed rivers was found to be free of obstructions, a concerning observation. The research stemming from this search for barriers found that Europe’s rivers have an estimated 1.2 million instream barriers meaning Europe has some of the most fragmented rivers in the world. The study detected thousands of large dams, but also a myriad of smaller barriers such as weirs, culverts, and sluices which had been disregarded and are the main culprits of fragmentation.

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Small Barriers, Big Problems

-Surely a fish can get over that small barrier!? What harm can a small weir really do? 

These are questions often asked by trout and salmon anglers I meet on rivers…

First off, all life stages need to move, from a 4cm fry to 100cm adult. Therefore, even small barriers can cause a lot of harm depending on the particular life stage moving within the fragmented river.



Imagine a scenario where an adult brown trout is moving upstream and is faced with numerous barriers – the first weir is passable and after 4 attempts the trout moves past. The second barrier encountered takes a little longer and eventually after a few days of failed jumping attempts the trout finally makes it past the weir, exhausted, the trout then encounters the third obstacle, a shallow culvert, after numerous attempts it proves to be too much as the fish has used up all its surplus energy and simply can’t kick through the shallow water running through the culvert, the trout falls back downstream to sub-optimal spawning habitat instead of making it to the optimal gravels upstream. 


Research is beginning to show that barriers may be selecting for smaller fish! What this means is that smaller fish may be more likely to make it through the shallow water running over certain barriers (more tail in the water to help kick through). Barriers such as culverts and weirs often have a thin layer of water running over the face or through the structure. These shallow water barriers may be stopping bigger fish from getting upstream as the water is just too shallow for them to kick through, as a result, we lose these fish along with their big fish genetics from the spawning gravels upstream. 

Another important point is that fish may get held up as they move downstream at small weirs and dams, the ponding effect upstream of even small barriers can often disorientate fish especially smolts moving downstream. This can make these sites predation hotspots, losing more fish before they make it to the sea is the last thing we need when their marine survival is so bad at the moment. 

The cumulative effect of obstacles encountered can slowly chip away at freshwater fish populations but will often go unnoticed. Just because we see the occasional fish ascend a barrier does not necessarily mean a barrier is completely passable. Obstacles may well be stopping a proportion of fish moving upstream and also causing problems for fish moving downstream that we simply will never see. 

What can we do? 

It is not all bad news. Mitigation efforts to improve river connectivity may not be as complicated as some fear, since many of these smaller barriers are obsolete and be easily taken out or removed. 

There is an increased awareness of barriers and blockages in rivers and this has been helped greatly by the excellent documentaries like Damnation and Blue Heart by Patagonia and the excellent Rising from the Ashes by Trout Unlimited with the focus being on the damaging effect of large dams. There is now hope that this new study will bring increased attention to the significant role that smaller barriers may play in disrupting a river’s ecosystem. 

It makes sense to focus some of our attention on small barriers that are easier to take out in comparison to a large dam that can often be quite politically divided. The science can help to direct restoration efforts and ensure we connect good habitat with other good habitats, not bad ones. 

The river-barrier atlas for Europe is an excellent accomplishment, but more efforts like this are now needed. After all, river barriers and their effects are not confined to Europe, and information on river connectivity tends to be even scarcer in many other parts of the world.

The increased public and angler awareness can really help. There are now apps available on phones where you can report the presence of small barriers by simply taking a picture.  Action is now being taken by many restoration groups who understand the damage even small barriers can cause. The next time you are on your home water, why not keep a note of any barriers you see, speak to your local fishing or conservation group, and help to raise the issue. This is a movement we must continue for the sake of our migratory fish. 

James Barry, Ph.d is a Research Officer on the Climate Change Mitigation Research Project with Inland Fisheries Ireland which aims to assess the impacts of climate change on stream habitats and fish communities. Check him out on Instagram @Riffle_to_Reef.


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