Fish passage and hydromorphology

February 2024

Eoghan Concannon

Hydromorphology considers the physical character and water content of water bodies. Good hydromorphological conditions support aquatic ecosystems i.e. hydromorphological elements such as water flow and substrate provide physical habitat for biota such as fish, invertebrates and aquatic macrophytes ( By supporting ecosystems, they are ultimately providing livelihoods for people.

Weirs, dams and other types of in- river manmade structures are plentiful around the world. For centuries these have been used to produce energy locally, to drive industry, transport, create reservoirs for drinking water, and more recently used for recreational purposes like boating, sailing, and fishing.

Large dam - Dragan Lake, Romania

There are more than 57,000 large dams worldwide. China has over 23,000 large dams. The US is the second most dammed country with some 9,200 large dams, followed by India, Japan, and Brazil (International Rivers). England and Wales have about 2,000 dams and there are around 800 in Scotland. While these structures still serve useful purposes in certain strategic locations, there are many that are now dysfunctional and not very useful for anything, and in some cases centuries old. This begs the questions how good are these structures for sustainable development?

In the Thames area plans for creating new dams for energy capture have been discouraged by state bodies like the Environment Agency.

Relatively low-lying river basins like the Thames don’t produce as much hydropower as areas that are more mountainous. Mountainous terrain can enable greater water speed and power thereby creating more energy - Portugal being a good example of that. Some other European countries, one of them being Latvia, now have communities and state agencies that have decided to remove dams for wildlife and people’s benefit because they haven’t produced as much power as hoped.

The rate at which large dams are completed has declined from around 1,000 a year from the 1950s to the mid-1970s to around 260 a year during the early 1990s. More than 3,700 hydropower projects were planned or under construction on the world’s rivers as of 2014. If built, they could block free-flowing rivers by more than 20% (International rivers).

Of course, weirs and dams still have their functions and purposes, but many are obsolete and no longer fit for use. These represent issues to the river ecosystem and the services rivers provide to society – damage to fisheries, water quality issues, habitat fragmentation as well as negative impact to recreation by posing serious risk to kayakers and swimmers, to name some. Given the maintenance and repair costs associated, there has been a big push to remove these structures where they are found to be obsolete, for wildlife and socioeconomic benefit. In 2022, 325 dams were removed in Europe (Dam Removal Europe).

Dam removal United States, Charlestown

Just across the English Channel in the Sélune river in France dams have been removed. Research director for the national research institute, Inrae, Jean-Marc Roussel, described the first results of this ecological restoration operation as ‘spectacular’ in reference to the return of sea lampreys, European eels and Atlantic salmon. Scientists also found that the temperature of the opened river had decreased by 2C. Laura Soissons, an Inrae research engineer, who coordinated the monitoring, with colleague Jean-Marc Roussel, noted that vegetation had returned to the banks, despite the scale of the construction site in recent years. And the sediments, once trapped by the dams, have resumed their natural transit.” (Water News Europe, January 2024)

Large dams have provoked opposition for numerous social, environmental, economic and safety reasons. The main reason for opposition worldwide is the huge numbers of people evicted from their lands and homes to make way for reservoirs. The livelihoods of many millions of people also suffer because of the downstream effects of dams: the loss of fisheries, contaminated water, decreased amounts of water, and a reduction in the fertility of farmlands and forests due to the loss of natural fertilizers and irrigation in seasonal floods. Dams also spread waterborne diseases such as malaria, leishmaniasis and schistosomiasis. Opponents also believe that the benefits of dams have frequently been deliberately exaggerated and that the services they provide could be provided by other more efficient and sustainable means (International rivers).

How Marsworth Reservoir now, with severely depleted water levels (Image: South Beds News Agency)

The argument for creating reservoirs in the Thames have been brought up recently as Thames Water and partners are now facilitating a public consultation for a new reservoir in Abingdon. We know that millions of litres of water are lost each year from existing infrastructure due to leaky old pipes, so why not address that issue instead of creating a new reservoir that will have a long term and adverse negative impact on local people and wildlife, and of course be ultimately profitable for the owner.

Dams may provide some benefits. However, the argument that they are a clean source of electricity is becoming ever more scrutinised by research and science.

Dams are often built to control floods, but on certain kinds of rivers they may make big deluges worse, a new study finds. The finding suggests river managers might need to rethink their flood control strategies on silty and sandy lowland rivers ( Dams trap sediment and release water at an unnatural speed, which cuts deeper into the river’s bed. This incision creates a roomier channel that can carry more water and prevent floodwaters from spilling over riverbanks (

In the local context, the Evenlode river catchment has dozens of weirs and dams, most of which are no longer in use and centuries old, except for Cassington which is a flow gauging weir for the Environment Agency. The Dorn and Glyme tributary system of the Evenlode has up to 60 old water mills and weirs (Environment Agency). For this reason, this sub catchment has been declared hydromorphically poor due to these obstructions which cause fish passage problems.

Weir removal can bring multiple benefits, compare to fish passes, ladders and rock ramps but as usual on the Evenlode there is a need to consider the reduced connectivity with the flood plain (due to historic and extensive drainage and over deepening) and whether that could be mitigated. Spending a lot of money on a fish pass makes it look like we accept the status quo of a heavily impounded channel and large barriers. That is a reasonable attitude on the Thames navigation, but it’s not believed to be the right approach on the tributaries, where the arguments for keeping the impoundments are much weaker.

To help provide solutions to this issue of fish passage, the ECP is currently planning works at two weirs on the Evenlode on its mid stretches. The ECP needs the help of local citizen scientists to monitor these projects to see how effective they are at fixing the river and its morphology. If you would like to get involved in this, please email

Charlbury Weir


International Rivers - Questions and Answers About Large Dams

Science Journal - Dams are supposed to prevent floods

National Geographic -Is building dams the way to save rivers?

Water News Europe - Sélune river revival after dam removal

Dam Removal Europe - Hydromorphology: What is it? 

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