Ellie’s Weir…ed Blog

In this post our GIS whizz Ellie Spilsbury outlines some of the work we have been doing to identify ways to improve the sustainability of the fisheries in our rivers and hopefully aid the return of salmon for the first time since the Industrial Revolution.

*Look closely and you will see hundreds of Minnows collecting at the bottom of this weir, unable to ascend. See the area in the water that looks dark brown; they are Minnows.*

Visit each of the three sections for more detail:

A familiar Story

Data analysis with a Salmon Splash of professional opinion

(Tr)outcomes expected

A familar story

Once upon a time, our River Aire had the highest Salmon population of any Yorkshire river. Then came the Industrial Revolution, which saw the wool and fabric industry boom throughout Yorkshire. Mills were constructed accompanied by weirs to harness our river’s energy. Although the mills are now closed and are becoming swanky new flats, the weirs often remain, isolating ecosystems that lie between them. Weirs disrupt the natural transport of sediment downstream, causing a build-up of silt and gravel behind the weir, which is detrimental to the habitat of spawning fish. Since 2011, one of the Aire River Trust’s goals has been to increase the connectivity of our river and its tributaries by removing or re-configuring weirs to allow fish passage. Following earlier work to install fish passes through and downstream of Leeds, significant steps towards this goal were made in 2022 with the successful construction of four fish passes as part of the DNAire project.

When we see water flowing over weirs, creating the sounds of waterfalls and visually pleasing white waters, it is easy to forget their man-made heritage and artificiality. It is hard to imagine seeing through the eyes of a migrating trout or salmon; every cell in its body instinctively directing it upstream to spawn, using both the stars and the earth’s magnetic field for navigation and then facing an unpassable wall of Yorkshire-dressed stone. It is often not just the height of the weir that presents the issue but the combination of weir height and the shallow depth of the concrete sill below the weir. The height at which salmon and trout jump is directly affected by the relative depth of the water at the foot of the barrier and the “hydraulic jump,” which boosts their leap.

The Environment Agency (EA) has identified around four hundred river obstacles within the Aire Catchment. However, we believe there to be many more. For example, the EA recorded two barriers to fish passage on Pitty Beck, yet on our Bradford Becks Walkovers, we found 11. This pattern is most likely repeated on each beck. Currently, tackling the removal of every weir in the catchment is unattainable. So, how did we prioritise them into a workable top twenty?

Data analysis with a Salmon splash of professional opinion

With help from The Rivers Trust, we are the first regional rivers trust to code an ArcGIS tool to accurately calculate the length of a river (including tributaries and forks) that would be opened and re-connected by the removal of every mapped weir in the Aire Catchment. Alongside this, we analysed ecological assessment data, invertebrate biodiversity, local community data (including deprivation), and weir visibility to the public. We assigned a score to each outcome and designed a weighted decision-making matrix that identified the weirs that scored the most highly. The data only tells us half the story, so we took our results to our expert team and discussed those weirs for which a solution in the short(ish) term might be feasible.

Once we had twenty feasible weirs, it was time to ground truth our ideas. The purpose of site visits is to add or, more often, diminish our confidence in the feasibility of the weir so that we only carry the most achievable sites to the next stage. We evaluated the weirs’ condition, site access, utility services or abstraction points, and landowner engagement by photographing and recording the area, our thoughts, and encounters.

The most surprising discovery for me was the actual size of a weir. After months of viewing photographs without visual perspective, weirs can appear to be half the scale of the real-life structure. Take a moment to analyse this photo: how tall do you believe it to be? See the very bottom of the blog for the upside-down answer.

(Tr)outcomes

We are fast approaching the end of the site visits and write-up stage. It is time to narrow our shortlist of twenty weirs down to four. So, it will be back around the table for our professionals to decide on the four “leak” proof projects to invest in. These four weirs will be subject to a comprehensive feasibility study and design process. I hope my next blog post will include more designs, machinery, hard hats and re-naturalised rivers.

River Worth Improvement Plan

Weir today. Gone tomorrow?

A group of postgraduate students from the University of Leeds have been visiting the riverside in Keighley as part of their “Engaging the Modern City” module. They’ve been keen to find out more about the river and what residents want to know about it. In response, they’ve produced the leaflet below.

On the front we will use several modules to present the issues we have investigated and a module at the back to recommend areas of PR activity around Keighley and to summarise our fieldwork
Jingzhe Zeng

A central theme they have been particularly interested in is the old weirs. What was their purpose? What do people hope might happen to them? Should they remain?

There used to be dozens of mills that thrived on the River aire, but now these mills have mostly been transformed into heritage for other activities.

The concern, however, is that the weirs that provided power to these mills still remain as part of the river channel, and from an ecological standpoint, they block the passage of fish that want to swim upstream for food, breeding, or refuge.

The ideal solution would be to remove these structures to make the river more level, or to build fish passes. For more details, check out https://aireriverstrust.org.uk/fish-passes/.

Please feel free to share your thoughts in the comments. Do you support the removal of weirs? Why or why not?
Jingzhe Zeng

Two fish leap in a graphic that divides text

Reconnecting the river for wildlife

The Aire Rivers Trust has just completed it’s Developing the Natural Aire project. Together with the Environment Agency, we have built fish passes to link 40km of the river to encourage the return of Atlantic salmon.

“It is fantastic to know that these fish passes in the upper River Aire are working as designed, and important fish species are rediscovering their key habitat

“As well as reopening rivers to fish migration and protecting ecologically important and endangered species like salmon and eel, fish passes are an amazing opportunity to reconnect river-resident species and the local community.

“Over the coming years, we look forward to seeing a growing proportion of trout, chub, barbel and salmon run spawning journeys higher up the river and a recovery in their populations.”
Thomas Somerville, Environment Agency’s Developing the Natural Aire Project Manager

How Fish Passes Work

As we come to the end of Developing the Natural Aire, SImon Watts explains why building fish passes matters.

All fish need to move to feed, breed and shelter. Even fish who spend their whole lives in the river can travel considerable distances looking for the ideal spot. Barbel have home ranges up to fifteen miles. Many freshwater fish migrate much, much further. For migratory species, like Atlantic salmon and the European eel, access to the sea is essential. The Industrial Revolution has left us with a fragmented river. Weirs built to harness the power of water for mills, factories and for navigation, block the way. Fish trapped in their sections are vulnerable to pollution or predation. And species like Atlantic salmon trying to return to our cleaner rivers cannot reach spawning ground in the shallow headwaters.

Why Do We Need Them

Whilst trout and Atlantic salmon can sometimes be seen leaping up waterfalls, weirs present a unique challenge. They are often long and steep. Their surface is smooth with little variation in the flow of water. The deep pools found at the base of waterfalls which fish can leap from are replaced by shallow aprons of concrete and stone. Many coarse fish species cannot jump obstacles.
The Aire Rivers Trust works with partners such as the Environment Agency and landowners to overcome these obstacles. The ideal solution is weir removal which restores natural river habitat (returning the artificial ponded section upstream back to a more natural system). But in some locations this proves impossible. Weirs form part of the urban fabric of our landscape. Some are valued for their heritage or for their amenity value. Buildings are built close to them and complicate their removal.

How Do They Work

Yorkshire Water explains why we need them and how fish passes work

Mid construction footage of the new fish pass at Kirkstall Abbey Weir as part of our Developing the Natural Aire project

An explanation of how an eel pass works from a Canal and Rivers Trust project similar to our Developing the Natural Aire

Where weirs cannot be removed, the solution is engineering. Records show rough fishways built in France during the 1600s. Here bundles of branches were used to create steps in steep channels to bypass obstructions. Engineered fish pass design has come a long way since the first one was patented in 1837 in New Brunswick, Canada. All designs seek to make weirs passable to fish. The majority of modern fish passes on the River Aire are Larinier Passes. Metal baffles in the base of them slow the flow over the weir that fish can swim or leap through.

The reconnection of 60km of habitat in the River Aire by the Developing the Natural Aire project will allow the return of Atlantic salmon, sea trout and other migratory fish. Monitoring conducted alongside the project has recorded a brown trout travelling from Armley Mills to Cross Flatts in Bingley (and most likely beyond) in a huge 80-kilometre journey to search for new areas to feed, breed and shelter in. This journey would have been impossible before Developing the Natural Aire as weirs like Kirkstall Abbey were completely impassible to fish.

River Worth Weir Walkers

Salmon Safari

Open Aireways