It’s Salmon migration season!

It’s that time of year when Salmon makes their final migratory push-up river to their spawning grounds. A very important part of the Salmon lifecycle.  

But on the Dee humans have made this journey much harder. So, let’s take a journey upstream with a returning Salmon which is heading for the Afon Trywerwyn.  

The first challenge is the long straightened dredged section of the Dee between the estuary and Chester. Dredged to reclaim land and allow boat access upstream, this section of the river provides no cover from predators and Salmon must stay here for several days while their bodies adjust from salt to freshwater.  

Then in Chester they come across Chester Weir, built first by the romans, Salmon can pass over in high tides and the fish pass on the side mean Salmon are only slightly inconvenienced by this weir.  

After a 50km clear run, Salmon come across Erbistock weir. Built to provide water for a mill, this Weir is now defunct. The only passage over the Weir is via the old fish pass in the centre, but unfortunately most fish are attracted by higher flows away from the weir, where they can spend days trying and failing to jump the weir. For Sea lamprey this is the end of their migration upstream as they are unable to use the fish pass.  

Erbistock weir

Just 3km upstream Salmon come to Manley Hall weir. This weir measures flow on the Dee and is important for flood modelling. Unfortunately, Salmon cannot pass this weir in low flows and so have to wait in this section of river for heavy rain, which could take weeks to arrive.  

After a clear run of 22km including through Llangollen, the next barrier is the iconic Horseshoe falls. Engineered by Thomas Telford and built in 1808. Adult Salmon can pass relatively easily through a low point on river left. This does however create a pinch point for predators. 

It’s then a clear run for Salmon until they reach the outskirts of Bala. In a stretch of just over 1km, Salmon have to pass 4 weirs in quick succession. Bala gauging weir, Bala sluices and weirs X & Y. Each is passable but each requires energy and specific river levels to pass. These weirs help control the Dee regulation ensuring the Dee can provide drinking water to Liverpool except weir x which is redundant.  

Weir X

Weir Y

Further upstream there was a redundant man- made weir. @lifeafondyfrydwy removed this in 2020, so this is no longer a barrier.  And finally there is a very big barrier: Celyn dam and Reservoir. Completely impassable to Salmon and when constructed in 1965 flooded not only the village of Capel Celyn but kilometres of spawning gravels.  

In total a Salmon migrating up the River Dee to the Trywerwyn has to pass over 8 Weirs, some are bigger barriers than others but if each barrier results in just a 3% loss in returning Salmon due to predation or exhaustion then this would mean approximately a loss of a quarter of the total number of returning Salmon, which do not breed.   

Hen Salmon lose a lot of weight migrating upstream and these weirs increase that loss of weight. For each pound of weight lost, a Salmon will lay 450-750 fewer eggs, reducing the total number of offspring to start the next life cycle.  

Man-made barriers block the migration of all fish species. We estimate over the Dee catchment there are more than 200 barriers to migrating fish. Removing or easing these barriers is key to helping fish species recover within the river. At Welsh Dee Trust we are working to remove as many barriers from the Dee as possible.