Our Fish Passes

Introduction and overview of our fish passes on the River Severn

The Unlocking the Severn project was created to restore connectivity for migratory fish on the UK’s longest river.

Our project was focused on providing the rare twaite shad with a way around the weirs that had for nearly 180 years been blocking the shad’s migration route to their historic spawning grounds between Worcester and the Welsh borders. This has restored connectivity on the river for the benefit of other endangered migratory species such as salmon, eel and lamprey, and in fact, all river fish.

Understanding Fish Passes

What is a fish pass and why are they needed?

The first in our ‘understanding fish passes’ blog series explains why barriers are a problem for wildlife and the role that fish passes can play.

Read Blog 1.

The scale of the problem

Find out how pervasive the problem of barriers are on our river systems and why we can’t simply expect fish to leap over them, or use the locks like boats do!

Read Blog 2.

How does a fish pass work?

Find out the technicalities of how a fish pass actually works – managing the flow of water to get fish up to a higher level upstream of a barrier.

Read Blog 3.

To provide fish passage upriver, we had to build 4 huge fish passes around navigation weirs on the River Severn between Worcester and Stourport.

The navigation weirs were built in the mid 1840’s to provide deeper, more reliable passage for large barges carrying industrial goods between the Black Country and the docks at Gloucester. But they had a catastrophic effect on migrating twaite shad populations and were bad news for all fish that need to move around on the river.

Weirs and fish passes in the middle River Severn

Diglis Weir & Fish Pass

The weir at Diglis was the first insurmountable barrier that shad encountered on their migration route.  Here we have built our largest deep-vertical-slot fish pass.  It’s 100 metres long, 8 metres wide and 5 metres deep.

At the time of construction, it was the largest fish pass of it’s kind in England and Wales.  Uniquely, it contains a underwater viewing window that allows us to film migrating fish using the pass and also give visitors their own chance to glimpse wild fish swimming by.

Find out more about how we built this massive structure on our dedicated Diglis Fish Pass page!

Bevere Weir & Fish Pass

Bevere weir is in a much more rural location and there was enough land alongside the weir to create a 100m bypass channel. This provides a gradually sloped ramp that fish can swim along to get past weir.

Blocks cast into the base of the channel slow the water and break up the flow.  This creates near-natural varied flow conditions, that fish, including the endangered twaite shad, can comfortably swim through.

Find out more about how we built this different kind of pass, and the challenges we had on site with flooding, on the dedicated Bevere Fish Pass page!

Holt Weir & Fish Pass

Holt weir is located next to a very steep-sided river cutting. So, building a fish pass here was no easy task.  There was no road access to the site and all the materials and plant equipment had to be delivered to site by boat.  This included all the diggers and a massive 100-tonne crane which was carried down the river on a massive floating pontoon.

This was certainly our most challenging construction site and as a result it was the last and final fish pass to be constructed.

Read more about the construction of this pass on the dedicated Holt Fish Pass page!

Image: Skynique

Lincomb Weir & Fish Pass

Lincomb weir was the northernmost complete barrier on the River Severn.  From the fish pass that we have installed, fish can now swim freely up river through Worcestershire and into Shropshire, finding gravel beds that are ideal for spawning.

This deep-vertical-slot fish pass is formed as a series of 11 ascending pools.  This kind of fish pass works well in the wide range of flow conditions experienced on the River Severn.  They are suitable for a broad range of fish species, all with slightly different needs and preferences for how they swim through a pass.

Find out more about the fourth and final fish pass and it’s construction on the dedicated Lincomb Fish Pass page!

Explore each fish pass in detail

Diglis Fish Pass


Bevere Fish Pass
Holt Fish Pass
Lincomb Fish Pass

Weirs in the Lower River Severn

Passable Weirs

The fish passes built by unlocking the Severn are all in Worcestershire.  Further down the river there are some other weirs.  Initial monitoring showed that sufficient numbers of twaite shad can pass these barriers, aided by the tides.

a group of school children stand in front of the notch at upper lode weir, watching for fish

Gloucester – Maisemore and Llanthony Weirs

At Gloucester the River Severn splits into 2 channels.  Both of these channels have weirs – Maisemore on the larger, western channel and Llanthony on the smaller eastern channel.  High tides cause the river to overtop these weirs and fish can swim straight over the top of these weirs at these times.

Find out more about the history Llanthony weir and lock here:

Gloucestershire Live Article on Llanthony Lock 

Herefordshire & Gloucestershire Canal Trust Restoration at LLanthony Lock

Upper Lode Weir, Tewkesbury

At Upper Lode weir on the Severn Ham in Tewkesbury there is another, smaller weir.  This weir has a notch cut into it as the riverbank, which the fish can use to swim over the weir.  The head drop (distance between the water level above and below the weir) is lowest at high tide, as high tides in the estuary cause freshwater to back-up within the river.

Video of the River Severn at Maisemore Weir

Video of the River Severn at Upper Lode Weir

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