The Avon Weirs
Remnants of the river's industrial heritage
A major project for the Clyde and Avon Valley Landscape Partnership was the construction of “fish ladders” or passes which would allow salmon to negotiate the two large weirs on the Avon at Millheugh and Ferniegair, and travel upstream once again.
But what exactly is a weir, and why do we need these elaborate constructions for salmon to get past them?
A weir is a structure usually made of concrete, stone or metal which is built in a river to back the water up, creating an impoundment (similar to a dam, but with the difference that some water is still allowed to flow over them). The water in the impoundment can then be directed through a channel, called a lade, along to a mill which would use the water for power.
Weirs were built on the Avon as early as the 1700s, and around 20 mills were known to have used them for water. Gradually, as industries which relied on mills declined, weirs were removed or fell out of use, and now there only a few left.
While weirs still allow some water to flow downstream, they are too big for salmon to leap, and so completely block their ability to travel upstream to spawn. Salmon begin their life in a river, migrating out to sea to mature, and once big enough, they return upriver to spawn (lay their eggs) and the cycle begins again. This has not happened on much of the Avon for several centuries due to the weirs. Salmon also spawn in the gravel deposits left on the bottom of rivers, and these deposits are prevented from moving downstream by the weir, and this reduces spawning habitat below the weirs too.
The weirs at Millheugh and Ferniegair still remain, and are a part of the industrial heritage of the river. The addition of the passes has now opened up many more kilometres of river for salmon, allowing them to travel upstream once again.
A heron flies in front of the new fish ladder at Millheugh (photo: Walter Smith).