The Industrial Revolution brought many changes to Yorkshire. The wealth created by the woollen industry allowed cities like Leeds and Bradford to grow and flourish. However, many of the “dark satanic mills” in William Blake’s famous poem that dot our Northern landscape where initially water powered rather than coal. There have been corn milling weirs have been on the River Aire since Norman times but it was the explosion in the number of woollen mills that led to larger, better maintained weirs being built.
For migratory fish these weirs pose a problem. Salmon may be capable of leaping waterfalls but the number of weirs on the River Aire and both their length and profile prevent the crossing them as they move up the river system except at high flow levels. These iconic fish do not feed during their migration to their spawning grounds in the gravel beds at the top of the river. This adaptation is probably to prevent such a veracious predator eating the previous year’s young (parr) who are still living in the river but it means that an adult making multiple attempts to cross a weir will eventually start to burn muscle mass and fertile eggs.
The fragmented habitats created by weirs also affect fish who migrate within the river system, such as trout. The fragmented and degraded river habitat leads to isolated populations of fish which are much more vulnerable to problems such as pollution or climate change. A fully connected river habitat is much more resilient due to the potential for natural recovery by migration from trout populations upstream, from side streams or from the sea.
Weirs stop the natural movement of sediment through a river system and upstream can often resemble the slow-moving waters of a lowland river. The temperature profile and oxygen content may not suit the coarse fish species naturally found in upland rivers and they can form fertile hunting grounds for predatory fish feeding on fish who find their movement through the river system blocked.
The Environment Agency and The Aire Rivers Trust Heritage Lottery Funded project DNAire – Developing the Natural Aire – feels very timely for the River Aire. The past two decades have seen considerable improvements in water quality as the result of the efforts of regulatory bodies like the Environment Agency and industry including Yorkshire Water. The historic damage done by industry and dome
stic sewage is largely gone and even fish species that are extremely sensitive to pollution such as Grayling have returned to our river. Our historic weirs may remain but both the Leeds Flood Alleviation Scheme and a hydroelectric turbine installed at Knottingley have seen fish passes installed on large weirs on the river. Whilst its believed that salmon return to their river they born in about 10% end up in neighbouring rivers and salmon have been recorded as far upstream as the weir at Newlay Bridge in Horsforth. By installing fish passes on the last four major weirs, using funding from the Heritage Lottery Fund supported by the National Lottery, the Environment Agency and The Aire Rivers Trust believe we will once again see salmon in our river again. It I hope you will join us throughout the DNAire project in celebrating their return and the natural heritage of the River Aire.
DNAire will see the creation of a virtual (and where possible a physical) trail along the River Aire to enable people to share in the return of the iconic salmon to the River Aire together with volunteering opportunities in river stewardship and citizen science. Activities for the public as part of DNAire will run until 2022 with the fish passes expected to be constructed in summer 2020.