Invasive species harming British waterways and how to report them

By Repairing eroded banks using natural materials
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1 Aug 2026

Invasive species harming British waterways and how to report them

Why invasive species matter in our rivers and brooks

Britain's brooks, streams and rivers are among our most loved natural features, and they shelter a surprising amount of life. Native white-clawed crayfish, water voles, mayflies, otters and salmon all depend on clean water and stable habitat. The trouble is that a handful of non-native species have arrived, often from garden ponds, aquariums or boat hulls, and they compete hard for food, shelter and space. Some carry diseases that native wildlife cannot resist. Left unchecked, these invaders can reduce a brook to a stretch of fast-flowing water with very little life in it.

The good news is that control work does happen, and it relies heavily on people like you. Volunteers, river trusts and local wildlife groups can only act where they know a problem exists. Learning to spot the main offenders, and reporting them through the right channels, is one of the most useful things any of us can do for our local waterways.

The main culprits: Himalayan balsam and Japanese knotweed

On the banks, invasive plants can be just as damaging as anything in the water. Himalayan balsam is probably the most recognisable. It grows tall, up to two or three metres, with hollow, reddish, sappy stems and distinctive pink helmet-shaped flowers in summer. It spreads explosively, since its seed pods burst and fling seeds several metres into the water, where they travel downstream. It dies back in winter, leaving bare banks that erode badly and smother native plants that would otherwise hold the soil together.

Japanese knotweed is the other big bank-side offender, easily identified by its bamboo-like stems with purple speckles and large, shield-shaped leaves. It is aggressive, hard to kill and can even damage buildings and hard bank defences. Giant hogweed also appears on riversides and is best reported rather than touched, since its sap causes painful burns in sunlight. If you find any of these, note the location carefully and avoid pulling them up yourself, as fragments and seeds can spread the problem further.

Signal crayfish and other water-borne invaders

In the water itself, the signal crayfish is a serious threat. It looks much like our native white-clawed crayfish but is generally larger, with a smooth, reddish-brown shell and a distinctive white or pale turquoise patch at the hinge of each claw. It burrows into banks, eats fish eggs and invertebrates, and carries crayfish plague, which is fatal to our native species. Many of our remaining white-clawed crayfish populations survive only in isolated headwaters, so seeing a signal crayfish in a clean, fast brook is worth reporting quickly.

Other invaders to know about include:

American mink — dark, slender and often seen swimming at dusk; they can devastate local water vole populations.
Zebra mussels — small, striped mussels that cling to hard surfaces and clog pipes and gravel beds.
Floating pennywort — a bright green aquatic plant forming dense floating mats that starve the water of light and oxygen.
Chinese mitten crab — a burrowing crab with distinctive furry claws, often seen in larger rivers and estuaries.
Topmouth gudgeon and other non-native fish — small fish that can outcompete native fry for food.

How to report what you find

When you spot a suspected invasive species, gather the basics before you report it: the date, an accurate location, ideally a grid reference or what3words, a clear photo and roughly how many you saw. Photographs are especially valuable for crayfish and plants, as experts can confirm the identification without needing to visit straight away.

In England, records can be submitted through the Environment Agency's incident hotline or the iRecord app and website, which routes sightings to the Local Environmental Records Centre. Natural Resources Wales and the Scottish Environment Protection Agency have their own reporting routes, and the GB Non-Native Species Secretariat provides identification sheets and reporting guidance for the whole of the UK. If you are a member of a local river group, tell them too — they often coordinate balsam pulling days and crayfish surveys.

Staying safe and stopping the spread

Good intentions can accidentally move invaders around. If you walk, fish, paddle or swim in more than one watercourse, follow the Check, Clean, Dry routine. Check all equipment, boots and clothing for mud, plant fragments and tiny animals. Clean everything thoroughly with hot water or a disinfectant suitable for the environment. Dry your kit completely, ideally for at least 48 hours, before visiting another waterbody. Crayfish plague and zebra mussel larvae can survive in damp crevices for days.

Never move crayfish, fish, plants or pond water between sites, and please do not release unwanted aquarium or pond species into the wild. If you keep non-native plants in a garden pond, bag and bin any cuttings rather than composting or dumping them near a stream.

Small actions, healthier brooks

Every accurate record helps. A single photo of signal crayfish in a Cumbrian beck, or a patch of Himalayan balsam on a Wiltshire stream, can trigger surveying, control work or a warning to neighbouring landowners. You do not need to be an expert ecologist; you simply need to look closely on your next walk, know the handful of species above, and pass on what you find. Our brooks have enough pressures on them already, and the people who can help tackle these invaders depend on the rest of us as their eyes and ears.

Conservation

A gentle walk along ancient mill streams

Breathable waders, grippy boots and a wading staff make stream walking safer, warmer and more enjoyable throughout the changing seasons.

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5 Comments

Robert Sonny says Jul 21, 2024 at 10:00am

I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
Reply
- John Abraham says Jul 21, 2024 at 10:00am
  
  I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
  Reply
  - Robert Sonnysays Jul 21, 2024 at 10:00am
    
    I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
    Reply
Robert Sonny says Jul 21, 2024 at 10:00am

I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
Reply

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