Invasive Species

The River Chess now hosts a number of species that are not native to the river and are adversely affecting the river and its indigenous wildlife.

Japanese knotweed

Fallopia japonica was introduced to Britain by horticulturists during the Victorian era, but quickly escaped from gardens into the wild. Growing aggressively, knotweed can tolerate a wide range of soil conditions and is often found along river banks.

Knotweed forms large, tall stands, outcompeting the natural vegetation, causing bank erosion and increasing flood risks. It can also cause damage to property and adversely affect land values. Knotweed must be controlled carefully as tiny portions of stem, crown or rhizome can re-grow and cause the infestation to spread.

There are a number of Japanese Knotweed stands on the Chess, which have been mapped by the Chilterns Chalk Streams project. The largest of these, on the Moor in Waterside, Chesham, is now undergoing a three-year control programme by Chesham Town Council to eradicate it.

Control for knotweed is usually the responsibility of the landowner or tenant. For more information on identification and control, download an Impress the Chess advisory leaflet.


Himalayan balsam

Impatiens glandulifera is another ornamental introduced to Britain by the Victorians that has escaped from gardens, becoming established in the wild. Himalayan balsam prefers damp soil and will rapidly dominate riverbanks forming large stands which outcompete native plants, lowering biodiversity. Being an annual (the UK's tallest), it dies back in autumn leaving bare banks that are vulnerable to erosion. Himalayan Balsam has spread at the fearsome rate of 645 km² per year in the UK.

Mapping by the Chilterns Chalk Streams Project has identified a number of stands on the Chess, particularly in Chesham. This is of great concern as Himalayan Balsam is already a big problem on other UK chalk streams, such as the Frome in Dorset. However, the stands on the Chess are currently small enough to be easily and cheaply controlled.

Control is best achieved by pulling up the plants before they flower (pulling during and after flowering risks spreading the plants further due to their explosive seed heads). Chesham Environmental Group are carrying out control annually at the Chesham sites with a view to wiping out the problem in two to three years and the Association is working to control it further down the river.

For more information on identification and control, download an Impress the Chess advisory leaflet. If you'd be interested in helping us with Himalayan balsam control, please contact our secretary on


Orange BalsamOrange balsam

The invasive potential of orange balsam, Impatiens capensis, is not clear, but it is commonly found along river banks and in marshy areas and has successfully colonised the entire Chess between Chesham and Rickmansworth. Currently orange balsam is not considered to be a threat to native species.  This is because orange balsam is a smaller plant that does not form large stands that outcompete other plant species in the way that Himalayan balsam does. However, substantial stands of Orange Balsam have been developing on sections of the Chess, making this species one to watch.




American mink

Mink (Mustela vison) were introduced to Britain from North America in 1929 for fur farming and feral mink are now present on a number of Chilterns chalk streams, including the Chess. Mink have a broad diet, including small mammals, birds, fish, amphibians and crayfish. Their predation of water voles was largely responsible for the 97% crash in the water vole population on the Chess between 2001 and 2003. Water voles are one of the UK's most endangered mammals - to read about them see our Wildlife page.

Mink footprint

Left: A footprint of a mink found on a mink raft on the Chess.

A water vole recovery scheme, combining habitat enhancement with mink control, had been successfully operating on the Chess since 2003 to reverse the decline in water vole numbers. The scheme was developed by the Bucks, Berks & Oxon Wildlife Trust and the Chilterns Chalk Streams Project and has been supported by landowners and fishery managers throughout the catchment, who have monitored for mink and trapped a number of them between 2004 and the present. Since implementation of the recovery scheme water vole numbers have recovered significantly.  The survey in 2011 revealed that the population was in fact slightly larger than its 2001 size before mink became established on the river. However, occasional sightings of mink continue to be made along the river and the water vole surveys in 2013 and 2015 indicated a substantial decline in the population once again, so it is essential that landowners continue to monitor for mink to protect water voles from the threat of local extinction.

Mink can be confused with otters, although mink are smaller and have a round, fluffy tail compared to the broad-based, flat tail of an otter.


Signal Crayfish - copyright Allen BeecheySignal crayfish

Signal crayfish (Pacifastacus leniusculus) were introduced from America for the aquaculture industry, but successfully escaped from locations such as the gravel pits in Waterside, Chesham, and made their way into the Chess. Being aggressive and larger they can outcompete the native white-clawed crayfish, but most devastatingly the invaders carry a fungus known as crayfish plague, which is deadly to the white-clawed natives. The last white-clawed crayfish was observed in the Chess in 1996. Signal crayfish can be seen along the length of the Chess and can be trapped if you obtain a licence from the Environment Agency, use appropriate equipment and obtain permission from the landowner. See more information on the EA's web site. Using the right type of trap is vital as water voles have been found drowned on the Chess in illegal crayfish traps.

Adult crayfish are easily identifiable because the undersides of their claws are bright red.