Serious pollution incidents can occur in our watercourses, spelling disaster for river ecology.  Regular monitoring is the best way of identifying such incidents early and enabling a rapid response.

The Angler’s Riverfly Monitoring Initiative (ARMI) was launched by the Riverfly Partnership in 2007 to enable anglers and other interested groups to actively monitor and protect their local rivers. It helps rivers to be monitored more widely and at greater frequency than is possible by the Environment Agency alone.

The ARMI monitoring technique involves pairs of volunteers taking 3-minute kick samples from the river bed each month, and recording the presence and abundance of eight pollution-sensitive invertebrate groups. The focus of the sampling is on ‘riverflies’ - mayflies, stoneflies and caddisflies. If invertebrate numbers drop below expected levels the Environment Agency are informed and will take action to investigate the suspected pollution incident. The Riverfly data held by each river group will also allow for long-term changes to be identified, and the regular presence of volunteers on the river will discourage potential polluters.

ARMI is already used widely in the UK as a 'neighbourhood watch' for rivers, practiced by anglers, environmentalists and community groups. It has proven effective in identifying pollution incidents and ensuring action is taken. In several cases this has led to prosecution of polluters by the statutory authorities and has prevented similar future incidents. ARMI will be recognised on all Water Framework Directive River Basin Management Plans in England and Wales.

In the Lea catchment, Riverfly monitoring groups are active on the Upper Lea, Mimram, Beane and Ash.


The Chilterns, Hertfordshire & Middlesex Riverfly Hub

The Chilterns, Hertfordshire & Middlesex Riverfly Hub is a partnership, set up in 2014 to provide ARMI training to anyone wishing to monitor the health of rivers in the Hertfordshire, Middlesex and Chilterns areas. Jointly led by the Chilterns Chalk Stream Project and Herts & Middlesex Wildlife Trust, the Hub coordinates Riverfly Monitoring across the area with the ultimate aim of setting up and supporting riverfly monitoring on every river in the Chilterns (including the Colne) and Hertfordshire.

The hub is able to provide training for volunteers through an annual programme of training workshops run by its five Riverfly Partnership accredited ARMI instructors. For more information, or to register your interest in becoming a Riverfly monitor, please contact the hub’s coordinators for This email address is being protected from spambots. You need JavaScript enabled to view it. or This email address is being protected from spambots. You need JavaScript enabled to view it..



The riverfly newletter is available at

There will be a Riverfly Open Day at Boxmoor Trust Centre on Saturday 5th October, 12-4pm. This will include a variety of speakers and a taster/refresher kick-sampling session in the Bulbourne. 

Please ensure to share this widely and register your attendance as venue capacity is limited:


The Herts & Chilterns Riverfly Hub is being supported by grants from Affinity Water, Natural England via the Lea Catchment Nature Improvement Area and Hertfordshire County Council.  Our partners are:


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