Culm Riverfly 2024: the best of rivers and the worst of rivers

Richard Horrocks, Riverfly Coordinator for the River Exe and Culm, sets out the results from the 2024 survey season.

The Riverfly Partnership assesses the health of the UK’s rivers by monitoring their invertebrates, which are at the heart of the freshwater ecosystem. Volunteers undertake regular surveys of eight invertebrate groups, to measure their abundance. The results tell us about the general water quality, spatial and temporal changes, and the effects of pollution. Whilst not as rigorous as official monitoring by the Environment Agency, the findings are more numerous and immediate (EA survey results can be up to five years old).

The national launch of Riverfly was in 2007 and the River Exe and Tributaries Association (RETA) undertook its first surveys in 2009. The network has since grown and by the end of 2024 had reached 64 sites which are each surveyed in the spring, summer and autumn each year.  So, we are potentially undertaking almost 200 surveys each year although, for practical reasons including the weather, we are unlikely to achieve all of these.

The first Riverfly surveys on the Culm were in 2011. There are now 14 sites including those on its tributaries: the Madford River, Sheldon Stream and Spratford Stream and 31 Riverfly surveys were undertaken in 2024. Each survey produces score that reflects the number of separate groups that are present and their abundance.  A score of 8 could mean that there a few specimens of each group or large numbers of a few groups. A survey can collect one specimen or thousands of a particular group such as shrimp (gammarus).  

Of the ten sites with a historic record, 19 surveys were below their average, two were average and only seven were above average.  Conclusions should not be drawn from a single year, as there is significant annual variation, but clearly any decline needs to be monitored.

Why does the water quality vary so significantly across the catchment?

The main reasons appear to be due to agriculture and sewage pollution which get progressively worse as the river progresses downstream and are particularly bad on some tributaries including the Spratford Stream that joins the Culm at Cullompton.

The principal agricultural pollutants appear to be related to livestock. The high numbers of gammarus suggests organic material and Citizen Science Investigations (CSI) show high phosphate levels. Water companies have been made to report on their sewage storm overflows since 2021 and the data shows that the Culm has been affected worse than other local rivers. Some overflows during extreme weather are permitted in law, but in 2023 (the latest available data) is recorded as having discharged for over 20,000 hours with five sites spilling for over 2,000 hours each; equivalent to over five hours every day. A third contributory factor could be road pollution. About 10km of the M5 drains directly into the river without any monitoring or treatment.  National studies suggest that the damaging impacts are only just emerging.

Identifying the causes of low water quality, and addressing them, requires specialist investigation and action only available to government-funded authorities such as the Environment Agency.  This, in turn, relies on a political will that has been lacking in recent years.  The value of people-led initiatives such as Riverfly has never been higher.

Thanks to all those Riverfly surveyors who go out to monitor fly life in the river. A new cohort of surveyors was trained by Richard on the Culm at Hemyock this summer; several of whom are now monitoring the neighbouring River Clyst. Pictured at the top of this article are Paula Fearnley and Ben Evans of the Broadclyst Environment Group.