Healthy, Safe Rivers for People and Nature

We hear from Dr Laura Bannatyne, Freshwater Research Manager at Earthwatch Europe about more than a decade of partnerships, a detailed picture of water quality, and a closer look at local concern, as well as hearing from Oxford City Council about safety around waterways.

Citizen Science Water Quality Monitoring in the Evenlode Catchment

The Evenlode Catchment

The River Evenlode is a 45-mile-long tributary of the River Thames that runs through the Cotswolds, rising out of the underlying limestone near Moreton-in-Marsh and flowing south-east towards the clay vales of the River Thames. The major tributaries are the Glyme and the Dorn, whilst smaller tributaries include the Sars, Littlestock, Chipping Norton, and Westcote Brooks. Additionally, there are lakes with high cultural value at Blenheim, Cornbury Park, Ditchley Park, and Glympton Park. 

A Beautiful Setting with a History of Declining Water Quality

The landscape in the Evenlode catchment has national and international significance as it includes the Cotswolds National Landscape (CNL), Sites of Special Scientific Interest, Conservation Target Areas, and the World Heritage Site of Blenheim Palace. The landscape is characterised by arable farming and livestock pastures, interspersed with a variety of important habitats including ancient oak-ash woodlands, limestone grasslands, lowland meadows, and fens. These habitats support a wide range of rare fauna and flora, including populations of nationally endangered curlews, water voles and tree sparrows. Bullheads, chub, roach, and trout are present in many channels, and brook lampreys have been spotted in the Glyme.

However, the river habitats in the Evenlode catchment are under threat from a combination of channel modification and pollution. Historically, the Evenlode’s watercourses have been widened, straightened, and over-deepened, separated from their floodplains and more vulnerable to flooding. The resulting loss of meanders and backwaters has reduced the variety of riverbed and bank habitats needed to encourage biodiversity, whilst the numerous weirs that have been constructed historically to power mills, now modify flows, and create barriers to fish movement. Recent droughts have placed further pressure on vulnerable aquatic ecosystems and the biodiversity of riparian zones (the transition space between the water and the land).

Nutrient pollution (nitrate and phosphate) poses the main threat to water quality in the Evenlode. Excess nitrate and particularly phosphate concentrations can produce algal blooms and plant overgrowth, which later rot and deplete oxygen concentrations. This impacts fish and other organisms, compromises the safety of water supplies, and limits the recreational use of water bodies. According to the Environment Agency (EA), none of the 18 waterbodies in the 430 km2 operational catchment has “Good Ecological Status” or “Good Chemical Status”.

Nitrate concentrations in the Evenlode catchment are typically high, the legacy of more than a hundred years of synthetic fertiliser use. However, changes in farming policies and the recent soaring costs of fertiliser mean that agriculture is a decreasingly significant source of nutrients in the Evenlode and its tributaries. By contrast, the burgeoning population has placed increasing pressure on sewage treatment works (STWs) in the catchment, which contribute 65% of the average phosphorus contribution across the Evenlode catchment. The Evenlode and tributaries including the Littlestock, Chipping Norton, and Hanborough Brooks are under intense and increasing pressure from treated and untreated sewage discharges, with the growing number of planned developments only expected to exacerbate the situation. Storm sewage overflows in 2024 increased from 2023, with untreated sewage released to water courses in the Evenlode catchment for ~17000 hours, including for more than 2000 hours (83 days) at five of the sixteen STWs.

Citizen Science: From Concern to Action

The observable decline in water quality prompted concerned organisations and community members in the Evenlode catchment to take action. The Evenlode Catchment Partnership was established in 2014 and comprises over 20 organisations and countless individuals, committed to improving the water environments within the Evenlode catchment through farming, flood management, and biodiversity conservation projects and interventions.

Earthwatch Europe leads the Water Quality Working Group, coordinating the efforts of around 60 citizen scientists who monitor the Evenlode and its tributaries on a monthly basis since 2014, using the FreshWater Watch method to take over 3600 nitrate, phosphate, pH (a measure of how acidic a liquid is), and turbidity (a measure of how cloudy a liquid is) measurements from the safety of the riverbank. These measurements, combined with observations of algae, give an indication of good, moderate or poor water quality.

Several citizen scientists have also been trained to undertake Riverfly surveys, entering the streams when water levels are safe in summer to monitor the number and variety of small invertebrates that live in riverbeds. These creatures are constantly affected by water quality, and so provide insight into long-term conditions.

In this way, volunteers have built up a picture of the overwhelmingly poor water quality in the Evenlode catchment, supplementing the more accurate, but less frequent and less detailed statutory monitoring carried out by the Environment Agency. The map below shows water quality recorded by Evenlode Catchment Partnership (ECP) volunteers over the past 12 months, with the Hanborough Brook highlighted.

Case Study: Hanborough Brook

The Hanborough Brook flows for about a mile from its source near Long Hanborough, through pastures and past a busy riding school and livery stable, before passing the Church Hanborough STW to join the waters that flow to the Wolvercote Mill Stream Bathing area. The Brook almost always has an unpleasant odour and at times during recent months had visible foam and scum on the surface, with deep deposits of silt material downstream.

The Church Hanborough STW serves Blenheim Palace and the 6000+ population of Freeland, Long, and Church Hanborough, and North Leigh. More housing is under construction, and further developments of several hundred units are planned. 

In 2023 and 2024, raw sewage was discharged into the Hanborough Brook (which is less than two metres at its widest) for ~1500 and ~2000 hours respectively. This typically takes place during the wetter winter months when the STW infrastructure cannot cope with the influx of rainwater and groundwater. On their website, Thames Water note that “An upgrade is planned for Church Hanborough STW. This will improve its ability to treat the volumes of incoming sewage, reducing the need for untreated discharges in wet weather, but we are not yet in a position to confirm the exact timing of this work. We expect Church Hanborough to meet all government targets for storm overflows by 2030 – 2035”.

In the summer months, particularly during the 2025 and 2026 drought, the brook is reduced to a trickle or stops flowing entirely: treated sewage makes up virtually all the flow in the Hanborough Brook downstream of the STW for several months of the year. These circumstances, together with previously recorded poor Fresh Water Watch water quality results, prompted local citizen scientists to ask: If more and more connections are added to church Hanborough STW, and if summers are becoming hotter and drier, due to climate change, what is the outlook for the environment and human health of continually discharging treated effluent into a channel with little to no summer flow?

Constituency data: Sewage discharges in England - House of Commons Library

Citizen scientists set up four monitoring sites in April 2026 to find out what the conditions in the Hanborough Brook are at present, ahead of the proposed developments that will connect hundreds more homes to Church Hanborough STW: One site was selected far upstream of the stables (Headwater/Upstream sonde), a second was positioned downstream of the stables but upstream of the STW, the third was about located 100 yards downstream of the STW and the last about 250 yards beyond that, near Whitehouse Farm. Measurements taken at least once a fortnight beginning in April.

As well as Freshwater Watch testing, volunteers collected E coli data using Roth RCards (a quick and simple to use test). Dissolved oxygen was measured by Earthwatch staff using a YSI handheld probe (an electronic, portable water quality meter) of the same type used by the EA.

The Power of Citizen Science

Although the study is not yet complete, the initial findings tell a powerful story, thanks to the efforts of local volunteers. Very little rain fell during the study period, so the graphs show the impact of treated, not raw sewage. The patterns may change again in winter.

The red line on the nitrate chart shows the acceptable nitrate concentration of 1.0 mg/litre. As mentioned, background nitrate is high in the Evenlode catchment due to historic fertiliser dosing of arable fields - it is already slightly above the acceptable level at the upstream sonde site - but downstream of the STW it jumps almost to the upper measurement limit of the Freshwater Watch method (10 mg/litre). It is even higher 250 yards further downstream (at White house Farm) due to the deposited organic material from the STW.

Phosphate tells a similar story, except that it is acceptable at the upstream reference site. The impact of the treated sewage is clear, and phosphate again continues to increase downstream towards the upper measurement limit of 1.0 mg/litre due to the deposited organic material.

E coli is measured in coliform units (CFU) per 100 ml. The acceptable limit is 900 CFU/100 ml, shown by the red line. Due to the low water levels and warmer temperatures experienced during the past three months, E coli was elevated at all three sites sampled. Nevertheless, the counts below the STW were so high that despite being part of the planned investigation, citizen scientists could not do any Riverfly surveys in that stretch of the Brook due to the risk of infections.

Dissolved oxygen should be in the 80% - 120% range in most healthy rivers. Again, low, slow-moving water and warm temperatures affected the Brook at all four sites, but the decline due to organic material is easy to see.

What Happens Next?

The citizen scientists will continue to collect data until the end of October when all the results will be reported, including the near-continuous data from two sondes. The ECP and Earthwatch, in partnership with Meteor Communications and Southwest Sensors, have been measuring temperature, turbidity, pH, ammonium (a nutrient), conductivity (the water’s ability to conduct electricity – a reflection of dissolved ions), and dissolved oxygen around the clock, with one sonde monitor installed at the most upstream site, and the other measuring these determinands plus phosphate about 100 yards downstream of the STW.

Meanwhile, the organisations and individuals of the ECP continue to engage with the EA and with Thames Water on these and other water quality issues, towards the goal of improving water quality in the Evenlode catchment.

If you would like to know more about Freshwater Watch, click here: About us | FreshWater Watch. If you would like to find out more about the Evenlode Catchment Partnership and its work, click here: Home - Evenlode Catchment Partnership

Stay Cool this Summer and Enjoy our Waterways Safely

The work of citizen scientists across the Evenlode catchment highlights the importance of healthy rivers, not only for wildlife but for the communities that live alongside them. Rivers and waterways provide places for recreation, support biodiversity, help manage flooding, and play an important role in helping us adapt to a changing climate. As people spend more time outdoors during warmer weather, they also offer valuable spaces to walk, cycle and connect with nature. However, the findings above are a reminder that water safety remains an important consideration when enjoying these environments.

On a hot summer’s day, being near trees, water and green space can make a real difference. A shady park, riverside path or canal towpath can offer some welcome relief from the heat, as well as somewhere to walk, cycle, relax or simply spend a bit of time outdoors. And as our summers become hotter, these spaces are becoming even more important.

Trees provide shade and help cool our streets and neighbourhoods, while rivers, wetlands and other green spaces can help manage heavy rainfall and reduce flood risk. Looking after and expanding nature is one of the ways we can help our communities adapt to a changing climate.

Enjoy the Water - But Stay Safe

Oxford and Oxfordshire have some beautiful rivers, canals and waterways to explore. But, however inviting the water might look during hot weather, open-water swimming comes with unavoidable risks. Oxford City Council does not advise swimming in Oxford’s rivers. Cold water shock, currents, hidden obstacles and poor water quality can all put swimmers at risk, including people who are confident and experienced in the water.

If you’re heading out near the water, take a look at the GO Active Outdoors – Waterways guide for ideas for walks and activities. You can also read Oxford City Council’s wild swimming guidance and Oxfordshire Fire and Rescue Service’s water safety advice before you go.

Help Look After Oxfordshire's Waterways

Healthy rivers are good for people, wildlife and our climate. There are plenty of ways to help care for them too. You can find out more about Friends of the Thames’ Clean Up Our River campaign, or get involved with the Canal & River Trust through volunteering and other ways to support its work.

So, while the warm weather lasts, make the most of Oxfordshire’s green and blue spaces: find some shade, enjoy being by the water, stay safe. And while you're there, take a moment to appreciate the role nature plays in helping us cope with a changing climate.