River pollution: Sewage levels in Dewsbury and Mirfield waterways revealed as River Calder ranked among the worst in England

A recent Guardian analysis found the River Calder – which runs through Mirfield and Dewsbury – to be the fourth worst in England for sewage spills in 2023, with 72 spills per mile of river, a total of 5,647.

Data released by the Environment Agency shows the frequency and duration of storm overflow spills in 2023, as recorded by event duration monitors (EDM).

Storm overflows are safety valves built into the combined sewer system. When the sewer system is under strain, excess sewage and rainwater can be discharged into rivers to prevent sewage from backing up into homes and streets.

The map reveals that in 2023, Mirfield’s Newgate combined sewer overflow (CSO) spilt once for 4.5 hours into the River Calder.

In Ravensthorpe, the Fir Cottage CSO spilt six times for 37.5 hours, also into the River Calder.

The Spenborough sewage pumping station spilt 109 times for 1,365.25 hours into the Calder and Spen rivers.

In Dewsbury, three sites spilt a combined 153 times into the River Calder – a total of 730 hours.

A Yorkshire Water spokesperson said: “We are committed to improving our region’s rivers and were disappointed about the number of discharges in 2023.

"This increase is due to the wet weather experienced in the 12-month period, which included 11 named storms.

"The weather experienced in the region in 2023 included a very wet summer and prolonged heavy rainfall towards the end of the year, resulting in groundwater infiltration into the sewer network.

“Met Office statistics show 1178mm of rainfall in our area in 2023, the highest level since 2013 and with the second half of the year one of the wettest on record.

"Overflows operate during prolonged or heavy rainfall and multiple storms in close succession can lead to increased discharges due to the storm capacity being used up.

“Our teams worked hard throughout 2023 to reduce discharges as part of our commitment to improve the operation of our network.

"As a result, our modelling indicates investment in our network and changes to our operations since 2021 equate to an improvement of 12,980 discharges when normalised against rainfall.

“Nevertheless, we know there is more to do, and we are making headway with a £180m programme to reduce discharges across the region by April 2025.

"Work is in progress on 62 projects, including some on the Aire and Calder, that will reduce discharges from some of the most frequently operating overflows, with more to follow later in the year.

"This is just the start of a long-term programme to reduce the impact of wastewater on the region’s watercourses and we have submitted plans to OFWAT that outline a further £1.19bn investment in overflow reduction between 2025 and 2030.”

The Environmental Agency data used is for storm overflows only and does not include emergency overflow data. The data map can be found here.