Sewage was pumped into Norfolk's rivers for the equivalent of more than 1,000 days last year, new figures show.
Water companies can pump waste into rivers and the sea during heavy rainfall, to stop drains backing up and flooding homes with sewage.
Government figures said the sewage was discharged into the Gaywood River at Grimston, near King's Lynn, for the equivalent of 148 days in 2021, during which there were 159 spills.
Actor Stephen Fry is among more than 1,000 people who have signed a petition calling for the Environment Agency to investigate the cause of the pollution and restore the river, which local councillor Rob Colwell describes as "devoid of life" in some stretches. The EA said it was investigating.
The River Ant and Cut-Off Channel were also affected for more than 100 days each, according to government figures.
But Anglian Water said a recording of 195 days of discharge into the Yare in Norwich was caused by faulty equipment and no spills had occurred.
It comes as the Environment Agency pledges to crack down on water authorities. Its chief executive, Sir James Bevan ,said: "Sewage pollution can be devastating to human health, local biodiversity and our environment.
"Storm overflows are designed to release excess storm water from the sewerage system into rivers or the sea during prolonged, heavy rainfall, to ensure they are not overwhelmed. Water companies should only do this under strictly permitted conditions.
"The latest data shows that water companies have been discharging sewage into waterways far too often, and that there is no room for complacency."
A report to MPs in January warned: "Cleaning up our rivers is important for public health and vital to protect wildlife.
"The build-up of excess nutrients like phosphorus and nitrogen from animal waste and sewage is reducing oxygen levels in rivers and in severe cases can cause fish kills."
Helen Baczkowska, acting conservation manager for the Norfolk Wildlife Trust, said: “Norfolk’s rivers and streams support a rich abundance of wildlife. In a time of biodiversity crisis, it is unacceptable to compromise their health, and risk losing more species as a result of human pressures such as sewage spills.
“Some of the rivers affected in 2021 include rare chalk streams such as the Gaywood River. There are only 210 chalk streams in the world, and most of the lowland steams can be found in Norfolk. Chalk streams have huge ecological value, their clear shallow waters making them havens for wildlife.
“In addition to issues like sewage, our wildlife-rich water courses are being negatively impacted by pollution, chemicals and silt entering from a variety of sources.
“Our rivers and streams are a vital home for wildlife and crucial source of clean water for our communities - it is time that we put these key functions at the head of decision-making, rather than seeing water courses as drains to remove unwanted substances from view.”
Anglian Water said: “Data from our 2021 CSO [combined sewer overflow] monitoring programme tells us our performance continues to improve, and the increasing visibility we have of CSO activity gives us even more opportunities to act faster in the areas where we can have most environmental benefit. But we know we need to do more.
"As part of our recently launched Get River Positive commitment we’ve set out a clear plan and demonstrable action in response to calls for a revival of rivers in England. Central to the pledges is to ensure that storm overflows are not the reason for unhealthy rivers in our region by 2030.
“We have a proven track record of investing in environmental protection and improvements, and any profit should be considered alongside the scale of our investment programme, which between 2020-2025 alone was our biggest ever at almost £6bn, £800m of which is targeted specifically at environmental improvements.
"It is the largest environmental programme of any water company in the UK."
Norfolk's top 10 rivers for number of days sewage was discharged
Grimston Water Recycling Centre, Gaywood River - 148 days, 159 spills
Horning Water Recycling Centre, River Ant, Dilham to River Bure - 133 days, 166 spills
Feltwell Water Recycling Centre, Cut-Off Channel - 126 days, 131 spills
Belaugh Water Recycling Centre, River Bure - 73 days, 96 spills
Mundford Water Recycling Centre, River Wissey - 71 days, 94 spills
Norwich Riverside, River Wensum - 70 days, 96 spills
Fakenham, River Wensum - 68 days, 79 spills
Burnham Market Water Recycling Centre, River Burn - 65 days, 105 spills
Watton Water Recycling Centre, Watton Brook - 50 days, 82 spills
Erpingham, Scarrow Beck - 47 days, 69 spills
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