Raw sewage repeatedly poured into the River Severn in Shrewsbury during heavy rain, a wildlife trust said.
Figures from the Rivers Trust showed sewage was released into the water more than 1,000 times in 2019.
Every river in the town had been affected and the wildlife trust has called for investment by Severn Trent Water to make changes.
Severn Trent said their system was designed to stop people's homes being flooded.
Pete Lambert, from Shropshire Wildlife Trust, told a Shrewsbury Town Council meeting old infrastructure was having a "significant negative impact on the river", according to the Local Democracy Reporting Service.
The trust wanted Severn Trent to move away from relying on combined sewer overflows (CSOs), when sewage and storm water is released into rivers in periods of heavy rain to prevent sewers backing up.
According to figures from the Rivers Trust, monitored CSOs in Shrewsbury released more than 1,000 times in 2019 for a total of more than 20,000 hours.
"The tributaries that run into the Severn in the vicinity of the town, including the Rea Brook, Bow Brook and Rad Brook, are all affected by discharges of this nature," he added.
Severn Trent said Shrewsbury's sewage system was designed to redirect waste water during heavy rain to a nearby water course to stop treatment works being overwhelmed.
"This is fully agreed with the Environment Agency, and by doing this we hope to make sure no-one has to suffer sewer flowing in their home, which is a terrible experience for anyone," it added.
The town council's finance and general purposes committee decided to invite Severn Trent to a public meeting to discuss the issue.