Severn Trent Water is to spend £250,000 in a bid to stop raw sewage running into a Birmingham stream.
The company, which made more than £200 million profits last year, claims contamination of Bourne Brook in Halesowen has increased in recent years because of ageing pipes and changing weather patterns.
The money will be spent improving a nearby underground storage point to prevent drains overflowing during heavy rainfall.
Project manager Scott Taylor said: "The discharge into Bourne Brook is unsatisfactory, so we are going to close the connection between the surface water sewer and the foul water systems.
"We are going to build concrete box culverts which will alleviate the problem and store more flood water."
Larry Barnes, operational manager at Severn Trent, said the main issue was the age of the drainage system.