Sewage has been flowing into the Mill Stream in Stamford over the weekend.
The Environment Agency and water company Anglian Water were called out to deal with the dirty water flowing into the stream surrounding The Meadows in Stamford.
Anglian Water media manager John Clare said: “A sewer in the town centre had become blocked by a build up of fat, causing sewage to back up and spill into the river through an overflow designed to take storm water.”
The incident was reported to the Environment Agency on Monday afternoon and an investigator attended the scene at 4.30pm that day.
An Environment Agency spokesman said: “When we arrived, we found that there was a blocked sewer.
“There was not a lot of flow on the river and the blockage was causing some changes to the colour of the water, which could indicate danger to fish and life in the side stream.
“Thankfully, there was no sign of any dead or ill fish.
“As it was a blockage, we contacted Anglian Water, who have now cleared it.”
The Environment Agency will return to the site this afternoon (Tuesday) to monitor the situation.
Mr Clare added: “Work to clear the blockage using a jetter lorry continued throughout the night (Monday) and this morning (Tuesday) and our staff will remain on site to clear debris from the river.
“Anglian Water takes its responsibilities to the natural environment very seriously and we work hard to prevent this kind of spill from happening.
“We would urge people to think carefully about what they are putting down plugholes, toilets and drains as the consequences of blocked sewers can be very serious. This is especially true in areas like Stamford with older sewers, where there are often connections between the foul and surface water networks.
Worried angler Rick Mitchell noticed the problem on Saturday. He reported the incident on Monday after failing to reaching anyone over the weekend.
Rick Mitchell, 49, of North Street, Stamford said: “It is outrageous.
“The days of sewage draining into the seas and rivers have long since past. This shouldn’t be allowed to happened.
“It is disgusting.”
Mr Mitchell raised concerns that children were fishing in the dirty area of the river and advised them to pack up for their own safety.
The Environment Agency incident hotline can be contacted on 0800 807060.