A clean-up operation is under way after an estimated 100,000 gallons of slurry polluted the Wellow Brook.
The collapse of a cattle slurry lagoon at an unidentified farm saw the waste make its way into the river just downstream from Radstock, killing countless fish.
Environment Agency (EA) staff are now working with the farmer to help avoid further pollution by trying to contain the rest of the waste.
Saturday’s spill has already caused major damage to the river around Wellow and Shoscombe.
The president of a local fishing club has spoken of his heartache after the incident which he fears may have ruined the river and the local environment for years to come.
Rob Whish, from the Avon and Tributaries Angling Association, which has exclusive fishing rights to many rivers and brooks, said: “One resident described it as a tsunami of slurry. It looked disgusting and smelt horrible.”
Mr Whish said a “significant” number of trout had already died and said a team of club members pulled out more than 50 dead fish in only half an hour on Sunday.
He added: “It is very difficult to be precise about the number of fish that will be lost and until the EA tests are complete it is hard to predict the real damage.
“It is desperately sad. Over the past few years we have worked on the Wellow Brook to improve the river and encourage more fish and wildlife. It is tragic for club members who have put in so much hard work.
“It is a disaster not just for the fishing club but because of the serious environmental damage it has caused.
“The critical question is the effect the pollution has had on invertebrates in the river.
“If they have been killed the river has been left sterile as there will then be no food for the fish or birds and may take years to recover.”
Fred Scourse, secretary of the Five Valleys Trust and vice-president of the association, said members of the club had been at the river since the incident.
EA spokesman Paul Gainey said he was unable to name the farm while investigations were continuing.
Mr Gainey added: “The Environment Agency has welcomed the increased investment many farmers have made in slurry storage and infrastructure that separates rainfall from slurry.
“We published specific guidance last autumn and are here to help advise farmers on the best way to handle their slurry through the rest of the winter.”