Caustic soda from the Dairy Crest plant in north Cumbria polluted half the length of the river Ellen, from Aspatria down to its mouth at Maryport. The waterway was left almost lifeless near where the spillage happened.
Dairy Crest may face prosecution for the pollution. The powerful alkali is used to clean vats and neutralise acids formed in cheese- making. It is understood that the accident was due to a a storage tank leaking and a sewer collapsing on Monday night.
The National Rivers Authority said that the dead fish included young and adult salmon and trout, minnows, lampreys and eels. Its spokeswoman, Jeanette Unsworth, said other life in the river had also been damaged, which could have long-term effects on the food chain.
Last night most of the pollution had reached the sea and been diluted to harmless levels. There are no drinking water intakes downstream from the creamery, but the rivers authority warned people not to touch the water because the pollutant was so corrosive.
The river, which flows 16 miles from the Lake District to Morecombe Bay, offers fly-fishing at a fraction of the price of other stretches of bank. A spokesman for Angling Times said: 'It is very important for anglers in that part of the world. It will be a tragedy for them if fishing has been destroyed for the rest of the season.'
The incident is one of three spillages into Cumbrian rivers in 14 months, and each has killed thousands of fish. Yet the national trend is positive.
The National Rivers Authority is about to report that rivers in England and Wales have improved markedly in the past three years after deteriorating through the 1980s.
Yesterday, another national rivers survey, this time carried out mostly by schoolchildren, recorded a small improvement in river quality since the last such exercise 20 years ago. More than 150,000 children checked 415 rivers and streams throughout Britain. They examined the wildlife they are able to support, the concentration of nitrate that is influenced by fertilisers, and the quantity of rubbish thrown in.
The Riverwatch survey, organised by county wildlife trusts, found that the number of rivers in the top categories for their water quality had risen by 5.7 per cent since 1971. Northern counties in England have fared best but some South-eastern counties have shown a decline.
When it publishes the findings of its own survey in two weeks, the National Rivers Authority will argue that the improvement shows that the public is now seeing distinct environmental improvements in return for rising water bills.
But it will also press the case for spending on further improvements to sewage works and sewage systems on hundreds of miles of severely polluted rivers in densely populated areas.Reuse content