Images posted to social media show the water running a thick brown colour downstream of Gouthwaite reservoir.
In a statement today the EA said: “We continue to investigate Yorkshire Water maintenance works near Pateley Bridge after members of the public reported what appeared to be a large quantity of silt affecting the majority of the River Nidd, downstream of Gouthwaite Reservoir on Saturday.
“Large quantities of silt in a river can harm fish and smother aquatic plants and invertebrates, starving them of light and oxygen.
“Whilst there is no longer any silt entering the river system, it may still be visible as it moves downstream. The EA continues to monitor the situation and assess the impacts to determine what further action may be necessary.”
The EA added that the category 1 status had been given to the incident due to the length of watercourse affected, but that this may change once the impact is understood.
Works being carried out by Yorkshire Water are thought to have potentially stirred up the silt that flowed downstream at the weekend.
In a statement Yorkshire Water said: “We’re working with the EA to investigate what has led to this incident in the river Nidd and support any mitigation. We’ve temporarily suspended some reservoir safety work that our partners, Mott Macdonald Bentley, were doing upstream at Gouthwaite reservoir while we continue these investigations.”
The affected part of the river is known for grayling and trout, but discolouration of the water caused by the silt has made it difficult to assess the impact on river life.
Speaking to local Yorkshire outlet The Stray Ferret, Dr. John Shillcock, president of Nidderdale Angling Club and a former ecology officer in the area said: “In my past experiences, quantities of sediment of this type block out light and can reduce oxygen levels in the river, harming or even killing river life including invertebrates and other organisms on which fish and other river life depend.
“We are awaiting reliable data on the level of sedimentation before we can assess what damage has been done to an extremely precious river system.
“We would like Yorkshire Water to work closely with the EA to collect as much data and samples as to work out the best course of action to minimise the immediate danger to river life, and to build a deliverable long-term strategy to enable the river to recover from this avoidable incident as quickly as possible.”