The estimate of the number of dead fish has risen by 4,000 and more than three miles of the river are affected.
The Environment Agency has said it now believes 10,000 fish have died after a pollution spill in Devon.
It increased its estimate from 6,000 after the incident, which was discovered last week at the River Mole near South Molton.
The agency tweeted to say the pollutant had been identified as “anaerobic digestate” – or “agricultural waste”.
It said over 5km (3.1 miles) of the river was affected and that its investigation is continuing.
In a separate incident, the Environment Agency told Sky News it is “urgently investigating” after pollution killed around 3,000 fish in the River Sheppey in Somerset.
Teams working between lower and upper Godney have been spraying hydrogen peroxide and aerating the water with “floballs”.
A spokesperson said: “Our teams have been out on site, urgently investigating the cause of this incident and have successfully identified the source.
“We are continuing our hard work to restore oxygen levels in order to protect wildlife.
“We have already seen an improvement in the area and are now surveying further stretches of the river and so far we have found no further pollution from the source.”
Source: Sky News 5th August 2019