It happened on the River Mole near South Molton
A formal investigation is underway after the pollution incident.
The Environment Agency says the pollutant has been identified as anaerobic digestate.
A 5 kilometer stretch of the the river was impacted and they've calculated that 10,000 fish died.
They say they can't share any further details at this time.
Alison Hernandez secures third term as police and crime commissioner
She'll continue tackling major issues on her to-do list
How Exeter's traffic troubles influenced council vote
LTN campaigner ousts Labour candidate
Labour tightens grip on Plymouth City Council power
They've gained nine extra seats
Radio Exe newsreader to abseil down Exeter Cathedral
Josh Tate will descend over 100 feet on Saturday
Labour retains control of Exeter City Council
Despite losing a seat to the Greens and an independent
