Environment Agency scientists say an oxygen crash has caused a fish kill on the Norfolk/Cambridgeshire border.
Fishery officers found dead and dying fish in the Delph at Welch's Dam, near Mepal, on Wednesday.
In a statement yesterday, the EA said low oxygen levels were 'a real concern'.
Water in the affected stretch will now be dosed with hydrogen peroxide which releases oxygen rapidly into water, helping fish to breathe.
The problem has been caused by flood water draining off the Ouse Washes into the Delph, which flows through Welney into the Ouse, near Downham Market.
Flood water has been held on the Washes later than usual, due to high rainfall experienced up until July. Warm weather has encouraged algal growth in this standing water, affecting levels of oxygen.
The Washes naturally drain into the River Delph, before it is pumped into the Ouse via Welmore Sluice.
It could take up to three weeks to finish pumping July's rainfall off the Ouse Washes and out of the watercourse.
Anyone who sees fish in distress should call 0800 807060.
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