Anglers in the area say the boggy landslide has polluted a key river for salmon fishing downstream.
Thousands of tonnes of peat and a stand of forestry were carried away when the earth on the Mourne Beg mountain moved on Thursday.
Members of the Derg Valley Community angling club say they've already reported one dead salmon downstream in "black peaty water" -- and believe more will turn up.
A spokesperson for the North's Department of Flooding say's it's aware of the incident and will take any necessary action.
The following was posted on Facebook by Derg Valley Community Angling Club.