A County Tyrone river turned black by pollution from a peat bog landslide may struggle to recover fully, an angling group has warned.
It happened at Meenbog Wind Farm, near Ballybofey, County Donegal, in the Republic of Ireland on Friday.
The quantity of peat that entered the waterway that runs into the Derg river is not yet known.
However, NI Water has suspended drinking water supplies connected to the river as a precaution.
The Ulster Angling Federation (UAF) said the extent of damage to river life will become clearer in the coming days.
"The pollution has made its way to the Derg which is black in colour with suspended peat pollution so no dead fish can be counted until levels drop and water clears," a UAF spokesman said.
"Needless to say this is a very significant pollution event, one of the largest in the history of Northern Ireland and Ireland and involving large acreages of bogland. It is one that will be difficult to reinstate."
The disturbed peat, and a large quantity of debris dislodged by the landslide, initially poured into the Mourne Beg river in Donegal.
The Mourne Beg is an important spawning river for salmon and part of the Foyle catchment, which is an EU protected site.
"It is likely that the Mourne Beg will have a complete fish kill. As regards the River Derg, the coming days will see what the extent of the fish kill will be as club members monitor the situation," the UAF spokesman said.
Local anglers said they are devastated by the incident.
Gary Irvine, of Derg Anglers, said with the high-water levels and discolouration from the peat, it could be several days before the full impact of any fish kill is known.
Club members are now actively monitoring the affect on the river.
The company constructing the wind farm in Donegal confirmed the slippage had happened on its site.
Invis Energy said it was "working with the relevant authorities to fully address the matter".
"There is no risk to public health," a spokesman said.
The UAF has called for an immediate halt to the wind farm's construction until an investigation is carried out.
Environment Minister Edwin Poots told the Stormont assembly on Monday that the peat slippage was being investigated by the cross-border Loughs Agency and NI Environment Agency.
Donegal County Council said its "full investigation" had a "focus on mitigating the impact of the event, preventing further pollution and determining the cause of the event".
The Loughs Agency said it was evaluating the environmental effect of the "significant" peat slide.
"As the extent of the incident continues to be assessed, it is essential that any fish fatalities identified in the Mourne Beg or River Derg are reported to Loughs Agency fishery officers," a spokeswoman said.
'Safe to drink'
Authorities on both sides of the Irish border have moved to assure the public there is no risk to drinking water quality.
NI Water said staff had "suspended the abstraction of drinking water from the Derg River as a precaution and had switched abstraction to the Strule until the situation had been resolved.
"We are closely monitoring the water quality of the River Derg," a spokeswoman said, adding they "can assure customers served by the Derg Water Treatment Works in County Tyrone that their water is safe to drink.
"Our staff have been working hard behind the scenes to ensure no customer was affected during this incident and we do not anticipate any further disruption to the plant.
Irish Water told BBC News NI there was no immediate concern for water services in County Donegal.