More than 140 fish were discovered dead in a Denbighshire river thought to have been polluted by sediment from a reservoir upstream.
Natural Resources Wales officers investigated after members of the public and the local angling group reported that the Afon Trystion looked discoloured in April last year.
Around 143 brown trout are thought to have been killed in the reservoir, which is upstream of Cynwyd, and then entered the river along with the sediment.
Natural Resources Wales say that the reservoir’s low water levels may have caused the movement of sediment from its banks and the reservoir bed, causing a "terrible incident" of pollution.
DoPower, which owns and operates the reservoir, was notified of the incident and worked alongside Natural Resources Wales to try and quickly improve the water quality.
Natural Resources Wales agreed an enforcement undertaking with DoPower to contribute towards the investigation costs and to carry out "vital improvement works."
DoPower is now set to pay £9,000 to the environmental charity Welsh Dee Trust to carry out fish habitat improvement work on the reservoir.
Rhys Ellis, Environmental Team Leader for Natural Resources Wales, said: “Our role is to ensure businesses can operate without harming people and the environment, this may involve prosecution, but in certain cases it can be in the public interest to look at options other than a court case.
"This approach is an example of NRW fulfilling its key role for the greater good of the wider community. The money is also spent in the local economy instead of going to the Treasury, which is the case with court fines.”
Peter Powell for the Welsh Dee Trust said: "This was a terrible incident that has caused significant impact to the wildlife of the Trystion. We are pleased NRW were able to identify the problem and a solution to prevent further escalation of the problem was identified.
“The donation will allow Welsh Dee Trust to make habitat improvements within the river, we hope this will speed up the wildlife’s recovery from this tragic event.“
Philip Barrett, Environmental Officer for Natural Resources Wales, added: “Incidents like this are damaging to the environment and can be costly to the individuals or companies involved. We will investigate reports of pollution and, where possible, take action against those responsible.
“Working together, we can protect and improve Welsh rivers."
You can report any pollution incidents to Natural Resources Wales' Incident Communication Centre by calling 0300 065 3000, emailing ICC@cyfoethnaturiolcymru.gov.uk or by clicking ‘Report an incident’ on the home page of their website.