Oil pollution in the Shannon south of Athlone, which has killed cormorants and fish, is being investigated by Westmeath County Council.
The incident happened at the weekend, initially affecting the Al River, a tributary of the Shannon in Athlone-Moate municipal district.
The local authority deployed a boom there, where the spill is believed to have occurred, in an effort to contain the oil.
A number of birds including other cormorants and cygnets were removed from the river by Athlone Society for the Protection of Cruelty to Animals, and washed in an effort to keep them alive.
A spokesman for Athlone SPCA said some of the birds had been taken for special cleaning to an animal sanctuary in Co Kildare.
Dogs who entered the river since Saturday also had to be cleaned.
Welfare officers told Midlands 103 radio that the oil spill in the river Shannon was very serious and it would be a difficult operation to remove it.
Their difficulties were added to by windy weather over the weekend which had broken up the oil slick.
As a consequence, oil had sunk to the bottom of the river where it caused more damage.