A dispute continues as to who is responsible for removing the silt and an “impasse” has developed.
The town river had been diverted in the 1840s to allow the construction of the cathedral, designed by Augustus Welby Pugin, one of the most famous of the Victorian architects.
St Mary’s was his favourite project, he said.
And the river, which empties into the largest of the Killarney lakes, is “unnaturally straight” in the section nearest the cathedral, a council meeting has been told.
Independent Alliance councillor Michael Gleeson said he had watched, since the 1970s, how “a blob” had built up into what was now a large island near the eye of Kingsbridge, the main bridge over the river.
Significantly, the OPW had flagged flooding in the area, due to the river.
The bridge is threatened, as one of its two eyes is being blocked up by a growing ‘island’ of silt.
“This build-up is continuing, so that ragwort, dock, and thistle — which are all banned under the 1936 Noxious Weeds Act — can grow,” the councillor said.
“That is what the NPWS [National Parks and Wildlife Service] appear to be protecting.”
Mr Gleeson called on the NPWS to use common sense and, finally, do something.
He warned that if the river was completely blocked and had to divert itself, the foundations of St Mary’s Cathedral could be at risk.
The bridge and the Deenagh river, at that location, are under the charge of the NPWS, the town engineer, John Aherne, advised.
He stated that the NPWS was ultimately responsible for the river’s maintenance and upkeep.
The council had no direct responsibility, but would request a meeting with the NPWS on the issue, Mr Aherne said.
Inland Fisheries Ireland had a role, too, the meeting was told.
On a wider note, the town engineer and councillors said there was not enough liaison between the NPWS and the council, and that a number of issues, including parking at tourist spots owned by the NPWS, as well as tree-cutting, needed to be addressed urgently.
Regular meetings are now to be requested by the council.
However, the NPWS, which has a large staff presence, as well as offices nearby, due to its role as managers of the Killarney National Park, disputes the council’s claims about any river-damage risk.
In a statement from the Department of Culture, Heritage and the Gaeltacht, the agency said it is not responsible for the Deenagh at that site and, furthermore, there were no issues associated with the river.
“There is no issue with the flow of the stream,” a spokeswoman said. “Silt has accumulated at one side of the stream, upon which vegetation has grown.
“This silt does not interfere with the flow of the stream, at present, as water levels are low at this time of year.”
The department insisted the land was owned by “another body”, but did not elaborate.
Meanwhile, there is also concern over build-ups of silt on the Flesk River, which flows into the Lower Lake. Locals say there is a danger of erosion of nearby river banks, as well as of flooding.