Jane Dodds has called for the dumping of sewage into Welsh rivers to be made illegal.
The Mid and West Wales MS said at the Senedd that the Wye is the second most polluted river in Wales after the Usk.
She said many MSs get their postbags filled with people writing about the state of the country’s rivers.
Ms Dodds, who lives in Hay-on-Wye, said: “I realise it's not a political point, but I am really disappointed that, once again, Conservative MPs voted against taking a tougher stance against sewage dumping by our water companies - something that would have helped us across the UK.
“I also would take issue on Dŵr Cymru. Dŵr Cymru executives get paid quite a lot of money - I think £600,000 was the last payment to a chief executive of Dŵr Cymru.
“This is at a time when Wales's water bills are the second highest across the country.”
Ms Dodds asked First Minister Mark Drakeford, in terms of river sewage, if he would support measures to make sewage dumping illegal.
But the First Minister said that if sewage dumping were to be made illegal, a question would arise as to what happens when storm overflows inevitably cause pressure on the system.
Mr Drakeford told Ms Dodds: “While I'm completely with the Member in wanting to see all of that addressed seriously and quickly, it is simply the fact that combined sewage overflows operate in a way that means that sewage doesn't come through the floors of private businesses and private homes.
“So if you banned it tomorrow, you would simply displace the problem and make the problem, I think, even less acceptable to Welsh citizens.
“The truth of the matter is that this is a complex issue where you can only make a difference by gathering together all the different contributions from those who have a part to play.
“That's why our action plan has buy-in from the Welsh Government, of course, but also the regulators, the developers, the farming unions and the water companies, and we will need to see it from communities themselves.”
Mr Drakeford said that in the case of the River Wye, 72 per cent of its pollution comes from agricultural sources.
He said: “While I agree that Dŵr Cymru must play its part, and do it effectively, unless you're prepared to tackle the other sources of river pollution, then you won't make the difference that Jane Dodds and I would like to see.”