Residents fear a project to redirect water from a reservoir could increase the risk of flooding along two rivers.
Natural Resources Wales wants a 25% rise in the flow of water into the Marchnant and Cownwy rivers from Lake Vyrnwy.
It said a higher and better flow would improve the ecology of the rivers.
However people living near Llanwddyn, Powys, are concerned the rivers - which often already break their banks - will cause even more flooding downstream.
Natural Resources Wales said the project involved reintroducing 25% of the "natural flow" to the Marchnant and Cownwy watercourses - dammed in the 19th Century - to help fill the reservoir, to improve the fauna and flora.
The project - which is being led by Hafren Dyfrdwy water company and United Utilities - will create new channels to split the flow of water between the lake and rivers.
But home owners said properties and land alongside the rivers already regularly experienced flooding.
"The Marchnant river already floods across the road and fills the gardens as it is," said Councillor Bryn Davies.
"The water that reaches the dams is about five times greater than what we see in the rivers now, so a 25% rise will double the flow of this river and guarantee that this flooding is more regular."
Lydia Johnston, who runs Cownwy Valley Produce from her home on the bank of the river near Abertridwr, urged United Utilities to halt the work, which is due to be completed before the end of March.
"The river already comes up onto the land here... that's without the extra water," said Ms Johnston.
"If we had extra water, I wouldn't want to think about the consequences. We'd lose a lot more land a lot more of the time."
The company said ecological and flooding assessments carried out show the work would have "very little" impact on water levels during times of high flow.
A spokesman for United Utilities said: "The planned works increase the percentage of the flows discharged to the river under normal conditions, but the work is not detrimental to the area."
There will be a public drop-in event on Monday 18 November between 16:00 and 19:00 GMT at Llanwddyn Community Centre.