Campaigners have said they will fight a proposal which could see a dam built in ancient woodland.
Natural Resources Wales (NRW) is considering a number of schemes to prevent flooding from Cadoxton Brook near Dinas Powys, Vale of Glamorgan.
One of those would see a 350 sq m (3,767 sq ft) dam built, but campaigners say it would destroy the nearby Cwm George and Casehill woods.
NRW said no decision has been made and it will consult with residents.
"There is no preferred solution," said Tim England, from Natural Resources Wales.
"We want to work with the community that's impacted by the schemes that we implement to make sure they are informing the decisions that we make.
"There's no easy decision. Every option we look at will have some local impact."
NRW said building it on the natural flood plain would be an effective solution for protecting homes downstream, some of which have a 3% chance of flooding each year.
But it added a number of options were being considered and any project was at least "a couple of years" away.
Hundreds of campaigners have already marched through the site in protest at the possible move.
Cathy Farr, from Save Dinas Woods, said: "This is a loved site. People come here, they bring their children, they bring their grandchildren.
"It's too precious to be destroyed when there are other options."
And Chris Matts, from Woodland Trust, said 50,000 people walk through the parks each year.
He added: "This woodland has been here in one way or another since the Ice Age - an area of ancient woodland.
"Across the UK there's only about 2% of that resource left so it's a really small amount and you can't replace it."
NRW said: "If we decide to move forward with an option, the next step would be to seek approval for funding from Welsh Government.
"If this is assigned we would then start work on the detailed design of that option, followed by submission of any required planning applications to the Vale of Glamorgan Council during the next couple of years."