Since Monday morning, about 130 farmers have been blocking access to the site, which been occupied for nine months by political activists wanting to bring it to a halt.
Farmers want the project to go ahead and say authorities are not doing enough to resolve the situation or to stop the activists from entering the site.
"They are not prisoners," said Philippe Jougla, president of the area's FDSEA farmers' union, in reference to the activists known to the public and among themselves as "zadistes", a complicated French play on words referring to their occupation of the site.
"If the zadistes want to leave, they can leave. But we are not allowing anyone to enter the site."
Opponents say they're determined to remain on the site, and police reinforcements arrived to prevent clashes between the two groups.
The police were determined "to avoid any incidents" between the two camps, said local prefet Thierry Gentilhomme.
"Pressure was high this morning, but we were able to create a 200-metre buffer zone between the zadistes and the farmers," a police officer at the scene reported.
The general council of the region is to make a decision on the government's proposals for the dam when it meets on Friday.
The dam made national headlines last October when 21-year-old protester Rémi Fraisse was accidentally killed by a police stun grenade.