The electrical generating company will be cleaning the dam at the Peñas Blancas hydroelectrical plant in San Ramón and has issued a warning that the flow of the river will be affected.
The work is scheduled to start today and continue through Sunday.
The river level is expected to rise at times to the same level that a strong rain might produce, said the company, the Instituto Costarricense de Electricidad. It urged residents not to take chances with the river.
The plan is to eliminate sediment due to human activity, landslides and rains.
The company has been making contact with residents in the nearby communities of Boca Arenal de Cutris and San Isidro de Peñas Blancas, it said. It also said that it would be making an effort not to damage the fish downstream.
The company maintains a continual biological program in the river as well as at other hydro sites.