Namdaecheon Stream in Gangneung, the actual location where the character Seong-jun warned Su-hyuk in the movie "A Man of Reason," is a river that flows through Gangneung and enters the East Sea. Near the downstream area, you can also enjoy the natural scenery and reed forests of Namdaecheon at the Ecological Observation Center.
Source: Korea Tourism Organization official English tourism database. Opening hours and details can change — check the official website before visiting.