“John Day is not open to the public, but visitors can view the dam from Giles French Park, located just downstream of the dam. Giles French is a great scenic viewpoint to get pictures and see the third largest hydropower dam in the nation. The Dalles Dam and Bonneville Dam, which are further west, both have visitors centers available to the public.”
Here are the details: http://www.nwp.usace.army.mil/Locations/Columbia-River/John-Day