The conservation group Columbia Riverkeeper demanded the documents in 2012 and the corps refused.
The corps had to decide in 2012 whether to undertake an environmental impact statement, as it had with two proposed coal terminals in Washington state, or conduct a less-intensive environmental assessment that would not include public input.
The corps opted for the assessment, and Columbia Riverkeeper sued to review the internal documents that led to the decision.
The proposed terminal would bring coal in open train cars from Powder River Basin in the Northern Rockies through Idaho and the Columbia Gorge. From there, the coal would be taken in barges down the Columbia River to a Pacific Ocean port for export to Asian markets.
The project has drawn criticism from environmental advocates, but proponents in Eastern Oregon argue the terminal would add jobs to an area that needs them.
Oregon Gov. John Kitzhaber and U.S. Sen. Ron Wyden have said the corps should opt for a more thorough environmental impact statement.
The judge said Tuesday that the corps was faced with public pushback when it opted for the environmental assessment. The documents are not exempt from disclosure if they simply reflect the agency's deliberations over how to present the material to the public, he said.
"I can see them saying, we're taking some heat," Papak said, "(and) how do we get it to the media in a way that is favorable to the corps' review."
Papak said it was clear that the environmentalists' attempt to get the documents was part of a larger campaign to direct public pressure at the proposed coal terminal.
Columbia Riverkeeper attorney Christopher G. Winter replied that the group is simply trying to inform the public of the decision-making process.