BEARS EARS NATIONAL MONUMENT, Utah — Parts of this sprawling region of red-rock canyons and at least three other national monuments would lose their strict protection and could be reopened for new mining or drilling under proposals submitted to President Trump by Interior Secretary Ryan Zinke on Thursday, according to congressional aides and others who have been briefed on the report.

Environmentalists, ranchers, tribal governments and Western lawmakers had been watching closely to see if Mr. Zinke would propose changing the borders of the Bears Ears National Monument, which President Barack Obama established at the end of his term, and other scenic and historic areas under federal protection.

In recent days, Mr. Zinke had been considering a dramatic reduction to Bears Ears, to approximately 160,000 acres from 1.35 million, according to multiple people familiar with the process.

No president has ever reduced a monument by such a large amount.

Shrinking the monuments would be widely seen as a direct blow to Mr. Obama’s environmental legacy, and would probably prompt the first major legal test of a century-old conservation law.