The order by Mr. Trump had set off a debate in Montana — and other spots around the United States — over the fate of these national monuments, which once they are set up limit how the land can be used for energy development or other commercial purposes.

Matthew Knox, a Montana rancher who lives in the Upper Missouri Breaks area, supported an effort to shrink the size of the national monument so that it would exclude the area he uses, including some leased from the Interior Department, to grow hay. That way he would not need to worry about some future president or government official deciding to discontinue hay leases.

“Monuments are created by executive order,” said Mr. Knox, who was busy welding a beam on a tractor that had snapped while his wife was cutting hay. “Presidents down the line should be able to review those orders and reconsider them.”

But many others were vehemently opposed to any changes, wanting to protect an area that is now accessible mostly only by canoe.

“It is such a big powerful thing,” said Mary Frieze, 71, a retired teacher, who spent a recent afternoon hiking the Upper Missouri River Breaks area, first along a series of cliffs and then gradually down right to the edge of the muddy river’s waters. “It is a dense quiet. Almost a religious moment, it is so strong and smooth — one that enhances your soul.”

Mr. Zinke expects to complete his review of the national monuments by Aug. 24. Already, he has indicated he is likely to recommend changes at Bears Ears National Monument in Utah, suggesting that it needs to be “right-sized.”

“There is no doubt that it is drop-dead gorgeous country and that it merits some degree of protection,” Mr. Zinke said in June, before adding that the Bears Ears site seemed too large and that different kinds of uses of the land are too restricted, meaning it is ”not the best use of the land.”