Interior Secretary Ryan Zinke left much to the public imagination when he submitted his review of the 27 national monuments designated since 1996. His eight-paragraph summary mentioned no monuments by name. Zinke did, however, seek to make one point clear: “...there is no doubt that President [Donald] Trump has the authority to review and consider recommendations to modify or add a monument.”

Consider modification, sure. But actually modify? That’s not as certain. The Antiquities Act explicitly mentions creation, not alteration. The president giveth, but can he taketh away? It’s happened before, most notably at Mount Olympus National Monument, which was created by President Theodore Roosevelt in 1909, and which was cut in half six years later. But there’s never been a legal challenge to monument chopping. That’s likely to change soon.

Zinke reportedly will recommend Trump downsize at least four monuments, and various tribal coalitions, environmental groups, and states have vowed to sue should that happen. It’s possible courts will decide that a president can’t alter monuments, thus preserving all Trump-targeted landscapes with a single decision. But if the monuments’ fates are decided on a case-by-case basis, here’s what Western law researchers say to watch for.

Bears Ears National Monument, Utah

The Antiquities Act grants the president authority to protect landscapes that hold “objects of historic or scientific interest.” According to the tribal group that proposed saving a swath of southeast Utah, Bears Ears National Monument holds more than 100,000 of them.

“There are thousands of sites that have been identified, and many more thousands that are thought to be there,” says Mark Squillace, a law professor at the University of Colorado Boulder. “If the goal of the monument was to protect all these sites, then without a very intense survey, you’d be hard-pressed to say that none of these sites are within the lands taken out of the monument.”

Zinke has already said Bears Ears is on the chopping block, and he will reportedly suggest downsizing the 1.35 million-acre monument to 160,000 acres. But given the volume of cultural sites and the previous administration’s diligence, Squillace says it would be hard for the Trump administration to prove that Bears Ears is larger than the Antiquities Act allows. A president must set aside the smallest amount of land possible to protect the designated sites, and the area of Bears Ears is 550,000 acres smaller than what the Bears Ears Intertribal Coalition had proposed.

Furthermore, if Trump shaves 90 percent off the monument he would signal that thousands of Native American cultural sites don’t deserve protection, even though a far more thorough Obama administration analysis determined that they did. If courts require either party in a lawsuit to justify which artifacts deserve protection, there’s a huge difference in the research record: a Navajo group began evaluating a potential monument in 2010, and the Obama administration spent another year reviewing the eventual proposal. Zinke, on the other hand, was ordered to complete his Bears Ears review in just 45 days.

Grand Staircase-Escalante National Monument, Utah

In 1998, two years after President Bill Clinton created Grand Staircase-Escalante National Monument, Congress passed the Utah Schools and Lands Exchange Act. The act authorized the federal government to trade the state of Utah $50 million and 145,000 acres of coal- and gas-laden federal land for 363,000 acres of state land inside the 1.7 million-acre monument. The land swap was seen as a pragmatic compromise at the time, one that fixed a patchwork of land ownership in the monument while providing Utah with new sources of revenue for schools. But that innocuous act might prevent Trump from shrinking the monument.

“When Congress passed legislation approving that land exchange, one could argue that Congress converted that presidential action into a Congressional action,” says John Ruple, who studies public-land law at the University of Utah. “It’s harder for the president to step in and say he can undo a Congressional act.”

There’s another issue facing a Grand Staircase reduction: the monument has already been litigated. An association of counties sued the federal government in 1997, arguing that Clinton overstepped the authority granted him by the Antiquities Act. U.S. District Judge Dee Benson issued a conclusive ruling in 2004: “The record is undisputed that the President of the United States used his authority under the Antiquities Act to designate the Grand Staircase Monument.”

“[The counties] challenged it as being too big. They lost,” Ruple says. “The court was very clear that landscape-scale monuments are appropriate.”

Cascade-Siskiyou National Monument, Oregon and California

Monuments can only be carved out of existing federal land, but what if that land is being held in trust for counties? Such a question arose when Obama added 48,000 acres to Cascade-Siskiyou National Monument. The parcels in question came under federal control through the Oregon and California Lands Act, which stipulates they be managed for “permanent forest production” and that timber royalties be funneled to local counties. Area timber companies and Oregon counties receiving O and C money sued after the expansion, arguing Obama violated the 1937 Act.

The O and C case presents a question similar to the Grand Staircase land exchange: Does the Antiquities Act grant the president power to alter an act of Congress? In this instance, says Michael Blumm of Lewis and Clark Law School, the courts have indicated that O and C land management must adhere to environmental regulations, and he suspects the Antiquities Act would fall under that category.

“We have 9th Circuit [Court of Appeals] decisions saying that the O and C Act didn’t exempt those lands from the Endangered Species Act, the Migratory Birds Act, or the Northwest Forest Plan,” Blumm says. Furthermore, there’s an argument whether the act requires timber harvest on the land. “The O and C Act talked about permanent forest production, not permanent timber production. It also said to manage the land for watershed health and for recreation.”

The monument itself is unique in that it was designated, originally by Clinton, to protect its astonishingly unique biodiversity. Obama expanded it only after scientists said it was too small to do that effectively. By that logic, the Trump administration would yet again face a challenge proving the monument is larger than necessary.

Gold Butte National Monument, Nevada

On July 30, Kevin Brown met with Zinke in Bunkerville, Nevada, site of the infamous standoff between the Bureau of Land Management and Cliven Bundy’s supporters, to talk about the most contentious topics in the West: public-land management and water. Brown is general manager of the Virgin Valley Water District, and his job is to supply drink to the thirsty city of Mesquite, population 17,500 and growing. Virgin Valley owns water rights to six springs about 30 miles south of town—an area that is now Gold Butte National Monument. Those springs could eventually supply 2,500 acre-feet of water each year, and Brown asked Zinke to shift the monument boundaries five miles south to provide unfettered access to the springs.

Should Zinke grant Brown’s wish, opponents will argue that the monument proposition clearly protects existing, valid water rights. No diversion exists yet, but the proclamation preserves existing rights-of-way, as well: Mesquite still has the option to build pipelines to the springs.

“The only legal effect would be that a right-of-way could not be issued if it were inconsistent or detrimental to the proper care of the [monument],” says University of Nevada, Las Vegas law professor Bret Birdsong. “It requires the BLM to take some additional consideration to the effect of a right-of-way, but it doesn’t prohibit BLM from granting those rights-of-way.” (Birdsong served in the Obama Interior department but was not involved with the Gold Butte proclamation.)

The preservation of property rights are rarely mentioned by monument critics. Just as water rights are still valid in Gold Butte, cattle-grazing permits remain in Bears Ears, and some logging is still allowed in Cascade-Siskiyou. (It does not, however, permit clear-cutting). Also lost in the discussion of whether presidents can alter monuments at whim is the reality that an incontrovertible, perfectly legal way to do so already exists—through an act of Congress.