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WASHINGTON — President Barack Obama this week plans to name Browns Canyon, in central Colorado, a national monument, a designation that adds a new layer of federal protection to the popular spot for whitewater rafting.

About 21,000 acres around the Arkansas River will be included in the listing. Obama will make the announcement Thursday in Chicago, where he’ll also declare a site in Illinois and one in Hawaii to be national monuments, according to the White House.

News of Colorado’s coming designation spread quickly through the state’s wilderness circles.

Bill Dvorak, who runs an outdoor-expedition business near the Arkansas River, called the region a “sanctuary for wildlife” and said it was about time the government preserved its beauty.

“I’ve been working on this for over 20 years, and I’m so damn thankful it’s happening,” he said. “It will do a good thing for the economy. It puts a star on the Rand McNally maps.”

The White House move comes on the heels of a failed effort last year by two Colorado Democrats to protect Browns Canyon.

U.S. Sen. Michael Bennet and then-U.S. Sen. Mark Udall tried to pass legislation that would have preserved a similar area near Salida, but the bill failed to advance through Congress.

In response, Bennet and Gov. John Hickenlooper urged Obama to take executive action. Specifically, the two Democrats recommended Obama use a law known as the Antiquities Act, which dates back to the administration of Theodore Roosevelt.

The measure gives broad power to the president to set aside federal land for protection — but without the need for congressional approval.

Obama has used the Antiquities Act 16 times, including the latest three in Colorado, Hawaii and Illinois. The one in Hawaii will preserve a site where Japanese-Americans were held in internment camps; the one in Illinois will honor the Pullman area in Chicago, a locale important to the labor civil rights movements.

In all, Obama has put more than 260 million acres of land and water under federal protection — more than any other president, according to administration officials.

It’s that use of executive power, however, that drew the ire of Republican U.S. Reps. Ken Buck of Windsor and Doug Lamborn of Colorado Springs.

“My message to the president is cut it out. He is not king. No more acting like King Barack. That is not how we do things in the U.S.,” Buck said in a statement.

Lamborn struck a similar tone.

“I am outraged,” he said in a statement. “This is a top-down, big-government land grab by the president that disenfranchises the concerned citizens in the Browns Canyon region.”

He said residents have raised concerns about how designating Browns Canyon as a national monument would affect the area — especially its effect on grazing rights and water rights.

“It is also important for people to note that national monuments created by presidential executive order under the Antiquities Act almost always become underfunded, neglected properties,” he added.

Before Thursday’s announcement, White House officials noted that the new protections for Browns Canyon would honor existing water and grazing rights — although details of the arrangement were not released Tuesday night.

Two federal agencies, the Bureau of Land Management and the U.S. Forest Service, will co-manage the land.

Lori Roberts, executive director of the Salida Chamber of Commerce, said her group has remained neutral through the proceedings to name Browns Canyon a national monument — although the chamber will promote it now.

But she said local ranchers have expressed some concern.

“I think there is a fear that ranchers’ rights will be lost,” she said. “It’s uncharted territory.”

Bennet said he was confident that the underlying order would balance existing arrangements with the new environmental protections.

“Coloradans have been very clear they wanted this protection, along with assurances that existing uses will be protected. We’re glad the administration heard those voices and provided those assurances,” he said in a statement.

Mark K. Matthews: 202-662-8907, mmatthews @denverpost.com or twitter.com/mkmatthews