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President Obama plans to announce Friday that he will set aside roughly half of the San Gabriel Mountains as a national monument, bringing stronger federal protections to the range to shield it from crowding and pollution.

Obama will designate about 350,000 acres of public land in the Angeles National Forest as the San Gabriel Mountains National Monument, officials said. It will be the 11th time Obama has used his executive powers to establish or expand a national monument without congressional approval.

To address fierce opposition from San Bernardino County officials and other critics, the new monument is expected to exclude portions of the range extending into the western reaches of the county, including the mountain communities of Wrightwood and Mt. Baldy. Some fear the new status will hamper growth and fire safety.

Stretching from Santa Clarita to San Bernardino, the San Gabriel watershed is within a 90-minute drive of 17 million people. It provides Los Angeles County with 70% of its open space and roughly 35% of its water. The rugged slopes and canyons attract more than 3 million visitors a year and are home to rare and endangered species, including Nelson’s bighorn sheep, mountain yellow-legged frogs and Santa Ana suckers.

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