WASHINGTON — The National Park Service is looking to stimulate summer vacations at national parks.

Interior Secretary Ken Salazar announced today that entrance fees at 147 national parks and monuments — including the Grand Canyon and Yosemite — will be waived on three weekends this summer. The weekends are June 20 to 21, July 18 to 19 and Aug. 15 to 16.

"During these tough economic times, our national parks provide opportunities for affordable vacations for families," Salazar said at a news conference at Cuyahoga Valley National Park in Ohio. "I encourage everyone to visit one of our nation's crown jewels this summer and especially to take advantage of the three free-admission weekends."

Most Americans live less than a day's drive from a national park, Salazar said. Last year, national parks attracted more than 275 million visits, generating an estimated $10.6 billion for local economies and supporting more than 213,000 jobs, he said.

For the Park Service, the free weekends will mean a loss of an estimated $500,000 a day from entrance fees that range from $3 to $25. A total of 147 parks and monuments charge entrance fees; the nation's other 244 parks are already free.

The waiver applies only to entrance fees and does not affect charges for camping, reservations, tours or concessions.