Los Angeles (CNN) -- By busloads and by church flocks, volunteers throughout the country spent Saturday performing the noble task of cleaning up humanity's home -- planet Earth.
"The nation's largest community improvement movement," as the volunteer groups are known, got their hands dirty and feet wet as they cleaned walkways and waterways around the United States.
Keep America Beautiful -- the nonprofit group that sponsored the Great American Cleanup "National Day of Action" -- organized trash-clearing activities in nearly all 50 states, as well as special large-scale events in 10 locales, including Los Angeles. In all, 10,000 volunteers were projected to participate.
"Oh my goodness. The volunteers, the energy, the enthusiasm. To watch the kids learn how to plant a tree was amazing to watch. It becomes theirs," said Gail Cunningham, senior vice president of Keep America Beautiful and managing director of the Great American Cleanup. "It brought nature alive for the volunteers."
One of this year's spotlight sites was the namesake waterway of the nation's second largest city, the Los Angeles River.
Saturday's activities coincided with the Friends of the Los Angeles River's 23rd La Gran Limpieza, which translates to The Great Cleanup.
About 450 people, including 250 from a local Mormon church, collected 400 bags of trash -- including a king-size mattress, an animal skull, and a discarded wheelbarrow -- from one stretch of the river, said Shelly Backlar, the river group's executive director. Volunteers were bused to cleanup sites.
"People are saying they can't believe what they are finding," Backlar said. "Everyone is smiling and really enjoying the river."
Added volunteer Judy Bloom, 48: "I spent a couple of hours picking up several bags of trash and it was primarily plastic, about 90%.
"There was an element of surprised in respect to how much this is caused by littering," Bloom said.
The other nine locales for the big-scale cleanups were Oakland, California; Cocoa, Florida; Indianapolis; Cobb County, Georgia; Hampton Roads, Virginia; Houston; Shreveport, Louisiana; Philadelphia; and Nashville.
Tidying up the country isn't a matter of just "waving a magic wand," said Cunningham. "It's understanding each person can make a difference."
"Each volunteer for one hour of work is worth $21.79, and we had 5.2 million volunteers last year" during a period from March 1 to May 1, she added. "The United States saved $113 million last year that would have otherwise paid government employees."
Last year's clean-up activities in 16,500 communities resulted in the removal of 177 million pounds of litter and debris; beautification of 177,000 acres of parks, public lands and open spaces; and planting of 166,000 trees and 1.5 million flowers and bulbs, according to Cunningham's group.
Last year, Los Angeles' La Gran Limpieza collected 25 tons of trash from cigarette butts to "river treasures," as Backlar put it, consisting of shopping carts, a hammock, a sleeping bag and a Barbie doll with dreadlocks.
From coast-to-coast, groups were relying on the volunteer spirit of the millennial generation and college students.
"There is passion for engagement and volunteerism from the millennials. They are active. It's inspiring to see," said Cunningham.
Christine Flowers, executive director of Keep California Beautiful, said Saturday's volunteer activities supplement state and local government efforts -- which cost taxpayers about $500 million a year -- to address illegal dumping and littering.
"When people get their hands dirty and get engaged, they care more for the environment and their community," Flowers said. "Volunteers learn something new and it gives them passion and fire to better the environment."