LOS ANGELES >> Politicians, government managers and nonprofit groups gathered Thursday to express their hope for a greener, more accessible Los Angeles River.

Lewis MacAdams, founder of Friends of the L.A. River more than 40 years ago, spoke of a new plan proposed by the Army Corps of Engineers to strip away portions of the river’s concrete undergirding and reveal its hidden attributes.

“The river is a rigorous mistress. When you tickle her with your deeds, you can hear her laughing beneath her concrete corset,” MacAdams said, reciting the line from a poem he’d written. Then, with the nearly full Deaton Auditorium in a hush, he added:

“Today, she is waiting to slowly take off the corset, as one takes back the sheets of a bed as one would for one’s lover.”

Nancy Steele, executive director of the Council for Watershed Health, the group hosting the seminar and the emcee, didn’t quite put it that way. But she also supports what’s called Alternative 20 of the plan, a $1.08 billion restoration that will add trees, parklands, vegetation, bike paths, pedestrian/bike bridges and recreation to an 11-mile stretch of the river known as the Glendale Narrows that runs from Griffith Park to downtown Los Angeles.

“There is excitement in the air now that the Army Corps has released its study,” she said in her opening remarks. “Concrete will come out. Streams will be daylighted. Wildlife will be able to connect up with their friends on the other side (of the river).

“But,” Steele summed up, “this will come at a high cost.”

The 51-mile L.A. River runs from the San Fernando Valley and Pasadena to the Pacific Ocean in Long Beach. During the 1930s, after several serious floods, the Army Corps of Engineers lined most of it with concrete for flood control purposes.

In the past seven years, the city of Los Angeles, working with Los Angeles County and the city of Burbank, which shares a portion of the L.A. River on its border, have been trying to convince the federal agency to support ripping out some concrete and adding amenities. The cities support the most expensive alternative.

“The river is the backbone of this city,” said L.A. Mayor Eric Garcetti.

The Army Corps prefers Alternative 13, which leaves out key projects wanted by the L.A. mayor but will cost substantially less, about $450 million. Under Alternative 20, the federal government would pay 54 percent of the bill, leaving 46 percent for local cities.

Two projects included in Alternative 20 — the greening of the Verdugo Wash Confluence and building a connection between the new El Rio De Los Angeles State Park to the river — are left out of other alternatives. Under Alternative 20, 671 acres would be restored by terracing its banks with plants and trees in what is now an industrialized flood control channel that very few Angelenos visit.

Other projects would include: greening the Arroyo Seco Confluence south of South Pasadena and restoring wetlands where the river empties into the ocean in Long Beach, said Carol Armstrong, a city of L.A. engineer who is familiar with the Army Corps study.

The Army Corps is holding a public hearing on its study at 5:30 p.m. Thursday at the Los Angeles River Center. 570 W. Avenue 26, L.A. 90065. Speakers and audience members were taking notes on what to say at that meeting.

“We need to convince the federal government that investment in ecosystem restoration along the L.A. River is worth it,” Armstrong said.

While Garcetti, who gave the keynote speech, said he’s met with members of Congress and the Obama administration in Washington, he urged the public to speak out and sign his online petition — www.lamayor.org/restore_the_river.

He said the first West Coast explorers, known as the Pobladores, followed the Los Angeles River as a source of life and community. They helped establish El Pueblo de Los Angeles, even though the mission was first built on the San Gabriel River, and later moved to higher ground in San Gabriel.

“Instead of a brick-by-brick mission, we have a block-by-block mission that once again leads us to the L.A. River,” Garcetti told the group.

Since he’s been in office, the trash in the L.A. River has been reduced by 90 percent, he said.

Much of the recent projects in L.A. came from Proposition O, passed by voters and raising $500 million for stormwater cleanup projects.

The city wants to see more money for L.A. River beautification. But the task of convincing a shutdown, dysfunctional Congress will be arduous.

“We will compete with nationwide projects,” Armstrong reminded the audience. “Here, in some cases, we are re-creating something that has been gone for a really long time. But we are the second-largest urban area in the country. Doesn’t L.A. deserve it?”