Where humans have altered the landscape, nature often does not reappear by itself.
Sometimes it needs a little help.
Since Friday, bulldozers and dump trucks have been at work in Pasadena in a section of the Arroyo Seco, a major tributary of the Los Angeles River that has been choked by 450 tons of jagged concrete thought to be debris from construction of the 210 Freeway.
For at least 30 years, the mess has littered the channel, one of the most heavily used recreation corridors in the Los Angeles area. Hundreds of people stroll, picnic and otherwise enjoy the area on any given day.
The work is taking place in two areas temporarily closed to the public: one just south of Devil's Gate Dam, the other just north of the Colorado Boulevard bridge. By the end of the month, the two 10-acre stretches will be reopened to hikers, equestrians, bird watchers, dog walkers and others who wander the embankments in search of open space and sunshine.
The concrete removal is part of a $1-million restoration project begun late last year by the city of Pasadena with the help of state funds. Talk of the project began 10 years ago, but it was pushed to reality by the Pasadena City Council after a persistent search for grant money. The project was funded through a state ballot measure, the Safe Drinking Water and Watershed Protection Act of 2000.
No one is sure where the concrete slabs came from. City officials say they hinder flood control because they raise the level of runoff in the arroyo, which threatens to erode pilings holding up the 210 Freeway.
"They were probably put there to hold up scaffolding for the freeway construction," said Lynne Dwyer, a contractor supervising the landscaping work. In 1973, floodwater washed out part of the freeway -- then under construction -- and could have left some of the debris.
Tim Brick, managing director of the conservation-minded Arroyo Seco Foundation, said the jagged concrete blocks and twisted steel reinforcing bars need to be removed not only to enhance flood control but also to improve safety and aesthetics.
"They are dangerous, they are slippery, they are obstructions," Brick said. "It is a very large form of litter."
The work is a key example of how Southern California's waterways can be restored to a wild state benefiting people who visit and fauna that live there, say those who helped make it happen. "The streams and rivers of Southern California have been greatly abused. This isn't just for the arroyo. We're saying we need to treat all the streams of Southern California with respect," Brick said.
"The Arroyo Seco is a spiritual, cultural and environmental force linking the San Gabriel Mountains to downtown Los Angeles," he added.
The waterway holds an important place in regional history. "It is the reason why downtown Los Angeles is where it is," Dwyer said.
More than two centuries ago, settlers camped by the confluence of the Arroyo Seco and Los Angeles River. They needed to be near a supply of fresh water, but they wanted to stay out of the floodplain. The pueblo that grew into Los Angeles was built on a mesa just above the floodplain.
The Arroyo Seco, which begins near Mt. Wilson and meanders 22 miles to its confluence with the Los Angeles River near Dodger Stadium, is not only blighted by concrete debris, it also is choked in places with nonnative trees and vegetation.
As part of the project, hundreds of invasive trees and truckloads of exotic plants have been uprooted since fall.
Eucalyptus, palms, carob, tree tobacco, periwinkle and ivy have been torn out because they crowd out native species and provide little or no benefit to wildlife, Dwyer said.
Black willows, sycamores, coast live oaks, ferns and succulents more suited to the climate and habitat are taking their place. Trucks fitted with nozzles have blown specially blended seed mixes along the banks to aid regrowth.
Though the project is not unique -- work has long been underway to naturalize sections of the Los Angeles River -- Brick says the scope and commitment by Pasadena officials serve as an important model.
"There are groups all over Southern California who would like to do the same thing," he said.
Dwyer added: "Everybody is looking at this -- how do you get nature back into the city? I think it's one project at a time."
Don Bremner, a Sierra Club leader who hikes the area frequently, welcomes the change. "People will appreciate it more as a natural area," he said.
For Dwyer, who grew up in the area, the transformation is like creating life itself.
"When I'm in this place, I see what's really beneath the concrete," she said. "You add water, you add seeds, a little soil, a little air, and voila! What you get is nature in the city."