First layers of soil to be laid on 101 Freeway wildlife crossing in California, the world's largest

Seema Mehta, Los Angeles Times on Mar 31, 2025

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LOS ANGELES — The wildlife crossing designed to help mountain lions, deer, bobcats and other creatures safely travel over the 101 Freeway between the Simi Hills and the Santa Monica Mountains will reach a major milestone on Monday, as workers lay the first layers of soil on the overpass.

The Wallis Annenberg Wildlife Crossing spans the 10-lane freeway in Agoura Hills and will become the largest such crossing in the world. It is designed to help animals avoid being killed while roaming in urban habitats. Although it is too late to help Los Angeles' beloved mountain lion P-22 expand his territory, the passage will allow mountain lions and other wildlife to range farther for food and mates.

Small puma populations have been isolated by the freeway, and their offspring were showing signs of birth defects.

"I imagine a future for all the wildlife in our area where it's possible to survive and thrive and the placement of this first soil on the bridge means another step closer to reality," Annenberg, a philanthropist, said in a statement.

"This extraordinary structure will serve not only animals," she said, "but it will reconnect an entire ecosystem and protect this global biodiversity hotspot — this moment marks another wonderful milestone toward that goal."

The Annenberg family's foundation was a major donor to the $92-million effort to make the bridge — which stands 21 feet and 8 inches above the freeway — a reality. Initially conceived more than three decades ago, construction of the 200-foot-long,165-foot-wide bridge began in 2022 and is expected to be completed in 2026.

"There's been a growing awareness in California as we're working to protect our nature, our biodiversity, that we can't just restore and protect habitat; We actually have to build connectivity between habitat," Wade Crowfoot, secretary for the California Natural Resources Agency, said earlier when the project was announced.

Monday morning, workers will begin placing soil — sandy loam mixed with lightweight volcanic aggregate — on the wildlife overpass. The process is expected to take several weeks and will require 6,000 cubic yards of soil, enough to cover three-quarters of an American football field in about 2.5 feet of soil.

Then, coastal sage, buckwheat, wild grape, wildflowers, milkweed and other native plants will be planted on the roughly one-acre habitat. Oaks and other trees and plants will be planted on 12 acres on both sides of the wildlife crossing.