When torrential rains occasionally pour down across Los Angeles, local residents worry about bad drivers, heavy traffic and being late for work. But one thing they can be grateful for is knowing that rainy days are a boon for local water facilities, where water is collected and can later benefit LA communities during drier times.
In Sun Valley, a series of multiple interconnected open-dirt basins – with just the right combination of natural porous and gravelly soil, along with some vegetation – are capturing local rainfall and runoff from the San Gabriel Mountains flowing down through Hansen Dam.
These “spreading grounds” behind fences may go unnoticed, but they serve a crucial purpose: to collect rain and stormwater, and all that water is allowed to seep into underground aquifers, replenishing the water supply for residents across Los Angeles County.
Thanks to 27 spreading ground facilities – including sites in Sun Valley, Pacoima and Sylmar – and 14 major dams and reservoirs county-wide, LA County captured over 120 billion gallons of stormwater during the 2025-26 storm season – from early October to April 15. That’s 10 times more water compared to the previous season, when only 11.9 billion gallons were collected.
This dramatic boost to the county’s groundwater supply was achieved by merging modern, high-performing infrastructure and, simply put, the luck of the weather, said Sterling Klippel, assistant deputy director of LA County Public Works’ Stormwater Engineering Division.
“Our average rainfall in downtown LA is about 15.4 inches. This year, we’ve had 16.9 [inches] so far” – totaling about 110% of the annual average, according to Klippel. By comparison, the previous season brought only 6.6 inches, less than half of the typical annual amount of rain.
When you do the math, said Klippel, the 120.3 billion gallons collected during the most recent storm season is “enough water [to meet the needs of] 3 million people for an entire year.”
“LA County roughly has about 10 million people, so … we’ve captured enough water for about 30% of the entire population of [the] county,” said Klippel.
“The investments that we’ve made in our system … bring them up to modern standards, [including] investments in these spreading grounds,” he said of the Hansen Spreading Grounds in Sun Valley. “We’ve spent hundreds of millions of dollars over the last couple of decades to make sure that the system [is] reliable and, when these storms come, that it’s fully operational.”
How Do Spreading Grounds Work?
Spreading grounds work in tandem with nearby dams (such as the Hansen and Pacoima dams) and mountains (including the San Gabriel Mountains) to capture stormwater during wetter weather periods, to help augment the local water supply during the always-inevitable drought years.
Driven by gravity, the water is naturally filtered as it percolates down through the porous layers of soil and rock, soaking into the ground to recharge aquifers and replenish groundwater. Depending on demand, the water can be pumped out and treated at a later date, primarily by the Los Angeles Department of Water and Power (LADWP) in the San Fernando Valley.
The water can then be distributed for municipal use – for everything from cooking to bathing to gardening – reducing the county’s reliance on outside water sources. About two-thirds of LA’s water is currently imported, including from the Sierra Nevada Mountains, the Sacramento-San Joaquin River Delta and the Colorado River.
Supervisor Lindsey P. Horvath, who represents the 3rd District – including portions of the valley – for the LA County Board of Supervisors (BOS), said she authored a motion in 2023 to help ensure LA County “had a clear, coordinated plan to capture and store more local water.”
“What we’re seeing at the Hansen Spreading Grounds, and across the county, is that when we invest in this infrastructure, it benefits whole communities,” explained Horvath in a statement to the San Fernando Valley Sun/el Sol. “Every drop we capture is water we can use in our homes, schools and neighborhoods.”
Reducing the county’s reliance on imported water helps lower costs over time and builds “a more reliable local water supply,” emphasized Horvath.
“The future of Los Angeles depends on how boldly and urgently we act to capture and manage our water today,” she stated.
And capturing more water locally can help safeguard the county against future droughts. Rising temperatures are making droughts and other extreme weather events – such as storms, heat waves and wildfires – more common, according to the California Climate Adaptation Strategy.
One major initiative now being developed is the Rory M. Shaw Wetlands Park Project in Sun Valley, named for a celebrated late civil engineer who worked for the City of LA. The long-term project, which is slated for completion in the next decade, is a collaboration with the LA County Flood Control District, the City of LA and the Sun Valley Watershed Stakeholders Group.
The key goals are to convert an inert landfill into a multi-purpose wetlands park to help reduce flooding, support water conservation, and feature 10 acres for natural water treatment, a 21-acre detention pond and 15 acres of open space and recreational areas for local community members.
Protection from Flooding
By managing local stormwater, the county’s water facilities are also reducing the likelihood of flooding, which helps keep surrounding homes, businesses and infrastructure safe, said Klippel.
Overall, continued Klippel, LA County has “one of the largest flood control systems in North America” – which includes dams, channels, spreading grounds and debris basins, which are engineered to help hold back potentially-destructive mudslides and debris flows, which can contain soil, rocks, trees and even cars or parts of building structures, including after wildfires.
“In the mountains, [when] it rains [and] storms up there, our dams are capturing that water, controlling that water and then, ultimately, we send it down to our spreading grounds, protecting communities from flood waters [and] mud flows,” said Klippel, noting that after the 2025 LA Wildfires, there were concerns “about mud flows coming down and impacting people’s homes.”
“[After] the recent fires we had, we had no major incidents involving flooding or mud flow,” he said, emphasizing that it took “a lot of work” behind the scenes to help keep neighborhoods safe. “A lot of this isn’t known by the general public, but these guys [are] out there doing the work.”
Klippel lauded the year-round, hands-on work by LA County Public Works’ maintenance staff.
“Our guys here, our maintenance crews, spend all year preparing these facilities so that you know when it does rain, we’re ready [and] these facilities are ready,” he said. The goal is to capture the maximum amount of stormwater for flood control and to increase the local water supply, to keep as much as possible from bypassing the system and pouring into the LA River and, from there, into the ocean – to help ensure, said Klippel, that “we aren’t wasting water.”
“It’s not only about flood protection; it’s also [about protecting] this precious water resource that we have,” he continued. “We always try to keep as much of it, save as much of it as possible.”
Horvath will convene the 2026 LA County Water Resiliency Summit in Alhambra on Monday, May 18, to explore long-term water resilience strategies. To learn more, go to: www.eventbrite.com/e/2026-la-county-water-resiliency-summit-tickets-1985327599293.