What Lots Of Rain Does To Our Rivers — And How Much Is Too Much

Over the first two weeks of 2023, heavy storms have drenched Southern California. We’ve had mudslides, flood warnings, and evacuations.

A lot of that water winds up in our rivers — sometimes, too much. The Ventura River actually flooded.

With more rain on the way, we wanted to know: How much water can our rivers handle? Where does all that water go? And what happens to all the stuff that ends up in those rivers that isn’t water?

How Much Rain Does It Take To Flood Our Major Rivers?

Los Angeles River: That answer is complex. Kerjon Lee, a spokesperson for Los Angeles County Public Works, said it also depends on where that rain falls.

“It's not just the amount of rainfall” — two or three inches over three days won’t do it — “but it's the intensity and where that rain falls in the L.A. Basin,” Lee said.

The L.A. Basin is protected by the county’s 14 major dams—plus four managed by the Army Corps of Engineers—and has 600 miles of open channel, with about a 3,000-mile network of storm drains, according to Lee. It’s designed to meet the federal standards for a 100-year flood. That means there’s about a 1% chance of a storm exceeding the limit of the L.A. River’s infrastructure in any given year. That’s a national stormwater infrastructure standard, and according to Lee, most of L.A. County meets that standard of protection.

The modern L.A. River, much of it clad in concrete, is actually designed against flooding. Once, it was its own ecosystem, and until 1900 it was the city’s main water source. People began building homes closer and closer to the river. In February 1938 a massive storm hit Los Angeles, the river overflowed and wrecked nearby homes, bridges, and office buildings. So the U.S. Army Corps of Engineers creating the concrete-lined channel of the L.A. River to mitigate future floods.

Southern Pacific's Dayton Avenue railroad bridge, near Elysian Park, collapsed during the flood of 1938 as the Los Angeles River swelled.

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Herman J. Schultheis/Los Angeles Photographers Collection/Los Angeles Public Library Collection

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A flood in the 1930s caused a bridge to collapse into the Los Angeles River.

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Courtesy Los Angeles Public Library

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Santa Ana River (Orange County): The same 1938 storm that devastated Los Angeles and caused the transformation of the L.A. River also wrought havoc in Orange County, where the Santa Ana River caused a massive flood. A series of ensuing engineering projects reduced that risk.

The Santa Ana River system did not flood after the January 2023 storms hit Southern California. The river actually surpasses FEMA’s 100-year flood infrastructure standard, according to Penny Lew, a senior civil engineer with Orange County Public Works. “The Santa Ana River is actually designed for a 190-year storm,” Lew said.

Ventura River (Ventura County): As for the Ventura River, that depends on the volume and intensity of the rain, as Assistant Director of Public Works for Ventura County David Fleisch explains. “There's not a set number, you know, if you get two inches of rain, it's gonna do this. Depends upon how, when it comes, and how fast it comes.”

What Is The 'Danger' Zone, And How Close Have We Gotten?

For the L.A. and Santa Ana Rivers, the storms Southern California has experienced in early 2023 have not met the 100-year storm standard.

“What we've been able to document so far is, I think it's a once-in-10-year storm,” Lee said. He said there’s still plenty of capacity in the L.A. River basin and within the county’s infrastructure for taking in that storm water.

As the Ventura River water levels crested on Monday, Ventura County officials watched the rainfall gauges throughout the county, particularly the last gauge before rainfall heads into the ocean. That last gauge is where the Ventura River flows through Foster Park.

When that gauge “gets above 19,000 cubic feet per second, we know we have to talk to the city about them considering evacuating the RV park that's right down by that ocean,” Fleisch said. The City of Ventura evacuated the park on Monday. Ventura County officials rescued 18 people from the floods.

And that leads to another important point: The rivers don't have to flood to be a danger to people, and especially for those who are unhoused and live near riverbeds.

How Long Before Water Levels Go Back To Normal?

Because the L.A. River is lined with concrete, there's a limited capacity for groundwater recharge, particularly in the lower regions of the river. But in the upper part of the watershed, “we are actually able to capture stormwater and store it and have it available for future reuse by water retailers by putting it into the groundwater system,” Lee said. In the San Gabriel Valley, Lee estimates that about 95-98% of the waterfall is able to be captured because of the porous grounds, which aids with recharging groundwater supplies.

As for the Ventura River, that depends on external factors such as the wind, sun and how much the ground has been saturated. “The ground is like a sponge. And once it absorbs all it can, then it continues to run off,” Fleisch said.

Since the ground is pretty much full with all the rain Southern California has received, Fleisch says it will take a while for that water to run off into the Pacific Ocean. “But there isn't really a formula that says, oh, in three days it'll be OK,” he noted.

When Rivers Flood, What Happens To All The Contaminants?

Contamination and pollution of the L.A. River has been a big challenge for Los Angeles County.

“If you get a plastic bottle of water and you throw that into the street when it's raining like we've experienced, and even when it's not raining, that bottle could end up in the catch basin, which connects to the storm drain system, and then those storm drains lead to water bodies like the L.A. River and the Pacific,” Lee said.

Motor oil, vehicle brake dust, fertilizers, pesticides, dog waste, and other materials people leave out on the street, Lee says, often do end up in waterways and contaminate the ocean and the L.A. River. And it's really hard to clean all that up. (The county does have a plan to revitalize parts of the river, although that plan has many detractors.)

The L.A. County Public Health Department advises residents to not enter beach waters for 72 hours after a storm. The City of Los Angeles Department of Sanitation advises avoiding L.A. River waters during, and within 72 hours of, a storm.

Throughout the year, Orange County’s operations and maintenance crews clean up trash and debris along the Santa Ana River channels, according to Orange County Public Works Communications Officer Shannon Widor.

“We have a network of all these smaller channels that go throughout cities, along freeways, and not all of them feed into the Santa Ana River," Widor said. "Those smaller channels feed into and connect to the Santa Ana River, so a lot of those trash impacts we see will oftentimes are initiated further upstream.”

The Ventura County River is largely surrounded by agriculture and people who raise animals. So, when rainfall hits, animal feces and road runoff material, like oil, mixes into the river.