Historic black-and-white photograph of the Los Angeles flood, with rushing water in the foreground and damaged structures behind it.

LA EcoExplorers Story

THE FLOOD THAT CHANGED THE SHAPE OF LOS ANGELES

A giant flood changed Los Angeles in 1938. This story shows why the city put a river in concrete—and what that decision meant for nature.

1 · March 1938

WHEN THE RIVER ESCAPED

After days of rain, the L.A. River burst beyond its banks and flooded neighborhoods across the region.

87people died

$900Mdamage in today’s dollars

The problem:The river could spread far beyond its banks during huge storms.

Look closer at what the flood did.

Aerial photograph of flooded homes and land in North Hollywood during the 1938 Los Angeles flood

Los Angeles · March 1938 Close-up photograph of the 1938 Los Angeles flood showing raging water and damaged rail infrastructure beneath a bridge.

NOW YOU’RE IN CHARGEWHAT WOULD YOU DO?How would you stop the river from flooding neighborhoods again?

2 · The city's answer

THEY BUILT A FLOOD CHANNEL

After the 1938 disaster, engineers rebuilt much of the L.A. River so huge storms would stay in one controlled path.

BIG IDEABuild a giant concrete flood channel.

1

STRAIGHTEN ITFewer bends.

2

ADD CONCRETEHard sides + bottom.

3

MOVE WATER FASTToward the ocean.

THE GOALProtect homes, streets + bridges.

Drag the line to compare.

Historic black-and-white photograph showing the Los Angeles River flooding beyond its banks.

Modern aerial photograph showing the Los Angeles River confined within a concrete flood-control channel.

DRAG TO COMPARE

BEFORE: THE RIVER FLOODS.Water can spread beyond the river into nearby land.

3 · The tradeoff

SOLVING THE FLOODS CHANGED THE RIVER.

Concrete helped protect Los Angeles from major floods. But it also created new problems for the river—and for people living around it.

NEXT: FOUR CONSEQUENCESHabitat · Water · Pollution · Green space

Aerial view of the modern concrete-lined Los Angeles River

THE MODERN L.A. RIVER

One solution. Four important tradeoffs.The next slides show what changed.

4 · Ecosystem loss

WHO LOST THE MOST?

When the river was paved, animals lost the places they needed to feed, hide, nest and reproduce.

WHAT DISAPPEARED?

1COOL SPAWNING STREAMSfor steelhead 2WILLOW THICKETSfor nesting birds 3QUIET WET POOLSfor red-legged frogs

THE BIG IDEAIf animals lose the places where they feed, hide or reproduce, they can disappear from the river.

Tap each animal to see what it lost.

THESE ANIMALS NEEDED MORE THAN JUST WATER.They needed habitat.

TAP ANIMAL 1 OF 3Start with the steelhead, then explore all three.

5 · Water issues SAME RAIN · DIFFERENT RESULT

SPONGE OR SLIDE?

A natural river can help save rainwater underground. A concrete river sends much more of it away.

1

NATURAL RIVERSPONGE

RAIN SOAKS IN REFILLS GROUNDWATER That means more local water saved for later.

2

CONCRETE RIVERSLIDE

RAIN RUNS OFF LESS GROUNDWATER More water leaves L.A. instead of becoming part of the local water supply.

WHY THIS MATTERS GROUNDWATER IS L.A.'S UNDERGROUND WATER BANK. Less soaking means less local water saved for dry days.

Trash-covered beach near the ocean with litter washed ashore

6 · Pollution

WHAT RUNS OFF THE CITY CAN END UP HERE.

Rain washes trash, oil, bacteria, and metals off streets and into storm drains. The concrete river can carry that dirty runoff all the way toward the ocean.

FAST WATER MOVES POLLUTION, TOO.Pollution can travel far from where it started.

Aerial view of the concrete Los Angeles River running beside a freeway and homes

7 · Communities

THE RIVER CHANGED DAILY LIFE, TOO.

The concrete channel did more than change water. It changed what people living beside the river had around them.

1

FEWER PLACES TO GOLESS PARK + RIVER ACCESS

Fewer natural places to walk, play and gather.

2

HOTTER SURROUNDINGSLESS SHADE + GREEN SPACE

Concrete gives less cooling than trees and riverbank plants.

3

CLOSER TO POLLUTIONTRAFFIC + DIRTY RUNOFF NEAR HOMES

Some neighborhoods along the channel carried more of the environmental burden.

THE BIG IDEA:A river design can affect people, not just water and wildlife.

The flood that changed the shape of Los Angeles THE CONCRETE FIXED FLOODING — BUT IT ALSO CHANGED THE RIVER.

Los Angeles solved a real safety problem after 1938. But the same concrete also destroyed habitat, reduced groundwater recharge, carried polluted runoff to the ocean and left many neighborhoods with less nature.

THE NEXT CHALLENGEHow can Los Angeles keep people safe while bringing more nature, cleaner water and healthier neighborhoods back to the river?

LA ECOEXPLORERS · LOS ANGELES