How Rain and Floodwaters Literally Built the Southland


Without rain, there would be no Southland. Over countless millennia, rainwater scoured the mountains ringing the region. It ground granite into sand. It sloughed off topsoil. Where it issued from canyons at the base of the mountains, this muddy soup fanned out, carrying courser and then finer sediments until it merged with the cold waters of the Pacific.

As the floodwaters drained, the sediments settled, filling the deep geologic basins that abut Southern California's mountains. Eventually, these stacked sedimentary layers grew thousands of feet thick, emerging from the waters of the Pacific as flat, broad valleys like the San Fernando, San Gabriel, and Pomona, or sprawling coastal lowlands like the Los Angeles Basin and Oxnard Plain.

In the twentieth century, the metropolis hit the pause button on this natural process of land reclamation. To protect lives and capital as the city expanded into areas prone to flooding or landslides, flood control projects reconfigured the region's hydrology. Check dams and debris basins now trap sediments in or near their mountainous origins. Storm drains and flood control channels funnel rainfall to the sea as quickly as possible.

Now, those sedimentary flatlands -- home to most of the metropolis's 18 million residents -- are slowly shrinking as wave erosion and (inevitably, probably) rising sea levels encroach on the coastal plain.

Through their composition and their printed captions, the photos above and below purport to document destruction. They certainly document real human suffering. But when viewed in the context of the region's geologic history, they show the opposite of destruction; they reveal the natural forces that -- with each new flood -- created a little more Southland.

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