LONG BEACH, Calif. — The winter rains finally arrived in Southern California, bringing drenching relief in recent days to a part of the nation suffering one of the worst droughts in history. But the El Niño storms brought something else as well: a reminder of lost opportunity, on display in this coastal city, as millions of gallons of storm water slipped down the usually dry Los Angeles River and out into San Pedro Bay.

After a year in which Californians cut water use by 25 percent, storm water has become the next front in what amounts to a fundamental restructuring of Southern California’s relationship with its intricate water network. More than 200 billion gallons of storm water, enough to supply 1.4 million households for a year, could be captured statewide — but instead end up spilling down sewers and drains and into the ocean, as was on display Thursday, in the hours after the rainfall ended, at the spot where the Los Angeles River ends here.

Nowhere is the disparity felt more than in parched Los Angeles, with its short winters and its overwhelming reliance on water imported from Northern California and the Colorado River. For nearly a century, since deadly floods in 1938 killed 97 people, engineers have focused on ways to flush storm water safely out of Los Angeles as quickly as possible. Now, officials want to capture that water.

“Something that was once viewed as a nuisance is now seen as a necessity,” said Eric M. Garcetti, the mayor of Los Angeles. “We haven’t done enough.”