LOS ANGELES — Twelve years ago, widespread destruction from Hurricane Katrina on the Gulf Coast helped compel federal engineers 2,000 miles away in California to remake a 1950s-era dam by constructing a massive steel-and-concrete gutter that would manage surging waters in times of torrential storms.

The nearly $1 billion auxiliary spillway at Folsom Dam, scheduled to be completed later this year, stands in contrast to the troubles 75 miles away at the state-run Oroville Dam, where thousands of people fled last week after an eroded spillway threatened to collapse — a catastrophe that could have sent a 30-foot wall of floodwater gushing into three counties.

Together, the two dams illustrate widely diverging conditions at the more than 1,000 dams across California, most of them decades old. The structures also underscore the challenge of maintaining older dams with outdated designs.

“Fifty years ago, when we were evaluating flood risk, the fundamental assessment was the climate was stable, not changing. We now know that is no longer true,” said Peter Gleick, chief scientist with the Pacific Institute, a California-based think tank specializing in water issues.

“We need to look at the existing infrastructure with new eyes,” he warned.

Back in 2005, Katrina’s deadly path became an arguing point for U.S. Rep. Doris Matsui, a California Democrat who was among those pushing Washington for improvements at Folsom Dam, perched 25 miles from 500,000 people living in Sacramento, the state capital.

“I used that, vigorously, to say we are the second-most at-risk river city in the nation,” Matsui said, after the Category 5 storm swept through New Orleans.