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Los Angeles - Twelve years ago, widespread destruction from Hurricane Katrina on the Gulf Coast helped compel federal engineers 3 200km 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 $1bn auxiliary spillway at Folsom Dam, scheduled to be completed later this year, stands in contrast to the troubles 121km 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 9m 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 The structures also underscore the challenge of maintaining older dams with outdated designs.
Emergency spillway
"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 specialising 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 US Rep. Doris Matsui, a California Democrat who was among those pushing Washington for improvements at Folsom Dam, perched 40km 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.
State officials now face questions about maintenance at Oroville Dam, the nation's tallest at 235m and why a decade ago they dismissed warnings from environmentalists that more needed to be done to strengthen its earthen emergency spillway.
After years of drought, Northern California has become waterlogged this winter from heavy rain and snow. Oroville Lake is brimming and water managers have been using the main spillway, which is lined with concrete, to lower the water level.
The emergency spillway is a brush-covered hillside below a masonry lip and had never been used until last weekend. When water gushed onto it, the ground began eroding and it was feared the intake lip could collapse and water would surge down the hill.
An investigation into what went wrong could take months.
Prolonged drought
John France, vice president and technical expert on dams for the engineering consulting firm AECOM, said the problems at Oroville should raise alarms across the country.
"Most of the dams in the United States are over 50 years old, when we didn't understand floods as well as we do now. So we have a number of dams in the US that have spillways that aren't large enough for the floods that they should be designed for," France said.
Arguments over water, how it's stored and who gets it are as old as the state and California has often been criticised for not doing more to improve its water systems as the population edges toward 40 million.
In a place known for alternating cycles of prolonged drought and biblical rainfall, the idea of asking reservoirs to store extra water for dry times frequently clashes with environmentalists, who want to see rivers flow freely.
At Oroville, opened in 1968, construction crews recently patched cracks on the main spillway and a state inspector judged the repairs "sound" in a February 2015 report. However, a gaping hole ripped open on that spillway two weeks ago, starting the series of events that led to use of the emergency spillway and evacuation orders for nearly 200 000 people.
Since then, crews dumped thousands of tons of rocks to shore up the damaged spillways and residents were allowed to return home on Tuesday after the lake level dropped.
Butte County Supervisor Bill Connolly, whose district includes the dam, has criticised dam managers for years.
"They never spend money unless they have to," he said. "If we were a federal facility, you don't see this happening."
Major storm
At 104m high Folsom Dam, operated by the US Bureau of Reclamation, the huge concrete-and-steel chute being constructed by the Army Corps of Engineers is expected to add as much as 40% capacity to the main spillway that controls water flowing from the reservoir behind it.
It's designed to allow safer releases during times of high water, precisely the challenge that led to fears of catastrophic flooding at Oroville.
Rick Poeppelman, chief of the Army Corps engineering division in the Sacramento district, said extensive data about probable maximum flood levels, not available decades ago, helped prompt the decision to build the auxiliary spillway at Folsom Dam.
When completed, the spillway can act like a second dam, allowing operators to release water through a series of gates and lower the reservoir level when a major storm is approaching. It includes a 500m chute. Another advantage is that the new spillway gates will be15m lower than those on the dam, allowing for earlier releases of water.
The government needs to test the spillway this year to see how it will work. Meanwhile, there are proposals to increase the height of the dam with the potential for extreme weather.
