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Unprecedented Climate Extremes: One Year After Record Drought, Lake Oroville is Spilling Over

We know that climate change pushes the weather toward extremes, but this is getting ridiculous. In California, in less than a span of 24 months, water levels at a key reservoir have shifted from record drought to a flood that’s now endangering the state’s water supply system. Unfortunately, it’s these kinds of extreme shifts that we’ve come to expect from human-forced climate change.

Record California Drought

During 2015, California experienced its hottest winter on record. The same winter was also California’s driest in 65 years. It was an extremely dry season that occurred during one of the most intense droughts ever to strike California (2011 through 2016). A period that included the worst dry spell ever to affect the state (2011 through 2014).

driest-period-on-record-for-california

(2011 to 2016 included the driest period on record for California producing extreme water stress for the state. Image source: The US Drought Monitor.)

A 2015-2016 El Nino brought hopes of rain. It also brought concerns that when the rains did finally arrive, they would come as deluges. This concern was driven by the fact that the warming atmosphere now holds an unprecedented amount of moisture. With much of that extra moisture bleeding off of the Pacific Ocean and with El Nino producing a tendency to both intensify the Pacific storm track and to aim rivers of moisture at California, these concerns appeared to be at least somewhat valid.

But, for the most part, the rain held off — increasing concerns that a drought that had already lasted for five years could continue. That an odd weather pattern called the Ridiculously Resilient Ridge might be a semi-permanent feature spurred by warming in the Arctic and a related movement of the Jet Stream northward.

Followed By Record California Flooding

The Jet Stream did flatten and the rains did eventually come, however. And when they did, it was at the oddest of times — when El Nino had long since faded and a weak La Nina — which typically brings drier conditions to the U.S. West Coast — was in full bloom. By January of 2017, the pattern had switched. And when it switched, it switched hard.

A massive river of moisture began to flow from the Philippines all the way to California during December of 2016. The atmospheric river linked up with a raging storm track running 6,000 miles across the Pacific from Japan to the U.S. West Coast. And this combined moisture flow and vigorous storm pattern has pummeled the U.S. West Coast for the better part of six weeks.

river-of-moisture-2017

(Throughout the winter of 2016-2017, a powerful, 6,000 mile long, river of moisture has produced a succession of strong storms running into California. This weather/climate feature is occurring in a record warm/moist atmosphere. The result has been that conditions in California have shifted from extreme drought to extreme flood. Image source: TPW Version 2.)

Some regions of California experienced their wettest January on record. Sacramento was one of these. Throughout California, records for the top ten wettest comparable periods were shattered. According to the Washington Post:

…by one important measure, there’s been more rain and snowfall in the 2016-2017 water year than any other season on record, to date. The California-Nevada River Forecast Center uses an eight-station index in the North Sierra to quantify the region’s precipitation. As of Feb. 12, these eight stations have received 68 inches — 226 percent of normal.

In the region of Lake Oroville — a reservoir that as recently as 2015 had dropped to extreme low levels — the rainfall has been particularly consistent and heavy. And it now appears likely that the winter of 2016-2017 will be the wettest on record for that region at least.

Weather Extremes Damage Critical Water Infrastructure

The Lake Oroville Dam had never seen so much water flowing into its backing reservoir since its completion in 1968. By January, Dam operators were already releasing considerable flows of water through its primary spillway to reduce pressure off the 800 foot tall structure trapping water within the reservoir. By February, more than 55,000 cubic feet of water per second was sent raging down the spillway in an effort to keep water levels below the over-topping line. Unfortunately, the spillway structures supporting the Dam have likely never seen so much continued stress from strong water outflows related to record high water levels. And as of last week, the powerful floods of water released from the Dam had damaged the primary spillway. The spillway’s concrete apron had eroded and initially produced a 300 foot wide sink hole near the top of the spillway that later expanded.

(Lake Oroville forced to use emergency spillway resulting in severe stress to key California water infrastructure. Video source: KCRA.)

Concerns about how an expanding sink hole in the reservoir’s wall could, in the worst case, breach the Dam wall and result in a catastrophic failure spurred operators to shut down and reduce water flows through the primary spillway. The abatement resulted in water levels at Lake Oroville rising to above 901 feet. This triggered an automatic over-spill into a second emergency spillway (the first time this has happened in the Dam’s history). But over-topping water also produced severe erosion — igniting more concerns of structural failure. And on the weekend of February 10th -12th, nearly 200,000 people were evacuated from the Dam’s outflow zone as a potential catastrophic structural failure could cause a 30 foot wall of water to rush through numerous downstream communities.

Over recent days, rainfall in the Oroville region abated — providing a brief window for repairs and reducing stress to the Dam. Round-the-clock emergency repairs in the form of bags of boulders used to buttress the Dam appeared to have shored up the Dam. Meanwhile, water levels within the Dam earlier this week were dropping by 4 inches per hour. Mandatory evacuation orders were lifted. And downstream residents began to trickle back in.

noaa-7-day-precipitation-forecast-lake-oroville

(More heavy rain on the way is likely to continue to produce a touch and go situation for the Lake Oroville Dam. If heavy rain continues through spring melt, the Dam could face considerable additional challenges. Image source: NOAA.)

However, the underlying weather conditions that caused so much damage to the Lake Oroville Dam have not yet changed. February and March are typically California’s wettest periods. And the massive river of moisture feeding into a powerful Pacific storm track continues unabated. Over the next 7 days, NOAA predicts that as much as twelve and a half inches of rain could fall on the Lake Oroville region.

Harmed by Drought, Harmed by Flood

So much rainfall will again likely necessitate considerable water outflows from the Dam’s damaged spillways — producing more stress to the already burdened structure. In addition, the arrival of warmer weather come March and April will add snow melt to the already considerable rainfall inflows coming into the Oroville system. To be clear, most experts still think that the overall risk of losing Oroville due to a complete failure of the Dam remain low. However, such a loss would be catastrophic to California.

more-heat-more-heavy-precipitation

(As the climate warms, it produces more record hot weather — which spurs increasing instances of drought. In addition, when precipitation does fall, it tends to come in the form of more heavy precipitation events where the rain that does fall, tends to fall more intensely over a shorter period. As a result, the human forced warming of the Earth is producing a general tendency toward more extreme instances of drought and flooding. Image source: NOAA/UCAR.)

The Lake Oroville reservoir provides drinking water to 23 million residents in California and irrigates 750,000 acres of farmland. In the outside worst case event where the Dam does fail, it would produce a water crisis for numerous residents and communities in addition to any damage caused by severe downstream flooding. But even if the Dam holds through the Spring, extreme deposition of sediment from heavy water flows running into the reservoir will also likely pose challenges to water access.

It’s a case of too much or too little. From 2011 through 2016 drought threatened Lake Oroville’s water supplies. Now it’s flooding. And unfortunately, with climate change, we can expect the weather in many regions to take on extreme characteristics or switch hard from one extreme to the other — as has been the case with California.

Links:

Climate Central

The US Drought Monitor

TPW Version 2

Stress Test isn’t Over for Lake Oroville

Record Rain is Straining California’s Whole Flood Control Network

KCRA

NOAA

Lake Oroville Critical to California’s Complex Water System

NOAA/UCAR

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