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RiverFlood

From Drought to Deluge

Jeffrey Mount January 10, 2017

The recent change in the weather is prompting many Californians to shift their worry over drought to fretting about floods. That’s an understandable response to California’s volatile climate, which is the most variable in North America. Most notable this year is the return of atmospheric rivers—river-like bands of moisture that periodically stream into California, often from the tropics. These storms are responsible for most of our large, devastating floods. They are also critical for our water supply, providing roughly half of our precipitation in normal and wet years. When atmospheric rivers don’t occur, we usually have a drought.

Here are a few key takeaways from this welcome stretch of wet weather:

  • This is likely to be the end of the surface-storage drought for most of the state. In other words, by next week almost all the major reservoirs will be at or above their seasonal averages (there’s a good summary here)—conditions we have not seen in six years. This is great news since reservoirs are the primary source of water for cities and farms.
  • Many multipurpose reservoirs—those that supply water, hydropower, and flood storage—are well above historic averages. To maintain their flood management capacity for future winter storms, these reservoirs are required by federal rules to release large amounts of water, which is why so many rivers below dams are running high.
  • It is reasonable to be optimistic that our reservoirs will fill this spring. We rely on melting snowpack to top up reservoirs in the spring and to provide roughly a third of the state’s water supply in an average year. The warmth of the weekend storms washed away some of this snowpack, particularly in the middle elevations of the Sierra Nevada (5,000–7,000 feet). Still, higher elevations accumulated a great deal of new snow with these storms, and more than two months remain in the winter snow season.
  • One wet year is not a drought buster. During the height of the drought, the state’s farmers and others turned to groundwater pumping to make up for surface water shortages. The water deficit in our aquifers is now immense. For example, in 2014 and 2015, surface water supplies to farmers in the Central Valley were cut nearly in half, causing them to make up most of this reduction through additional groundwater pumping or land fallowing. It would take many successive wet years—and more intentional groundwater capture and storage—to restore aquifers to the condition they were in before the onset of drought.

The wet beginning to 2017 is a welcome relief from the past five dry years. Full surface reservoirs take pressure off water users and water regulators. But the rains did not wash away California’s major water challenges. There are big decisions ahead—many to be made this year—about how to sustainably manage groundwater, improve storage, resolve the problems of the Delta, and arrest the decline of our native fish and wildlife. Maintaining momentum on these issues is as critical now as when the reservoirs were low.

Learn more

Read California’s Water (PPIC briefing kit, October 2016)
Visit the Policy Priorities for California’s Water YouTube page
Visit the PPIC Water Policy Center flood resources page

Photo by Carson Jeffres


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