California’s Drought is Driving Reservoirs to Near Historic Lows in 2021

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California has entered its second significant drought in the last decade. Below average snowfall, lower than average winter precipitation, and the recent heatwave have resulted in many reservoirs in the state that are starting out the summer months at very low levels.

California’s water year 2020 was the 13th driest in terms of statewide precipitation and the 5th driest in terms of statewide runoff. For example, in the Northern Sierra water region, rainfall for the current water year is just 23.1 inches compared to the 1966-2015 average of 51.8 inches.

The U.S. Drought monitor categorizes areas depending on the severity of the drought and the outlook is currently very dire for California. The entire state is currently experiencing some level of drought with 94.8% of the state in at least the severe drought category (D2). 85.4% of the state is in the extreme drought category (D3) and while about a third of the state is experiencing the highest category of drought: exceptional drought (D4).

June 15 drought conditions map for California.
June 15 drought conditions map for California. Source: U.S. Drought Monitor

The lack of rainfall and snowmelt feeding the state’s water system has already triggered a declaration of drought in 41 of California’s 53 counties. Snowpack in the state had rapidly decreased since April 1 to near zero.

With very low soil moisture levels after two seasons of below-average rainfall, much of the snowmelt was absorbed by parched soil instead of replenishing reservoirs. Many reservoirs are well below average in water storage. Across the state, the average reservoir storage at the end of May was 67% of average.

California’s Vanishing Snowpack

California relies on spring and summer snow melt to replenish many of its reservoirs. Snowfall on the Sierra Nevada, Rocky Mountains, and other ranges slowly melts and flows as freshwater down into river basins each spring and summer.

This snowmelt eventually makes its way to many of California’s reservoirs to help the state with its water needs during the dry summer months. Normally, the Sierra Nevada snowpack makes up about 30 percent of California’s water supply.

This year, the Sierra Nevada received just 59% of normal snowfall by April 1, 2021. Driven by warm spring temperatures, the snowmelt began 3-4 weeks early in 2021 and by June, snow cover in the mountains was essentially zero, leaving no runoff to replenish the state’s waterways.

This time-series of satellite images shows the rapidly melting snow cover in the Sierra Nevada.

Satellite images showing snowmelt in the Sierra Nevada mountains from March 31 - July 7, 2021.
Snowmelt in the Sierra Nevada mountains from March 31 – July 7, 2021. Source: NASA’s Terra satellite

What is a Reservoir?

A reservoir is a big natural or man-made lake that serves as a water supply. California has more than 1,500 reservoirs as part of its water grid managed by various federal, state, and local agencies.

NASA recently released satellite images for two of the state’s dams showing how low the water levels are.

Lake Oroville’s Water Level Has Dropped

As of June 21, 2021, Lake Oroville is at 34% capacity, with just 1,219,189 acre foot of water compared to its ability to store 3,537,577 acre feet of water.

These side-by-side satellite images show how much the water level in Lake Oroville has dropped since 2019. Between June 1019 and June 2021, the water level dropped 190 feet (58 meters) from 895 to 705 feet above sea level. Water levels are dropping so low that officials are warning that Lake Oroville’s Edward Hyatt Power Plant might have to shut down for the first time since it was built in 1967.

The light outline in the 2021 satellite image is the exposed bank of the lake from the water level dropping.

The light outline is known as the “bathtub ring”. The lighter area is caused when calcium carbonate and other mineral components, many of which are different salts in the water, bind to the sandstone and form a white imprint. The high water mark is near the top of the white mark. When reservoirs are at or near capacity, the bathtub ring is cover by water.

Side by side satellite images showing the water level in Lake Oroville between June 2019 (left) and June 2021 (right).
Side by side satellite images showing the water level in Lake Oroville between June 2019 (left) and June 2021 (right). Images: NASA, Landsat.

The historic low for Lake Oroville is 645.11 feet which was reached in September of 1977 after a dry season.

Shasta Lake

Shasta Lake is also experiencing lower than average water levels. Shasta Lake is California’s largest reservoir and third largest body of water.

Shasta Lake is currently at 40% of capacity with a water level that has dropped 106 feet (32 meters) in elevation since June of 2019.

Shasta Lake in June of 2019 (left) compared to June of 2021. Images: NASA, Landsat.
Shasta Lake in June of 2019 (left) compared to June of 2021. Images: NASA, Landsat.

Southern California Reservoirs

The Copernicus Sentinel-2 mission captured the state of three Southern California reservoirs in Angeles National Forest located north of the city of Los Angeles. The green hillsides in June of 2020 are now showing scars from the Ranch 2 and Bobcat fires which burned during the summer 2020.

The three reservoirs, Morris, Cogswell, and San Gabriel, all show the tell-tale “bathtub rings” of lower than average water levels.

Side by side Sentinel-2 images showing Angeles National Forest in June 2020 (left) and June 2021 (right).
Reservoirs in the Angeles National Forest in Southern California in June 2020 (left) and June 2021 (right). Satellite images: contains modified Copernicus Sentinel data (2020-2021), processed by ESA, CC BY-SA 3.0 IGO

California River Flow Affected by the Drought

Many California rivers have also been affected by the drought. This map with callouts shows natural flow compared to the average for several of California’s rivers for 2014, 2015, and 2021 (all drought years).

Full natural flow at DWR forecast points on selected California rivers shown as a percent of average to date. Map: California Department of Water Resources.
Full natural flow at DWR forecast points on selected California rivers shown as a percent of average to date. Map: California Department of Water Resources.

References

Carlowicz, M. (2021, June 17). California reservoirs reflect deepening drought. NASA Earth Observatory. https://earthobservatory.nasa.gov/images/148447/california-reservoirs-reflect-deepening-drought

Carlowicz, M. (2021, July 14). Sierra Snowbank short on funds. NASA Earth Observatory. https://earthobservatory.nasa.gov/images/148565/sierra-snowbank-short-on-funds

Drought + heat = increased impacts. (2021, June 17). Department of Water Resources. https://water.ca.gov/News/Blog/2021/June/Drought-Heat-Increased-Impacts

This article was originally published on June 21, 2021 and updated on July 14, 2021 with new information.

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