Snowmelt is Starting in California’s Sierra Nevada Mountains

Caitlin Dempsey


Following a precipitation season with well above average rain and snowfall in California, researchers are keeping a close eye on the anticipated flooding from melting snow.

California’s Sierra Nevada mountain range

The Sierra Nevada mountain range, stretching over 400 miles from north to south along the eastern edge of California, plays a critical role in the state’s water resources.

The Sierra snowpack acts as a natural reservoir, storing water throughout the winter months and gradually releasing it as the snow melts during the spring and summer.

This runoff flows into rivers and reservoirs, providing water for millions of Californians and supporting agriculture, hydropower generation, and ecosystems.

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Approximately 30 percent of California’s water supply comes from the Sierra snowpack, making it an essential component of the state’s water management strategy. The snowpack’s health is closely monitored, and SWE measurements are used to forecast water availability for the upcoming months.

Snow among conifer trees.
Snow among the red first in the Sierra. Photo: Nate Stephenson, USGS, public domain.

Snowpack in the Sierra in 2023

The snowpack in California’s Sierra Nevada mountains on April 1, 2023 was more than four times the average (439%). Many of California’s mountainous region had snowpacks well above average for this time of year. (April 1 is considered the peak of snow pack in California by researchers)

NASA reports that mountains in the central section of the Sierra had a snowpack that was 284 percent of the average, while the northern slopes had a snowpack that was 271 percent of the average.

This map, created by NASA, compares the current snow water equivalent (SWE) for the mountains that fringe the eastern side of California’s Central Valley. Researchers use data collected from Earth observation satellites to map and calculate snowpack levels.

A faded shaded relief map of the Northern part of california showing snow water equivalent with dark blue areas having a SWE closer to two meters.
Snow water equivalent in the Sierra Nevada mountain range. Map: NASA.

What is snow water equivalent?

Snow Water Equivalent (SWE) is the amount of water that would be left if a snow layer were to melt completely. It helps us understand how much water is stored in the snow and is important for managing water resources and predicting floods.

Ground measurements of snowpack levels and snow water equivalent

The California Department of Water Resources (CA-DWR) use ground measurements to calculate snowpack levels. Their April 3, 2023 survey of 130 snow sensors across the state showed that the snow water equivalent was 61 inches (1.55 meters), amounting to 237 percent of the average for that date.

What affects the Sierra snowpack each year?

Several factors affect the amount of snow that accumulates in the Sierra Nevada each year, including weather patterns, temperature, and precipitation.

The top satellite image shows the snowpack in the Sierras in California in 2022. The bottom satellite image shows the snowpack in the Sierras in 2023.
What a difference a year makes. Snowpack in the Sierra Nevada mountains on April 1, 2022 (top image) was far less than the snowpack in the Sierra on April 6, 2023. Images: NASA’s Terra satellite, public domain.

Climate change has also emerged as a significant factor affecting the snowpack, as warmer temperatures can lead to a decrease in snow accumulation and an increase in rain, reducing the overall water storage capacity of the snowpack.

The snowpack’s size and water content can vary greatly from year to year, with some years experiencing abundant snowfall and others seeing meager accumulations.

These fluctuations can have significant impacts on California’s water resources and make planning for water management a complex and challenging task.

2022-2023 was a season of increased atmospheric rivers

Overall, California experienced about 39 atmospheric rivers for the 2022-2023 water year.

Atmospheric rivers are long, narrow bands of concentrated moisture in the atmosphere that transport large amounts of water vapor across vast distances. These phenomena are typically several thousand kilometers long and a few hundred kilometers wide.

Atmospheric rivers are responsible for transporting water vapor from tropical and subtropical regions towards the mid-latitudes, often leading to heavy precipitation when they make landfall on coastal areas.

Atmospheric rivers are responsible for bringing a significant portion of California’s precipitation each rainy season.

A true color satellite image showing an atmospheric river off the coast of California.
Satellite image taken January 4, 2023, at 1:20 p.m. of an atmospheric river affecting California. Image: NOAA-20 satellite, public domain.

For the 2022-2023 wet season, California experienced numerous atmospheric rivers rated in the strong category.

Strong atmospheric rivers are intense, long bands of concentrated moisture in the atmosphere that transport large amounts of water vapor across vast distances. They can lead to heavy precipitation, causing significant rainfall or snowfall events, often resulting in flooding, landslides, and other natural disasters when making landfall.

The Southern Sierra Nevada in California has experienced 11 moderate atmospheric rivers, which is twice the average of 5.5, along with four additional strong atmospheric river events.

The snowmelt is coming

In increase in precipitation arriving via atmospheric rivers with colder winter temperatures is what has help to propel snowpack levels in the Sierra this year.

But with increased snowpack comes increased snowmelt.

As the weather turns towards more mild spring days and the amount of warm sunlight increases each day, the snowpack lying in the Sierra has started to melt.

Researchers have been forecasting a higher than average runoff this year for rivers due to the increased snowpack and snow melt potential. Related: Mapping Snowpack and Forecasting River Rise in California

As the snow pack starts to melt, some of the water will evaporate, some of the water will be absorbed by the ground, but a large amount of the snow will become runoff. This runoff enters California’s streams and rivers but also will increase flooding, particularly at lower elevation such as the Central Valley.

Already, dry lakes such as California’s ancient Tulare Lake are started to re-emerge in the Central Valley as a result of flooding from the increased rains and snow melt in the state.

Many of California’s reservoirs are already at above average or are nearing capacity. As the snow melt in the Sierra Nevada starts, river and reservoir levels will continue to rise.


California’s Snowpack is now one of the largest ever, bringing drought relief, flooding concerns. (2023, April 3). Department Of Water Resources.

Cassidy, E. (2023, April 19). A boom year for Sierra Nevada snow. NASA Earth Observatory.

Hecht, C. (2023, April 17). Epic snow from all those atmospheric rivers in the west is starting to melt, and the flood danger is rising. The Conversation.

Sierra Nevada SWE reports. (2023, April 21). Institute of Arctic and Alpine Research.


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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.