Aerial image at sunset of the Sierra Nevada mountains covered in clouds and snow. Photo taken on June 9 by Megan Nguyen.

By Megan Nguyen

As recently as this weekend, winter storms have brought much snow to the Sierra Nevada after five years of drought. Warm temperatures have begun to melt the mountain snow that will flow down the valley through a network of rivers. The recreation opportunities seems endless: Mammoth Resort announced they plan to stay open until July 4th or even longer. Whitewater enthusiasts are enjoying flows not seen for a decade in rivers from the North Fork American to the Merced and beyond.

But though the rivers may be tempting this summer, the high flows and cold temperatures make rivers deceptively dangerous. Rivers with high flows and cold temperatures can claim the lives of even the most experienced and skilled swimmers. Three people on the Kern River have already died and 24 were rescued. And with the large snowpack, these fast-flowing, cold conditions are likely to continue long into the summer season.

Having been in drought for the last five years, we are not used to seeing these much colder and larger flows so late in the year. Water year 2017 (October 1, 2016-September 30, 2017) has surpassed the wettest year on record (1982-83) in the Sacramento and San Joaquin watersheds. According to NASA, this winter has brought more snow than the last four years combined.

Satellite imagery showing snowpack comparison between 2015 and 2017. Source: NASA

But these extremes aren’t unusual for California. California has a Mediterranean climate, where almost no rain falls during summer. During the cooler winter, there is large variability in yearly precipitation – the most extreme in the nation – resulting in a wide potential for flood or drought in any year.

Nevertheless, California species, both human and nonhuman, have adapted to these extremes. Snowmelt recession is an important environmental cue for species mating such as the foothill yellow legged frog in the Sierra Nevada. These frogs have adapted to California’s seasons and are genetically wired to lay eggs during the spring snowmelt when river flows recede and water temperatures increase.

A female foothill yellow-legged frog (Rana boylii) waiting to lay eggs (gravid). Source: Ryan Peek

Humans have also taken advantage of California’s wet winter patterns. Snowpack reserves are an essential and natural form of water storage. On average, the snowpack provides about 30 percent of California’s water supply as it melts in the spring and early summer.

In addition, the long snowmelt season provides surface water long into the dry season. The average snow water equivalent (SWE) measures the amount of water contained within the snowpack. It can be thought of as the depth of water that would theoretically result if the  entire snowpack melted instantaneously. The snowpack measured on April 1 is the standard that typically measures peak snowpack.

As of June 1, 2017 the Central Sierra snowpack was 72% of the April 1 average, which is a dramatic increase from the last five years (Table 1.) With a SWE of 20.9” on June 1, 2017, there is enough snowmelt to keep reservoirs and rivers swollen for months to come.

Snow Water Equivalents for the Central Sierra snowpack provided by the California Cooperative Snow Surveys. Source: CDEC

However, rising temperatures induced by climate change may result in drier summer conditions and more precipitation as rain than snow. Also, earlier snowmelt may threaten California’s water supply and species dependent on snowmelt cues. Throughout the American West, scientists have already seen snowmelt starting earlier compared to historic trends, as well as an overall decrease in the average amount of snow.

We are glad to see the mountains still capped with snow as they serve as an important water storage resource for California. Be prepared for snowmelt to stream down the mountains for many more months to come. Below you will find some useful resources to help you find information such as river stage height and temperature so you can be aware of river conditions and have fun this summer season.

1. Daily Snowpack Readings
2. Current River Conditions
3. Daily Statewide Hydrologic Update

Megan Nguyen is a GIS researcher and Outreach Coordinator at the Center for Watershed Sciences. Her work and interests revolve around a variety of topics such as drought impacts, flood mitigation, environmental policy, and education outreach.

Further Reading

“Snowpack Statewide Water Content is below Average.” 2017. Department of Water Resources.

Mote, P.W. et al. 2005. Declining mountain snowpack in Western North America.

Stewart, I.T., et al. 2004. Changes toward earlier streamflow timing across Western North America.

Peek, R., H. Dahlke, S. Yarnell. 2016. Linking water source signatures with native amphibian breeding timing in a Northern Sierra Nevada watershed. Hydroecology C10. Presentation for Annual Meeting at Society for Freshwater Science, Sacramento CA.