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How an ‘unripe snowpack’ will impact Lake Tahoe this summer

SF Gate logo SF Gate 4/25/2019 Amy Graff
a view of a snow covered mountain: Aerial view of Lake Tahoe on a sunny winter day in California. Sierra mountains covered in snow are visible in the background. © Provided by Hearst Newspapers Aerial view of Lake Tahoe on a sunny winter day in California. Sierra mountains covered in snow are visible in the background.

For the third year in a row, Lake Tahoe is expected to fill.

This is noteworthy for the sixth-largest lake in the United States that flirted with record-low levels amid a five-year drought that ended in 2017.

Even more good news for the West's water supply: Tahoe's water level is likely to reach its peak late in the season as a robust snow pack slowly melts through summer, feeding the reservoir and the Truckee River continuously for months to come.

"There won't be a lot of time between when it fills and when it starts snowing again," explains Chad Blanchard, the U.S. District Court water master who tracks water between Lake Tahoe and the lower Truckee River. "That means we have more water to carry over and if we do go back into a dry cycle, it'll prepare us better."

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Straddling the California-Nevada border, Tahoe is a renowned outdoor playground enjoyed by people from around the world. It's also the main water source for the Reno-Sparks, Nev., area. The lake is fed by 63 tributaries that drain 505 square miles known as the Lake Tahoe Watershed. With a vast surface area of 191 square miles, Tahoe requires an immense amount of water to fill, especially because roughly 100 billion gallons of water evaporates annually.

Blanchard explains the lake reaches its peak when the amount of water flowing into the lake matches the amount lost due to evaporation.

"As temperatures warm, evaporation is starting to climb, but inflow is also climbing rapidly with the melting snowpack," he says. "The lake will rise as long as the inflow is greater than evaporation."

Depending on the amount of rain and snow that fell in winter, evaporation typically equals inflow any time between April and August. At this point, the flood gates on the Lake Tahoe Dam are closed and water is no longer released into the Truckee River. The lake is "topped off," so to say.

Blanchard can't say exactly when the lake will peak but he suspects it will happen later in the season, maybe late July or early August. That's because the watershed is currently under a near-record snowpack after a winter of severe weather.

He describes the current state of the snowpack as "unripe," as it's massive and cold. While snow has melted at lower-elevations below the lake, the higher level snow above 6,200 feet is still well-established.

"Things are starting to pick up, and the lake is starting to see more inflows from snowmelt, but it has been limited considering how late we are," says Blanchard. "We're just starting to see some of the higher stuff melt. We'll see what happens after about a week worth of warm weather."

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When the lake hits its peak, it will likely be at full capacity, give or take a couple inches. The lake's natural rim is at 6,223 feet above sea level. The lake can store an additional 6.1 feet in its reservoir, which would push it to 6,229 feet, its legal maximum limit.

The dam at Tahoe City, the lake's only outlet, regulates the upper 6.1 feet above the low water mark. This winter, small amounts of water have been released into the Truckee River since February.

As of Tuesday, the lake level was at 6,227.94 feet with only about a foot of rise needed to hit full capacity. In coming weeks, Blanchard will be watching the weather and the snowpack closely to determine how much water to release and prevent flooding on the Truckee River. It's a tricky dance to ensure as much water as possible is stored.

RELATED: Readers' photos show off Lake Tahoe, brimming with water and beautiful

With two of the three past winters seeing record-breaking snowfall, Lake Tahoe reached full capacity in 2017 and 2018.

After an extended period of drought, the lake saw a significant rise in 2017. That year, the water level shot up 6.5 feet. The equivalent of three feet was also released through the dam. In 2017, the lake peaked in early July. There was less snow the following year, but the lake still filled with significant carryover from the year before. In 2018, the peak was in May.

"A lot of things are driving the lake's level," says Blanchard, but long story short, it comes down to how much water is coming in and how much is coming out.


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