The process of the Sierra Nevada spring snow melt sped up this week as a heat wave brought triple-digit temperatures to parts of the Western United States.

A thawing snowpack that's massive after a brutal winter fed rivers and reservoirs with high-flowing runoff.

One place that saw a stunning impact from all the runoff is Lake Tahoe.

More than 12 billion gallons of water poured into the lake over the past week.

That's a staggering amount of water, and resulted in the lake level rising four inches since June 16.

That rise occurred while intense heat increased evaporation rates from the lake's surface. What's more, water managers have been releasing water from the lake into the Truckee River for the past 120 consecutive days to make room for snow-melt runoff.

"It's not typical to spill at all," says U.S. District Court Water Master Chad Blanchard. "It's only on the big years when you have to release water."

To further understand the significance of a four-inch gain in a week consider that during the spring snow melt season at the height of the California drought in 2015, the lake rose two and a half inches across several months.

With all this water, the lake is now only three inches from filling — and will likely reach full capacity in mid-July, Blanchard says.

If it fills it will be for the first time in 11 years, and the total amount it will have risen across the water-year between Oct. 1 and Sept. 30 will be record-breaking.

Straddling the California–Nevada border, Tahoe is the sixth largest lake in the United States, an outdoor playground for people around the world, and the main water source for the Reno-Sparks area in Nevada, as reported previously on SFGATE. The renowned ecological wonder 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.

Lake Tahoe's natural rim is at 6,223 feet above sea level. The lake can store an additional 6.1 feet in its reservoir and climbs up to 6,229.1 feet at full capacity, its legal maximum limit. The only outlet, a dam at Tahoe City, regulates the upper 6.1 feet above the low water mark, and this winter water is being released into the Truckee River as billions of gallons flow into the lake.

When thunderstorm clouds moved over lake Tahoe on June 18, 2017, it created the perfect tableau for a gorgeous sunset.

As of Friday morning, the lake was at 6,228.84 feet, and Blanchard says they are releasing 800 cubic feet per second this week to prevent flooding. Releases in June have reached as high as 1,600 cubic feet per second.

The lake still needs some 30,000 acre feet of water to fill and a lot more than that will be dumping into Tahoe in coming months.

"All that high snow lasts so long," Blanchard says. "We're trying manage it to ensure that we fill, but we have to be able to adjust and compensate for variations in the actual runoff compared to the forecast. We could have filled it months ago, the inflow can be so much higher than your release capacity."