Weather Window | Tahoe-Sierra mega droughts

TAHOE/TRUCKEE, Calif. — The Sierra Nevada water year for 2014 ended on Sept. 30 and the snowfall and precipitation totals aren’t pretty.

The 194.5 inches of snowfall measured last season at the Central Sierra Snow Lab tied with 1924 as third least snowiest since 1879, well under the 409 inch seasonal average. Water is more important than snow and according to the 8-Station Northern Sierra Index established in 1922, the winter of 1924 is the also the driest of record. The index is comprised of eight weather stations from Highway 50 north to Mt. Shasta City. The data from these locations are averaged to provide an overview of how much precipitation fell in any given year over the Northern Sierra.

DRY AS A BONE

The winter of 1924 was the opening salvo in what became one of the worst droughts in California history, as 1925 and 1926 also ranked in the top 10 for least snowiest winters. Low water levels at Lake Tahoe in the late 1920s and early 1930s led to massive pumping from Big Blue past the dam and into the Truckee River for Nevada agriculture. Precipitation in 2014 (a combination of rain and snow melted for its water content) came in at 39.5 inches, ranking it as the 19th driest since 1871, significantly better than the meager 18.9 inches measured at the Snow Lab in 1924. The annual average precipitation at the lab is about 51.5 inches.

Last winter a persistent ridge of high pressure over the eastern Pacific Ocean shunted most Pacific storms away from the Tahoe region. With the westerly zonal flow blocked off California, the jet stream took a more north to south route across North America, which set up last winter’s weather pattern of fierce storms and repeated cold waves in the eastern U.S.

During most winters the semi-permanent Pacific high pressure system migrates away from its position off the California coast, which allows wet winter storms to move in with rain and snow.

CALIFORNIA CRISP

Last winter’s precipitation and snowfall numbers are sobering, but the Tahoe-Sierra region is no stranger to drought. Every so often stubborn high pressure rears its head and dries California to a crisp. A lack of winter storms from 1975 to 1977 nearly dried up the state’s reservoir system. Only 183 inches of snow fell at the Snow Lab in 1977, the second lowest amount on record. The positive aspect of that drought is that it was relatively short-lived and it led to permanent water conservation in California, particularly in Bay Area cities where officials realized that they could implement comprehensive policies to reduce urban consumption by 40 percent.

We’ve had five significant droughts since the Dust Bowl era, the most recent being 2007-09. That two year dry spell cost 21,000 agricultural jobs in California’s farming regions. One of the worst droughts in the past century occurred between 1988 and 1992 when Tahoe’s water level fell to three feet below its minimum level cutting its flow into the Truckee River for more than two years. Lake Tahoe is currently projected to go below that level later this month.

STUMPED

Droughts have always impacted the region, some worse than others. During the early 1930s, Tahoe’s falling surface level exposed tree stumps off the beach near South Lake Tahoe. A University of California at Berkeley scientist, Samuel T. Harding, examined the stumps and determined that they had lived for 100 to 150 years before rising water levels submerged them. The stumps are between 4,850 and 5,530 years old; other stumps have been found deeper in the lake from Emerald Bay and Fallen Leaf Lake to Stateline. They range up to seven feet tall, three and a half feet in diameter, and up to 6,300 years old. The existence of the ancient trees suggests that between 4,000 and 5,000 years ago a much drier climate dropped Tahoe’s water level to far below the natural rim for at least 1,500 years.

Extreme droughts that occurred thousands of years ago don’t seem too threatening today, but there is evidence that desiccating dry spells are still part of our contemporary climate regime. Two prolonged dry spells occurred between 1,200 and 600 years ago. During those mega-droughts, many lakes and rivers in the Sierra Nevada dried up for decades, a disastrous scenario that makes complaints about closed boat launch facilities seem trivial by comparison.

Eventually wetter and cooler conditions returned and for the past 4,000 years the climate in the west has been generally stable. The period of modern settlement in the Sierra Nevada (about the last 150 years) has been one of the wettest intervals in several millennia. We can only hope that it stays that way.

Tahoe historian Mark McLaughlin is a nationally published author and professional speaker. His award-winning books are available at local stores or at thestormking.com. You can reach him at mark@thestormking.com. Check out his blog: tahoenuggets.com.