The atmospheric river has just delivered sufficient water for rafting on the South and Middle Forks of the American River in 2015! What is an “atmospheric river”? How can we know what rafting flows will be like in 2015? How does this relate to California’s drought? If you are interested in the answers to these questions, read on!
Understanding the patterns that determine California’s weather has been a subject of heated debate among whitewater boaters for decades. No one I know explained them better than Ken Brunges, one of the founders of Mother Lode, who understood that the precipitation that makes rafting possible on the Forks of the American River usually comes in 3 to 4 large storms each year. Miss one of these storms and it is a dry year, get an additional storm and it is a wet year. More than one in either direction means drought or flood. The point was that each storm is so large that the overall outcome in California is particularly uncertain and until the rainy season is completely over, huge errors in prediction can occur. One of the best examples of this phenomenon occurred in 1991 when a “critically dry” year suddenly became a “wet” year as several large storms rolled in during the famous “Miracle March”.
Climate science has yet to overcome the unpredictable nature of California’s weather, but it has recently helped to explain it. We now know why California’s water supply is not delivered gradually over time like a drip irrigation system, but rather arrives in a few massive pulses of moisture that resemble a firehose. These pulses are being carried by the Jetstream across the Pacific Ocean from Hawaii. This “atmospheric river” of moisture can vary tremendously in width, from 20 up to 300 miles, and have lengths up to 1000s of miles. This configuration makes it look and function like a “river” transporting up to 10 million acre feet of water vapor in a single storm- 7.5 times the volume of the Mississippi! This vapor ultimately condenses into precipitation which falls either as rain or snow depending on how cold the pulse becomes as it approaches the higher elevations of the Sierras.
The reason these “rivers” eluded detection for decades, even after weather satellites became common, is that the water vapor that composes them is essentially invisible to both conventional satellite photography and radar. Only when weather data gathering became more sophisticated in the 1990s was the significance of these “rivers” fully appreciated. It is now evident that they help explain why the State of California receives most of its yearly precipitation in a period of only 5-15 days each year. The magnitude of these storms is truly world class and they are as big and wet as the largest storms that hit Hurricane Alley!
(Source: Dr. Alexander Gershunov of Scripps Institute as quoted in “Hungry Water” submitted May 4th, 2012 by Emily Underwood to Johns Hopkins University in conformity with requirements for the degree of Master in Arts in Science Writing. More of this thesis will appear soon in our blog, it is great reading!)
Given that it is only early February and the unpredictable nature of these “atmospheric rivers”, how can we already know that dam controlled rivers such as the South and Middle Forks of the American River will have rafting flows this year? The key reason is that the way we manage the flows from dams has been refined in recent years. Although the volume of water necessary to facilitate whitewater boating represents a small fraction of the total water released, it is the timing of the releases, not the total volume, that is adjusted. The result is that on the South Fork of the American River where the majority of California’s whitewater rafting occurred in 2014, rafting flows are provided based upon a collaborative agreement between SMUD, PG&E and the Federal Energy Regulatory Commission (FERC) that results in excellent recreational flows, even in a drought year.
So how much precipitation have we seen so far this season and how does it look for rafting in 2015? The National Oceanographic and Atmospheric Administration (NOAA) is one gold standard source for precipitation data in California. Its website documents the current “water year to date” (WYTD) which begins each October 1st and ends September 30th. Surprisingly, despite the fact that January was the driest such month on record, as of January 31st, 2015, Sacramento had 105% of normal precipitation! This illustrates the power of the atmospheric river which delivered its “pulse number one” in December with 160% of “normal” precipitation.
I am pleased to report that “pulse number two” has now arrived in the first week of February! Although NOAA will not provide new figures for the WYTD until the end of February, rest assured, we already know the answer to at least one pressing question: Rafting on the dam controlled South and Middle Fork is essentially guaranteed for this summer!
Other questions still remain. Will we get pulse three or even four and if so, how cold will they be? Will they end the drought? Since approximately 30% of our overall water supply is stored in the snowpack, and the snowpack is currently at very low levels (25% of normal), the drought is definitely not over! Since the North Fork of the American is not dam controlled and is exclusively fed by snowmelt, it will require much more precipitation as snow to be runnable this season. Although the snow water content is ahead of this time last year, as time goes by getting sufficient additional snow to raft the North Fork becomes increasingly improbable. Nevertheless, as we learned in 1991, we simply have to wait to see what happens. Regardless of the outcome this season, given the water deficit in underground water supplies, increasing demand and ongoing climate change, we should plan on conserving water again this summer and in all summers to come.
What we do know is that rafting flows on the dam controlled South and Middle Forks of the American are now predicted. We invite you to come enjoy them with us during the 2015 season. It’s guaranteed to be great fun!
Doing our rain and snow dance in Coloma-
See you on the river,
Scott, Charlie the RiverDog, and the MaLode Crew