Will 2017 be the wettest year ever for California?

2017 has been a fun year for those of us who enjoy winter storms - with notable and important exceptions in Oroville and other places where floods threaten lives and livelihoods.

It has been really  wet. Water year 2017 (measured October '16 through September '17) may turn out to be the wettest year on record - going back to 1922.

 split_rock.jpg

As a rafting guide for Whitewater Voyages, I got my passengers got a little wet at Split Rock Rapid on the Merced River during the spring snowmelt of 1979. That section of river was exciting but seemed safe enough. I am glad to this day, however, that I declined to run the lower "Quarter Mile" section - where being swept over North Fork Falls is a distinct possibility at high water. 

Photo Isabella Rosekrans

There are different ways to measure how wet California's water year is. I like to use the 10-river index: the total runoff of 10 principal rivers in the Central Valley ranging from the Sacramento River in the north to the San Joaquin in the south. This metric ignores the coastal regions as well as the mostly dry southern half of the state, but it incorporates most of the water that our cities and farms depend on, as well as the rivers that feed the Bay-Delta estuary.

Figure 1 compares the runoff we expect in 2017 along these 10 rivers, listed from north to south, to their average values and to their values from 1983 - the wettest year to date in the hydrologic record. The values for 2017 in Figure 1 include uncertainty - where 50% indicates what we "expect", and the 90% and 10% values provide a range of possible outcomes - since we know neither how wet the spring will be nor exactly how much snow is in the Sierra.

At this point it looks like the largest of these rivers - the Sacramento (measured at Bend Bridge) will not set any records, but other Sacramento Valley Rivers - the Feather, Yuba and American - will indeed have the highest flow in recorded history.

The Tuolumne River's "expected" flow in 2017 trails the record value in 1983 by only 1%. The "10%" value for the Tuolumne in 2017, as currently projected, would surpass the 1983 value by 10%. So it is a very close call as to whether 2017 will be the wettest year ever on the Tuolumne. 

The total runoff for these rivers in 1983 was calculated to be 55,744,000 acre-feet, and the "expected" total for 2017 is presently only 54,305,000 acre-feet - about 2.5% less. So according to State forecasters, 2017 is unlikely to surpass 1983. But it surely could. 

 Figure_1.jpg

For 2017, the "90%" value means 9 times out of 10 there will be more flow. "50%" is the median value - i.e. it is equally likely to be wetter or drier. The "10%" values means there will be more flow only 1 out of ten times. 

Figure 2 shows how much of this flow occurred between October and March, and how much is expected to occur after April 1 (with a 50% probability). Note that the majority of the runoff in the northern Sierra (Sacramento, Feather, Yuba and American Rivers) has already taken place. In the southern Sierra, where the mountains are higher, more of the precipitation has fallen as snow, and runoff will take place later in the year. Waterfalls in Yosemite and elsewhere will be spectacular well into July.

 Figure_2.jpg

The spring and summer in the Sierra will be wonderful indeed. Many trails will not be open until late in the year, however, and it is essential that hikers be prudent when deciding whether to cross certain streams. It will be a great year for rafters and kayakers as well, but they also need to be especially careful not to venture out when flows are at unsafe levels.