A "What-If?"

Spring, 1933 - The area of California known as "Long Valley"1 has been shaking for months. Many of the numerous hot springs in the area have been getting hotter and several ranchers have been surprised to discover cattle, dead of asphyxiation, laying in low hollows in the land.

(several of them have felt "woozy" when leaning down to check out their dead cows as well, though no one has died of it yet2) Geological map of Long Valley, from http://lvo.wr.usgs.gov/GeologicMap.html

June 16th, 1933 - Residents of the Long Valley area are awakened by the shock of a massive, 7.2 earthquake that rocks the region at 6:32 a.m. and is felt as far away as San Diego. It is the last thing most of them will ever feel.

In Bishop, the area's largest "city" (at two-thousand people3), thirty-five miles to the south, nearly every major structure collapses. The survivors crawling from the shattered buildings look northward in open-mouthed shock as a dark-gray pillar of cloud rockets upwards, filling most of the northern sky.

The Long Valley Caldera has awakened from its seven-hundred and sixty-thousand year nap.

June 17th, 1933 - Had this world yet experienced such, they could have compared the explosive eruption of the Long Valley Caldera as being the equivalent of four Mt. St. Helens's.4

As it's a half-century too soon for that, all they can compare it to is a good slice of hell.

By now the ash cloud stretches east to the Utah border5 (the sun doesn't rise in Tonopah this day, but ash falls), south to Death Valley, and north nearly to Carson City. Due to winds and the Sierra Nevadas, it hasn't spread much more than thirty miles westwards6, but from Sacramento residents see a dark wall like a second, much taller Sierras, rising above the real ones to the east.

By now, the number of survivors from the Mono Lake to Lone Pine area is down to the couple of hundred who managed to move south or north away from Long Valley fast enough. Few are uninjured - and everyone is coughing like long-term victims of emphysema from the ash. Several thousand are feared dead - rightly - from the initial eruption and people everywhere in the shadow of the ash cloud are fleeing in terror from it. Such fleeing, of course, is causing its own set of casualties... Los Angeles Aqueduct - image from http://www.laep.org/uclasp/ISSUES/bringing_water/title.htm

In Newspapers and on radio across the world, the number one story is the eruption. Down in Los Angeles, though, while as shocked by the loss of life as anyone else, bigger worries are now beginning to worm their way through the Department of Water and Power. By early afternoon, the last trickle of water flows through the Los Angeles Aqueduct into the reservoirs in the Mojave - and it is more ash-gray mud than water.

The Los Angeles Aqueduct is - for most of its length - a simple open ditch. Now the northern half of that ditch is a line of muddy-ash a hundred miles long covered with even more ash and most of the actual pipeline sections - along with diversion dams, control structures and the like - have either been massively damaged in the quakes, buried in ash, or - in most cases - both.

The DWP quickly realizes that Los Angeles, a city of a million-and-a-half people, now only has water sources to support less than a third of that number7 - and more won't be coming soon.

This won't be immediately apparent to most. Los Angeles is just coming out of the rains of spring (though it was a slightly lower than average year - 11.88 inches) and all the reservoirs and aquifers are full. But by the end of summer, that definitely won't be the case.

The DWP starts drafting emergency rationing notices and informs the City Council that they have to have a meeting - now. Meanwhile, hasty conferences with geologists suggests that - even if the eruption ends the next day, it could be years before the Owen's Valley can be used as a water source, more years if you add in repair time on the aqueduct - decades, if you factor in that the eruption is both hardly likely to end "the next day" and equally likely to be far from the last eruption in the area.

Within limits, Los Angeles can get some water from other sources and conserve what it does have:
Agriculture in the city (mostly in the San Fernando Valley) will quickly find their allotments curtailed.8 Industry, too, is going to find itself on the short end of the water stick.
L.A. can buy some water from surrounding communities with their own water sources.9
In a little over two years, Hoover (Boulder) dam will be completed and the Colorado River Aqueduct can be rushed to completion. This took until 1941 OTL, though, so I don't expect it until 1937, '38 at the earliest - and that assumes all the requisite reservoirs and piping at the L.A. end can be completed in time.

It's the height of the depression and Los Angeles's economy - indeed, all of California's economy - has just taken one more big, big hit. Meanwhile, much of Nevada is an ash-shrouded ghost town (and even western Utah's seeing a drop in population). The Ashcloud, June 18th, 1933

As the month of June wears on, the eruption subsides to the occasional earthquake10 and a fluctuating - though much, much smaller - column of ash from the many throats of the caldera. Occasional streams of lava flow across the ash-caked Long Valley floor. Dark, churning pools of ash, rock, vegetation, and water that Los Angeles thought was theirs form new lakes of black in the hollows of the area, dammed by huge mounds of ash and rills of hardened lava. The entire topology of the valley has been mixed in a volcanic blender and - now only vaguely resembling the green bowl that it once was - continues to change day by day as the fires beneath continue to belch forth upon the land.

Now what?