WATER & THE SNOWPACK
A state that stores a third of its water as snow, and bets the year on a handful of storms.
The largest reservoir has no dam
THE APRIL 1 SNOWPACKCalifornia's built storage — every dam, every reservoir — is real and large. But the single biggest store of water in the state is the Sierra Nevada snowpack, and it is not engineered at all. Snow accumulates from November through March, reaches its maximum around the start of April, and then releases slowly through spring and early summer, arriving in the rivers and reservoirs at exactly the time of year when nothing is falling from the sky.
That timing is the whole point. California's precipitation arrives almost entirely in the months it is least needed and none of it arrives in the months it is most needed. The snowpack is the mechanism that bridges the gap — a natural delay line that holds winter water until summer. Roughly a third of the state's supply passes through it.
Which is why April 1 is the most-watched date in California hydrology. The snowpack's water content on that date is the closest thing the state has to a balance check, and the surveys that measure it have been run since 1929.
Atmospheric rivers
THE FEW STORMS THAT DECIDE EVERYTHINGAn atmospheric river is a long, narrow corridor of concentrated water vapour in the lower atmosphere — typically a few hundred miles wide and thousands of miles long, carrying moisture out of the subtropical Pacific toward the West Coast. A strong one transports more water than the Mississippi. When it makes landfall and the Coast Ranges or the Sierra force it upward, that vapour condenses out.
They are not a curiosity; they are the water supply. A handful of these events deliver a large share of California's annual precipitation, and the difference between a drought year and a wet year is usually not a broadly wetter season — it is whether four or five of these arrived, and where they aimed. The scale below, developed at Scripps, ranks them by how much good and how much harm a given event is likely to do.
Scale after Ralph et al., Scripps Center for Western Weather and Water Extremes. Ranking combines vapour transport with how long landfall persists — a moderate AR that stalls does more damage than a strong one that moves through.
The failure modes
HOW A WATER YEAR GOES WRONGWarm storms
The same inches of precipitation are worth far less arriving warm. A storm with a high snow level rains on ground that should be collecting snow, so the water runs off in days instead of releasing over months. A wet year with warm storms can still leave summer short.
Rain on snow
The flood scenario that matters most. A warm atmospheric river arriving over an existing snowpack adds its own rain to meltwater it releases, and the two together can produce runoff far beyond what the rainfall alone suggests. The 1997 New Year's flood is the reference case.
A dry January
The wet season is short and front-loaded. A dry December and January cannot be made up in spring, because by March the storm track is already retreating north. Mid-winter is where a season is won or lost.
Dry antecedent soils
After a drought, the first big storms are absorbed rather than running into reservoirs — the ground takes its share first. This is why a single wet year rarely ends a multi-year drought in the reservoir numbers.
Dust and early melt
Dust deposited on the snowpack darkens it and it absorbs more sunlight, melting weeks earlier than it otherwise would. Early melt means the water arrives before the demand does, and passes through.
Whiplash
The pattern that is hardest to manage: severe drought and record wet back to back. Infrastructure and land are set up for one regime at a time, and rapid swings between them break both — dry ground sheds water, burned ground sheds debris.
The orographic engine, in numbers
WHERE THE WATER ACTUALLY LANDSComputed from the same NOAA normals the rest of the site uses. Precipitation against elevation, for every place on the western slope of the Sierra and its lee — which is the single transect California's water supply depends on.
| Place | Elevation ft | Annual precip ″ | Annual snow ″ | Side of the crest |
|---|
Live water data comes from the state
This page explains the system. It is not a live gauge. For current snowpack, reservoir levels and river stages, use the California Data Exchange Center (CDEC), the Department of Water Resources, and the California Nevada River Forecast Center for flood forecasts. Flood warnings come from the National Weather Service.