Guides
Western water, explained
How the numbers on this site actually work — acre-feet, allocations, drought classes, reservoir pools. Each guide is built on the same live data as the trackers, so the examples are real and current.
Measurement & units
What is an acre-foot?
An acre-foot is the amount of water it takes to cover one acre one foot deep: 325,851 gallons, or 43,560 cubic feet. It is the unit the American West runs on, roughly a year of water for two average California households.
Reservoir storage vs capacity vs "percent of normal"
Reservoir storage is the water actually in the lake, in acre-feet. Percent of capacity is that storage against the reservoir's full pool. Percent of normal compares it to what is typical for this date, and it is the number that actually tells you whether a reservoir is in trouble.
Streamflow basics: what cfs actually means
cfs stands for cubic feet per second, the standard measure of how much water is moving past a point in a river. One cubic foot is about a basketball of water, and 1 cfs flowing for a full day adds up to about 1.98 acre-feet.
California's water projects
How CVP allocations work (and when they're announced)
A CVP allocation is the percent of a contractor's contract water the federal Central Valley Project expects to deliver that year, not a percent of what the farm or city needs. It is announced by Reclamation around late February and revised through spring as the snowpack and reservoir picture firms up.
How SWP allocations work
An SWP allocation is the percent of its Table A contract amount each State Water Project contractor can expect that year, set by California's Department of Water Resources. It is announced around December 1, starts deliberately low, and is usually revised upward through spring.
CVP vs SWP: California's two big water projects, explained
The CVP is the federal Central Valley Project, run by the Bureau of Reclamation and serving mostly Central Valley agriculture and wildlife refuges. The SWP is the State Water Project, run by California DWR and serving mostly Southern California cities. They share the same Delta, which is why Delta conditions constrain both.
Drought, snow & water rights
How to read the US Drought Monitor
The US Drought Monitor rates drought on a five-step scale from D0, abnormally dry, to D4, exceptional drought, and publishes a new map every Thursday. It is a blend of many indicators and expert judgment, not a single formula, and it is not a reservoir or groundwater gauge.
What is snow water equivalent (SWE)?
Snow water equivalent, or SWE, is the depth of water you would get if you melted the snowpack where it sits, measured in inches. It is the number that matters for water supply, because a deep dry snowpack can hold less water than a shallow dense one.
How to read percent-of-median snowpack (and why it breaks at melt-out)
Percent of median compares today's basin snowpack to the 1991-2020 median for this same date, so 100 percent means a normal snowpack for the day. The catch is that near melt-out the normal itself falls to almost nothing, so the percentage blows up and stops being meaningful, which is why this site reports the season peak once the snow is nearly gone.
What is SNOTEL?
SNOTEL is the USDA Natural Resources Conservation Service network of automated snow-measuring stations across the western mountains, roughly 900 of them. Each station weighs the snow sitting on a pillow to measure its water content, and those readings are the backbone of western water-supply forecasting.
Reservoirs & the water year
What is carryover storage (and why September 30 matters)
Carryover storage is the water still banked in a reservoir at the end of the water year on September 30. It is the insurance policy that carries a system into next year, and it is one of the first numbers water managers cite heading into a dry winter.
The water year, explained
A water year runs from October 1 to September 30 and is named for the calendar year it ends in, so Water Year 2026 ends September 30, 2026. It starts in fall because that is when the West's precipitation season begins, and nearly every water statistic is anchored to it.
The Colorado River system in charts
The Colorado River is banked in two giant reservoirs, Lake Powell and Lake Mead, which together hold most of the system's water. The river was divided in 1922 into shares that add up to more water than it reliably produces, and that structural gap is why Powell and Mead have trended down for two decades.