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.
What is an irrigation district?
An irrigation district is a local public agency, run by a board its landowners or voters elect, that holds water rights or water contracts and delivers that water to the farms inside its boundary. How it holds its water, its own senior rights or a share of a federal or state project, decides how much its growers get in a dry year.
Why does the San Luis Reservoir level swing so much each year?
Because pumps fill it, not a river. The federal and state water projects pump surplus Delta water into San Luis Reservoir in winter and spring and draw it down through summer for farms and cities south of the Delta.
Drought, snow & water rights
How to read the US Drought Monitor: D0 to D4
D0 abnormally dry, D1 moderate, D2 severe, D3 extreme, D4 exceptional. What each US Drought Monitor class means, and why percent in drought starts at D1.
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.
Will El Niño help the Lake Mead water level?
Not reliably. In 46 winters of records, April 1 snowpack in the Upper Colorado River Basin, which feeds Lake Mead, showed no link to El Niño or La Niña.
What states does the Colorado River flow through?
The Colorado River flows through Colorado, Utah and Arizona, then forms the Arizona-Nevada and Arizona-California borders before it crosses into Mexico. Seven states share its water: Colorado, New Mexico, Utah and Wyoming in the Upper Basin, and Arizona, California and Nevada in the Lower Basin, with a treaty share for Mexico.
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.
What is a water year? October 1 to September 30
A water year runs October 1 to September 30 and is named for the year it ends in, so Water Year 2026 began in October 2025. Why the West counts this way.
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.
What is dead pool at Lake Mead and Lake Powell?
Dead pool is the level below which a dam can no longer release water downstream by gravity: 895 feet above sea level at Lake Mead and 3,370 feet at Lake Powell. Minimum power pool, where the turbines stop, sits higher, at 950 feet at Hoover Dam and 3,490 feet at Glen Canyon Dam.
Does rain raise the Lake Mead water level?
Barely. An inch of rain landing on Lake Mead adds about an inch to its surface, while the level itself is set by Colorado River water released from Lake Powell upstream and by what Hoover Dam lets out below.
Lake Mead water level by year
Lake Mead stood at 1,037.53 feet on Oct 15, 2026, its lowest level since 1937. Every year since 1935 is charted below, with its low and high.
Why are Elephant Butte water levels so low?
Elephant Butte Lake is low because the Rio Grande has delivered less water than the reservoir gives up in irrigation releases and evaporation, year after year since 2000, and the 2026 snowmelt barely reached it. Elephant Butte Reservoir stood at 4,295.96 feet above sea level on Oct 15, 2026, 111.04 feet below full pool, with 94,701 acre-feet in storage (5% of capacity).