# 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. _Updated Jul 9, 2026._ Three different numbers get used to describe a reservoir, and people mix them up constantly. Storage is the raw volume of water in the lake, measured in acre-feet. Percent of capacity is that storage divided by the reservoir's full pool. Percent of normal compares today's storage to what is usual for this exact date. They can tell very different stories about the same reservoir on the same day. ## Why percent of capacity misleads Percent of capacity feels intuitive and is often the wrong lens. Reservoirs are drawn down and refilled on a seasonal cycle by design. A flood-control reservoir is supposed to sit low through winter, holding empty space to catch storms, then fill in spring. A lake at 60 percent of capacity in November can be exactly where it should be, while the same 60 percent in July might be a warning. Capacity alone cannot tell the two apart, because it has no sense of the calendar. ## Why this site leads with percent of normal The honest question is not how full is the reservoir, it is how full is it compared to a normal year at this point in the season. That is why The Waterline leads with storage against the trailing five-year median for the date. Being 51 percent below the median for early July is a real signal. Being at 45 percent of capacity, on its own, is not, because the normal for that date might be 45 percent. ## Drawdown is not a crisis A falling storage line through summer is usually the system working as intended, not a reservoir dying. Operators release water to meet downstream demand and to open flood space for the next winter. The line to worry about is not the drop itself, it is a reservoir sitting well under its normal-for-the-date band, especially heading into the dry season with little carryover. ## Worked example: one reservoir, three numbers [Chart: Lake Mead storage](https://the-waterline.com/reservoir/lake-mead) ([data](https://the-waterline.com/reservoir/lake-mead.md) — chart image: https://the-waterline.com/charts/reservoir/lake-mead/chart.png) Take Lake Mead, charted above. Read it three ways. Its storage is the acre-foot figure on the page. As a percent of capacity, that storage sits against Mead's enormous full pool, which is why a partly full Mead still looks like a small share. As a percent of normal, the more useful read, you compare today's line to where it usually sits this time of year. The same reservoir can feel comfortable on one measure and alarming on another. When you see a reservoir headline, always ask which of the three numbers it is quoting. ## Related data + guides - [Reservoir storage tracker](https://the-waterline.com/reservoir) - [What is carryover storage?](https://the-waterline.com/guides/what-is-carryover-storage) - [Full pool, dead pool, minimum power pool](https://the-waterline.com/guides/full-pool-dead-pool-minimum-power-pool) --- Canonical URL: https://the-waterline.com/guides/reservoir-storage-vs-capacity