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What Does "Normal for the Date" Mean for Water Levels?

By the Water Level Atlas team · Last reviewed Oct 21, 2026

"Normal for the date" compares today's level or flow with the same day of the year in past years. If today's reading sits in the middle half of those past values (between the 25th and 75th percentiles), it is normal for the date. Below that it is below normal, and above it is above normal.

Why compare with the same date?

Most lakes and rivers follow a yearly rhythm. Rivers often run high after snowmelt or spring rain and low in late summer. Many reservoirs are filled in spring and drawn down in autumn or winter (see why lake levels change and winter drawdown).

So a single number means little on its own. A river flow that is ordinary in April might be unusually high in September. Comparing with the same date removes the season from the picture and shows whether today is unusual for this time of year. Agencies use the same idea. The Minnesota DNR's stream conditions page, for example, rates flow as low, normal or high "for what is typical this time of year".

What is a percentile?

A percentile tells you where a value sits among past values. USGS describes it like this: a river flow at the 90th percentile is equal to or greater than 90 percent of the flows recorded on that day of the year in all past years of measurement.

A worked example with round numbers: imagine a gauge with 20 years of records for May 1. Line up those 20 values from lowest to highest. The 25th percentile is about a quarter of the way up the line, the 50th percentile (the median) is in the middle, and the 75th percentile is about three quarters of the way up. If today's value is higher than roughly 18 of the 20 past values, it is above the 90th percentile.

What do the labels mean on our pages?

We use five bands, based on where today's value falls among past values for the same date:

The middle band follows the convention USGS uses on its drought pages, where a percentile between 25 and 75 is considered normal, below 25 is below normal and above 75 is above normal. Our "much below" and "much above" labels split off the lowest and highest 10 percent.

Where do the past values come from?

Lakes and reservoirs. We use the daily history we have stored for that lake's gauge. For each past year we take the average level over a short window around the date (three days either side), then calculate percentiles from those yearly values. The current year is left out. We only show a normal range when at least 5 past years are available for that date. With fewer years, the page shows no normal band rather than a weak one.

River gauges. We use the U.S. Geological Survey's own day-of-year statistics. USGS calculates these from all approved historic observations for each day of the year, as percentiles of daily mean values. Provisional data are not used in those statistics (see provisional vs approved data). Because they come from the full approved record, river gauges often have many more years behind their normal range than lakes do.

Why is "normal" not the same as "average"?

The "Normal for" number on our pages is the median (50th percentile), not the mean. USGS lists the median and the mean of daily values as separate statistics. They can differ a lot. One huge flood year can pull the average up far above what a typical year looks like, while the median stays in the middle of the pack.

"Normal" also does not mean "ideal" or "safe". It only describes what has happened most often in past years on this date. A lake can be normal for the date and still be below its full pool if it is usually drawn down at this season (see what is full pool). A river can be above normal and nowhere near flood stage. Flood status is a separate measure set by the National Weather Service (see flood stages explained).

Can normal change over time?

Yes. Each new year of data shifts the percentiles a little. Ranges built from a short record move more than ranges built from decades. Changes in how a reservoir is operated can also make older years less like recent ones. That is why our pages show how many years the range is based on, where that is available.

Using this on Water Level Atlas

Lake and river pages show a badge such as "Normal for the date" or "Below normal for the date" next to the current reading, plus a "Normal for" figure with the median for today's date. The charts shade the normal range (the middle 50% of past years) behind the measured line. River pages also list the "Normal range" for today's date with the number of years it is based on. Our methodology page has the full details.