How we read the numbers
Every number on this site is a published reading from an official agency. This page explains how we compare it with normal for the date, how we decide a record, and what we never say. Updated .
What a reading is
A river gauge measures the height of the water above a fixed zero point on its scale, not the depth of the river. Each operator sets the zero of its own gauges, so two gauges on the same river cannot be compared by their numbers alone.
Lakes, reservoirs and some gauges give heights above a named datum, such as mean sea level, NAP in the Netherlands or IGLD 1985 on the Great Lakes. Each page names the datum its source gives.
We never convert units or datums. A value stays in the unit its source publishes, and we compare it only with values and levels of the same series.
When a reading is current
Each page shows the time of its newest reading, in the local time of the place. We work out the usual interval between a source's readings from its last 30 days, and use at least 2 hours for PEGELONLINE, Rijkswaterstaat, the USGS and the National Weather Service, 3 hours for the Great Lakes stations, and 26 hours for the Environment Agency and for sources with one value a day. A reading counts as current while it is no older than twice that interval.
A page without a current reading keeps its last one and says since when there has been none. When a source is later than it promises, the page says so: after 6 hours for the hourly sources, after 1 day for the Environment Agency, after 4 days for the U.S. Bureau of Reclamation, and after 3 days for the history pages, which are rebuilt once a day.
The normal range for the date
For each date we take the daily values of the 15 days centred on it in every earlier year. A year counts when at least 12 days in that window are complete. With at least 10 years that count the date has a normal range: the 10th to the 90th percentile of all those values. The current year never counts.
For sources that report through the day (PEGELONLINE, Rijkswaterstaat, the Environment Agency and the National Weather Service), a day's value is the mean of its hourly means over the UTC day, and the day is complete with readings in at least 20 hours of the day. The other sources give their own daily value.
A reading below the 10th percentile is below the normal range, and the page says when it is in the lowest 5 percent or lower than every earlier year for the date. The same holds above the 90th percentile. The lists of places furthest from normal rank by that first, then by how far the reading lies beyond the 10th or the 90th percentile, measured in widths of the normal range.
Records
A record is the lowest or highest value in our own series for a place, counted from the first day we hold for it, which every record names. For sources that report through the day a record is a single reading; for the others it is a daily value.
Where an agency publishes its own record for the gauge, the page shows both, and says so when ours goes beyond the published one. The records page lists only places with at least 10 years of daily data, with the records of the last 7 days first.
Official levels
Each page shows the levels the agency publishes for its own gauge, in English with the agency's own name or code: the equivalent low water level (GlW) and the mean water levels that PEGELONLINE gives for German waterways, the flood categories of the National Weather Service, the typical range of the Environment Agency, and the shortage levels of Lake Mead from the Reclamation decision that sets them. A level is compared only with readings of the series it is set on.
Provisional and raw data
Most readings are provisional or unchecked raw data: PEGELONLINE publishes unchecked raw data, recent USGS data is provisional, and Reclamation's values are provisional unless it says otherwise. Agencies can correct values later. Our pages take the corrected values on the next build, and a record set by provisional data says so.
What we do not show
We show no forecast values for German gauges: the forecasts PEGELONLINE relays come from the Federal Institute of Hydrology (BfG), whose terms allow only private and own use. ELWIS and the BfG publish them.
Nothing on this site is navigation, safety or flood advice. We never say whether it is safe to be on or near the water, and we never rate a flood risk. For shipping, use the official notices on ELWIS; for flood warnings, use the official service of your country.
Pages and lists
A place has its own page once it has a current reading and either a normal range for the date or an official normal range from its agency. Other places with a reading appear as a row on the lists of their water, state and country. A page once published stays, also while its source is quiet.
A list needs at least 2 entries, and a list longer than 200 rows continues on further pages.
Changes
- : First published.