How to read river levels, lake levels and reservoir figures

A gauge reading is not the depth of the water

Most river gauges measure the height of the water surface above a fixed zero point. In the US that number is called gage height or stage; in Britain and Ireland it is usually just the river level. The zero is chosen when the gauge is installed, and it is often set a little below the lowest water anyone expects, so readings stay positive.

That is why a river can read 2 ft and still be deep enough to drown in, or read 15 ft at a spot where the channel is barely wider than a road. The number only makes sense compared with other readings from the same gauge. Comparing the stage at two different gauges, even on the same river, tells you very little.

Occasionally you will see a negative reading. It does not mean the river has run dry. It simply means the water has dropped below the zero chosen decades ago, which happens on some large rivers during long droughts.

Where zero sits: datums and elevations

The fixed reference is called a datum. A local gauge datum is just a convenient zero for one site. Many agencies also publish the elevation of that zero, so the reading can be converted into height above sea level.

In the US, elevations are tied to national datums such as NAVD 88, and older records often use NGVD 29. Big lakes and reservoirs are usually reported this way: when Lake Mead is listed at something over 1,000 feet, that is the height of its surface above sea level, not its depth.

In Great Britain the reference is Ordnance Datum Newlyn, written as mAOD (metres above Ordnance Datum). Northern Ireland uses Belfast datum, and the Republic of Ireland uses Malin Head, written as OD Malin. Canada uses the CGVD2013 vertical datum, though many older stations still report against earlier ones. When a gauge is moved or resurveyed, the datum can change, and long records then need adjusting before old and new readings line up.

What counts as normal

A level is only high or low compared with something. The most useful comparison is the same gauge on the same date in past years. If a site has 30 years of readings, you can rank today against every mid-October in that record.

We describe that range with percentiles. The median is the middle value: half of past years were higher on this date and half were lower. The 10th percentile marks the level that only one year in ten has been below, and the 90th the level only one year in ten has topped. Between those two lines is what we call the normal range. Below the 10th percentile counts as low, above the 90th as high.

Some agencies publish their own version. The Environment Agency in England shows a typical range for each station, and the US Geological Survey colours its maps by percentile class. A short record is a weaker guide than a long one, so we show how many years sit behind each comparison where the data allows.

Flood stage and warning levels

Being well above normal is not the same as flooding. Flooding depends on the banks, the floodplain and what has been built on it, so agencies set separate thresholds for each site.

In the US, the National Weather Service sets the categories. Action stage means it is time to prepare. Flood stage is the level at which the water starts to affect roads, land or property. Moderate and major flood stages follow, each tied to specific impacts at that location. The NWS also issues river forecasts with an expected crest for many gauges, which we show when they exist.

In England, the Environment Agency issues flood alerts, flood warnings and severe flood warnings for defined areas, with Natural Resources Wales and SEPA doing the same in Wales and Scotland. In Ireland, the OPW runs the national gauge network and Met Éireann issues weather and flood-related warnings. In Canada, flood forecasting is provincial, so thresholds and names differ from one province to the next. In every case, the official warning for your area matters more than any single number on a chart.

Reservoirs: elevation, storage and percent full

Reservoirs are reported in up to three ways. Elevation is the height of the water surface, usually above sea level. Storage is the volume of water held. Percent full is storage divided by capacity.

The three do not move in step. Most reservoirs are wider at the top than at the bottom, so the last few feet near full pool hold far more water than the same few feet near the bottom. A lake that is down 50 ft can still hold a large share of its water, or very little, depending on its shape.

Capacity has its own fine print. Some agencies count only live storage, the part that can actually be released, and leave out dead pool, the water below the lowest outlet. Capacity also shrinks slowly as sediment builds up, and a survey can lower the official figure overnight. Percent full is always relative to whichever capacity the agency uses.

Units and quick conversions

US agencies report heights in feet and flow in cubic feet per second (cfs). Nearly everyone else uses meters and cubic meters per second. One foot is 0.3048 m, and one meter is about 3.28 ft. One cubic meter per second is about 35.3 cfs.

Reservoir volumes in the western US are given in acre-feet: the water that would cover an acre to a depth of one foot, about 1,233 cubic meters or roughly 326,000 US gallons. Elsewhere you will see million cubic meters (often written hm³), and British water companies often talk in megalitres, where one megalitre is a thousand cubic meters.

How often the data updates, and why it lags

Most automatic river gauges record every 15 minutes. They do not always send each value straight away: many transmit in batches, so a reading can reach us an hour or more after it was taken. Reservoir operators often publish once a day, sometimes only on working days.

Fresh readings are provisional. Agencies check them later and may correct values affected by ice, weeds, debris or a faulty sensor. In winter, northern US and Canadian gauges sometimes show flat or odd values because ice is pressing on the sensor or the channel is frozen over.

When a gauge goes quiet, the cause is usually mundane: maintenance, a power or radio failure, or damage during a flood. We mark readings that are too old to call current, so you can tell a steady river from a silent gauge.

Unit conversions