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What the units on a river gauge mean

Every reading on FloodRadar comes with a unit: m, mASD, mAOD, m3/s and a few others. They come from the Environment Agency, Natural Resources Wales and SEPA, who run the gauges. Here is what each one means.

The one thing to remember: most river levels (m and mASD) are measured from a zero set at that gauge, not from the river bed and not from sea level. So a reading only means something next to the same gauge's own normal range, warning lines and record, which every station page shows. 2 m at one gauge and 2 m at another tell you nothing about which river is higher or deeper.

mmASD metres above stage datum

The level of the water above a fixed zero point at that gauge, called its stage datum. When the gauge was built, the zero was set at a convenient height, usually close to the river bed or to the lowest level the river is expected to fall to, so that readings stay above zero. A reading of 1.2 mASD means the water is 1.2 metres above that gauge's zero.

A river in cross-section with a gauge board. The gauge's zero is near the river bed; the water surface is 1.2 metres above it. gauge zero (stage datum) 1.2 mASD water surface gauge
The reading is the height of the water above the gauge's own zero, not the depth of the river.

mAOD metres above Ordnance Datum

The height of the water above Ordnance Datum: the mean sea level that Britain's maps are measured from (in mainland Great Britain, Ordnance Datum Newlyn, set from the tide gauge at Newlyn in Cornwall). Gauges on tidal rivers, lakes, reservoirs and some lowland rivers report in mAOD.

Land rising from the sea. A tidal gauge near the coast reads below zero at low tide; an inland river gauge reads 41 metres above Ordnance Datum. 0 mAOD (mean sea level) 41.0 mAOD inland river gauge −0.4 mAOD tidal gauge at low tide
mAOD is height above sea level: large inland, around zero at the coast, below zero at low tide.

Why two gauges' levels cannot be compared

Each gauge's zero is set at its own height, so the same reading can mean very different things.

Two gauges both read 2.0 metres. At the first the river is in flood near the top of its banks; at the second it is at a normal level well inside its banks. Gauge A reads 2.0 m near the top of its banks its zero: near the bed Gauge B reads 2.0 m a normal day, well inside its banks its zero: below the bed of a deep channel
The same 2.0 m: a flood at one gauge, an ordinary day at another.

That is why FloodRadar colours a reading by that gauge's own history: its typical range, its "high for this river" line and its record, rather than by the number alone.

m3/sMl/d flow

These are not heights. They measure how much water is passing the gauge.

Water flowing past a gauge. One cubic metre per second is one cube of water a metre on each side passing every second. 1 m³1 m³1 m³ 3 m3/s = three cubes like these past the gauge every second gauge
Flow is volume per second, not height.
UnitMeansTo picture it
m3/scubic metres per second, often called "cumecs"1 m3/s is 1,000 litres a second: about a bathtub every 0.15 seconds
Ml/dmegalitres (million litres) a day; used mostly at pumping stations and outfalls1 m3/s = 86.4 Ml/d

Flow rises and falls with level, but not in a straight line: when a river spreads over its floodplain, a small rise in level can carry a lot more water. Flow gauges are often at the same site as a level gauge; the station page shows the other one under "At this site".

mBDAT metres below datum

Used for groundwater boreholes. It is the depth of the water below a fixed point, usually the top of the borehole. It counts downwards, so a bigger number means the water is lower: 12 mBDAT is 12 metres down, and a fall to 9 mBDAT means the water table has risen 3 metres.

A borehole from the ground surface down to the water table; the reading is the depth from the top of the borehole to the water. datum: top of the borehole 12.0 mBDAT water table
mBDAT counts down from the top: a smaller number means wetter ground.

mm millimetres of rain

On a rain gauge, the depth of rain that fell, as if none of it soaked away or ran off. 1 mm of rain on one square metre is 1 litre of water. As a rough guide, 10 mm in a day is a wet day, 25 mm or more is a lot for most of England, and much more is ordinary in the western hills. The weather map's place card says how unusual a forecast total is for that place.

Reservoirs are shown as % full: how much of the reservoir's usable storage is filled.

The units and their definitions come from the Environment Agency, Natural Resources Wales and SEPA, who run the gauges; the diagrams are simplified and not to scale. A station page's "Unit" line shows the unit its readings use.