Basics · Water level
How to read a water level
A water level only becomes useful once you know the unit, the measurement location, the time and a comparison. These four details prevent the most common misreadings.
The level is an altitude – not the water depth
For many Swiss lakes the level is given in metres above sea level (m a.s.l.). A value of 394.50 m a.s.l. on Lake Constance means the water surface at this station is 394.50 metres above the reference datum used. It does not mean the lake is 394.50 metres deep. The actual depth depends on the lake bed and differs everywhere.
Other networks report a local level in centimetres. Its zero point is station-specific. So 250 cm on one river cannot be compared directly with 250 cm on another. Even two stations on the same water body can use different reference datums.
Also check the water body in the station name: a station on a tributary describes that stream or river at that location, not automatically the lake. A gauge on the Freibach at Rheineck is therefore not a Lake Constance level. To read the lake surface elevation, choose a station in the lake itself.
Four things belong with every value
Read the station name and unit first, then the timestamp. Depending on the source, official stations typically report roughly every 10 to 60 minutes. If the latest value is much older, the transmission may be disrupted. Seepegel therefore shows the measurement time next to the value and labels the source.
- Location: where was the value recorded?
- Unit and datum: m a.s.l., cm or another unit?
- Time: is the value current or delayed?
- Trend: is the level rising, falling or fluctuating?
Why trend and seasonal median matter more
A single value has little meaning without comparison. The 24-hour trend shows short-term movement; seven or thirty days reveal slower developments. Lakes usually respond in a more damped way than their tributaries. Wind can also shift the surface between opposite shores temporarily.
Where official references exist, Seepegel compares the current level with the median of historical daily means for the same calendar day. The FOEN reference often covers 1991–2020; the period stated on the chart is what matters. The median is the central historical value, less sensitive to individual extremes than an average. Being 20 cm above it only means that the level exceeds that day's reference. It is not a flood warning.
When a level becomes safety-relevant
For hazards, official thresholds, warning levels and situation reports apply. They are water-body specific: an absolutely high number can be normal, while a small-looking local level can already be critical. Use Seepegel for overview and trend, but in case of possible danger always the linked original source and the warnings from federal, cantonal and municipal authorities.