How Rivers and Floods Are Measured
Follow water level from a streamgage to calculated discharge, flood stage and wider satellite flood mapping without confusing these distinct products.
Explore flood and river conditionsWhat the data really means
Use these four anchors before interpreting the related live visualization.
- Stage
- Water-surface height at one gauge
- Discharge
- Volume of water passing per unit time
- Flood stage
- Locally defined threshold, where available
- Inundation map
- Observed, modeled or forecast spatial extent
From evidence to the PlanetVexa view
Each stage changes what can be claimed. PlanetVexa keeps the source record, derived interpretation and visual presentation visibly distinct.
- 01Stage
Water-surface height at one gauge
- 02Discharge
Volume of water passing per unit time
- 03Flood stage
Locally defined threshold, where available
- 04Inundation map
Observed, modeled or forecast spatial extent
A streamgage observes water at one cross-section
A streamgage records the height of the water surface relative to a local reference. Pressure sensors, radar or stilling wells can measure stage repeatedly, while telemetry sends provisional observations for rapid use. The number belongs to that river location, datum and timestamp.
Stage alone is not the amount of water moving through the channel. Channel shape, slope, vegetation and changing sediment determine how a given height relates to flow.
Discharge is calculated from field measurements
Hydrologists measure channel width, depth and water velocity across a section to calculate discharge. Repeated visits across low and high flows establish a site-specific rating curve that converts routinely observed stage into estimated streamflow. Floods can reshape the channel, requiring the relation to be checked and revised.
A near-real-time discharge value is therefore derived from an observation and a maintained relationship. Provisional values can change after quality review.
Flood threshold and high flow are not synonyms
A flood stage is established for some gauges by the responsible forecasting authority using local impacts and channel context. A percentile describes how unusual current flow is compared with the historical record for that date. High percentile flow does not automatically mean an official flood warning exists.
Many gauges have no published flood stage, and one threshold cannot describe every upstream or downstream impact. PlanetVexa retains the category and issuing source instead of inventing one.
Satellites and models expand the spatial view
Optical and radar satellites can detect water beyond normal boundaries, while hydrologic models estimate river flow and possible inundation across larger areas. Clouds limit optical imagery; vegetation, terrain, sensor resolution and classification affect flood mapping. Model output remains a forecast or analysis, not a gauge reading.
PlanetVexa separates gauge observations, remote-sensing detections, modeled discharge and official warnings. The page provides context, not emergency guidance; local authorities remain the source for protective action.
Questions people ask
Is river stage the same as discharge?
No. Stage is water height at a gauge; discharge is flow volume derived from velocity, channel geometry and a maintained rating relation.
Does high streamflow mean a flood warning exists?
No. Historical percentile, flood stage and an official warning are different products.
Can a satellite map every flooded street?
No. Resolution, revisit time, vegetation, terrain, clouds and classification uncertainty limit flood detection.
Read the official material
PlanetVexa summarizes these sources in original language and links to the responsible institutions for definitions and operational context.