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Answer: Streamflow, or discharge, is the volume of water moving past a point in a river over time. USGS measures stage (water height) continuously at streamgages, then converts it to flow using a stage-discharge relation built from periodic direct measurements of width, depth and velocity.

Stage vs. flow

Stage, or gage height, is just how high the water surface is. It is easy to record automatically, but what people usually need is flow, such as cubic feet per second.

The method

1. Record stage continuously at a gage.
2. Hydrographers periodically measure discharge directly, crossing the river and measuring depth and velocity across sections.
3. Pair the measurements with stage to build a rating curve.
4. Convert the continuous stage record to continuous streamflow.

History and scale

USGS opened its first streamgage in 1889 on the Rio Grande in New Mexico, and now operates over 8,200 continuous-record streamgages. Data are published in the National Water Information System.

In the water cycle

Streamflow carries precipitation that runs off land, plus groundwater seepage, back toward the ocean.

Claims

• id: C1 claim: The height of the water surface is called stream stage or gage height. source: S1 evidence: "The height of the surface of the water is called the stream stage or gage"
• id: C2 claim: USGS started its first streamgage in 1889 on the Rio Grande. source: S1 evidence: "started its first streamgage in 1889 on the Rio Grande River in New Mexico"
• id: C3 claim: USGS operates over 8,200 continuous-record streamgages. source: S1 evidence: "The USGS operates over 8,200 continuous-record streamgages"
• id: C4 claim: Streamflow refers to the amount of water flowing in a river. source: S2 evidence: "to refer to the amount of water flowing in a river"

Sources

• [S1] How Streamflow is Measured. USGS. https://www.usgs.gov/special-topics/water-science-school/science/how-streamflow-measured . License: US government work, public domain. Accessed 2026-11-04.
• [S2] Streamflow and the Water Cycle. USGS. https://www.usgs.gov/special-topics/water-science-school/science/streamflow-and-water-cycle . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_y45wahqv,
  author = {{iinventedlight}},
  title = {{How streamflow is measured}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/streamflow-measurement/},
  note = {Permanent ID iil:note:y45wahqv. License: CC-BY-4.0.},
}

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