Streamflow Glossary

Key terms used in river monitoring and USGS stream gauge data.

CFS — Cubic Feet per Second

The standard unit of streamflow in the United States. One CFS is the volume of water that would fill a cube 1 foot on each side, flowing past a fixed point every second. That's roughly 449 gallons per second. A small stream might flow at 10 CFS; the Mississippi River at St. Louis averages about 175,000 CFS; during major floods it can exceed 1,000,000 CFS.

Discharge

The volume of water flowing past a cross section of a stream per unit of time — synonymous with streamflow. Expressed in CFS (cubic feet per second) in the US, or cubic meters per second (m³/s) internationally. USGS computes discharge by multiplying the measured stream velocity by the cross-sectional area of the water.

Gage Height (Stage)

The water surface elevation measured at a stream gauge, expressed in feet above a local datum. Gage height is not the same as depth — it's measured from a fixed reference point that may be above or below the streambed. Gage height is what triggers flood warnings; when gage height exceeds the flood stage threshold, flooding typically begins.

Rating Curve

A site-specific mathematical relationship between gage height and discharge at a stream gauge location. USGS hydrologists periodically measure the actual discharge at various water levels and use these measurements to build a curve that converts gage height readings into discharge (CFS) values. Rating curves change over time as the channel shape evolves.

Percentile Rank

A comparison of today's streamflow to the historical record for the same calendar date. A percentile rank of 50 means today's flow is at the median — half of all historical years on this date had lower flow. Below the 10th percentile is Very Low; above the 95th percentile is Very High. Percentile rank is the most useful indicator for answering "is the river high or low right now?"

Very Low < 10th

Low 10–25th

Normal 25–75th

High 75–95th

Very High > 95th

Flood Stage

The gage height at which a river begins to overflow its natural banks and cause damage to property, infrastructure, or agriculture. Set by NOAA/NWS for each individual gauge location based on historical observations. There are four thresholds: Action Stage (prepare but no flooding), Flood Stage (minor flooding), Moderate Flood Stage (significant damage), and Major Flood Stage (severe, life-threatening flooding).

Action Stage

The water level at which emergency managers and river authorities need to take preparatory action — monitoring conditions, alerting downstream communities, or pre-positioning equipment. No flooding is expected at action stage, but conditions warrant attention.

HUC — Hydrologic Unit Code

A standardized watershed identification system used by USGS. The US is divided into 21 major regions (2-digit HUCs), which are subdivided into smaller and smaller units down to individual sub-watersheds (12-digit HUCs). Each stream gauge is assigned a HUC code indicating which watershed it drains. HUCs are useful for understanding which areas contribute water to a given gauge location.

NWIS — National Water Information System

The USGS database and web portal for water resources data. NWIS contains real-time and historical streamflow, groundwater, water quality, and precipitation data from thousands of monitoring sites nationwide. All data on StreamFlowData.com is sourced from the NWIS API, which is freely available at waterdata.usgs.gov/nwis.

Daily Value (DV)

The mean daily discharge computed from sub-daily instantaneous readings over a 24-hour period. Daily values are the basis for computing long-term statistics and percentile ranks. The USGS DV service provides decades of historical daily mean discharge for most gauges.

Instantaneous Value (IV)

A single measurement at a specific point in time — the "current reading" displayed on station pages. USGS publishes new instantaneous values through the day. StreamFlowData.com refreshes them hourly, and they are what we display for current conditions.

Drainage Area

The total land area that drains water to a particular point on a stream, measured in square miles (mi²). All precipitation that falls within the drainage area eventually flows to the gauge location. Larger drainage areas generally correspond to higher discharge; the Mississippi River at its mouth drains over 1.2 million square miles.

Datum

The reference elevation from which gage height is measured, expressed in feet above the National Geodetic Vertical Datum (NGVD29) or North American Vertical Datum (NAVD88). Adding the gage height to the datum elevation gives the absolute water surface elevation above sea level. The datum is stable at most gauges but may be adjusted after major surveys.