Chicago, IL

Weather Forecast Office

Hydrology Terms and Definitions

Action Stage

Action is taken for possible high water. Water may cause minor impacts and be a nuisance to persons near the stream. Local governments or agencies may take actions to reduce property damage and danger to life.

Use Caution!

Graphic showing a river at action stage, with water flooding low forest land near the river.

Minor Flood Stage

Some property flooding and public threat may occur. Roadways, trails, park land, and private property near the stream may become flooded.

Take Action!

Graphic showing a river at minor flood stage, with water flooding a roadway near the river.

Moderate Flood Stage

Flooding of structures and main roadways may occur. Residences and numerous roadways near the stream may become flooded. Evacuations may be necessary. Disruptions to daily life.

Take Action!

Graphic showing a river at moderate flood stage, with water flooding structures near the river.

Major Flood Stage

Extensive flooding of structures, main roadways, and other critical infrastructure may occur. Schools, hospitals, police stations, fire stations, residences, businesses, and roadways may become flooded. Evacuations may be necessary. Significant disruptions to daily life.

Take Action!

Graphic showing a river at major flood stage, with water flooding numerous structures near the river.

Low Stage

Water is low enough to be a public threat, cause impacts to commerce, or damage property. Boating or shipping traffic may be restricted. Water supply intakes may be affected.

Normal Pool

The typical water level upstream of a lock or dam. For structures where the water level can be adjusted, the normal pool is the general target water level, as indicated by an operations manual or permit. For structures without manual adjustments, the normal pool is the spillway water level.

Bankfull

Water begins to overflow the lowest natural banks along this stretch of the stream. Very minimal impacts confined to areas near the stream are possible.

Stage

Water level above an arbitrary reference elevation called the datum. Not the same as depth!

Flow

Estimated rate of water moving past a given location. Also referred to as streamflow or discharge. Most runoff becomes streamflow in area streams.

Datum

Arbitraty reference elevation for stage observations. Adding the stage value to the datum will provide the water elevation. The water elevation is the same as the datum when stage is zero (0).

Note that different datum systems exist which may elevation differences of inches to feet.

Learn more about datums from the National Ocean Service.

Precipitation

Precipitation is water that falls from the atmosphere to the Earth’s surface. This includes rain and snow.

Precipitation is the component of the water cycle monitored most closely by hydrologists when trying to determine the threat of flooding.

Evaporation

Evaporation refers to water that changes from a liquid to a gas and moves from the Earth's surface back into the atmosphere. Evaporation is one part of evapotranspiration, which also includes transpiration.

A large portion of the precipitation that falls on area river basins over a year is removed through evapotranspiration.

Transpiration

Transpiration refers to water that is pulled out of the soil and released into the atmosphere as a gas by plants. Transpiration is one part of evapotranspiration, which also includes evaporation.

A large portion of the precipitation that falls on area river basins over a year is removed through evapotranspiration.

Runoff

The portion of liquid precipitation that does not soak into the soil but instead moves over the Earth's surface toward streams. During winter, frozen precipitation may be stored on the soil's surface as snow cover. Warmer temperatures melt snow cover, and some of this water may turn to runoff.

Precipitation that does not turn to runoff or get stored as snow cover instead goes to infiltration.

Infiltration

The portion of precipitation that soaks into the soil. During winter, frozen precipitation may be stored on the soil's surface as snow cover. Warmer temperatures melt snow cover, and some of this water may undergo infiltration.

Precipitation that does not soak into the soil or get stored as snow cover instead turns to runoff.

Drainage Basin

A drainage basin, also known as a watershed or catchment, is the area from which water flows to form a stream. A basin is defined by its outlet. All precipitation (rain and snow) that falls within a drainage basin eventually flows to the outlet point, unless it is first removed by evaporation and transpiration.

River Reach

A river reach is a stretch of a stream or river that behaves in a generally similar way. Flood impacts and flood impact categories (such as minor flood stage) are determined for the entire river reach tied to a specific river gauge.

More Information