National Weather Service United States Department of Commerce
Graphic indicating observations
Keeping track of the different components of the water cycle is important to hydrology. For example, monitoring observations of current conditions helps scientists better understand where flooding or drought may occur.
Rain gauges may be used to keep track of precipitation and stream gauges may be used to keep track of how much water is in rivers. Other observations may include information about soil moisture, drought, snow cover, and river ice cover.

River Gauges


River gauges are used to monitor how much water is in streams. Typical measurements include stage and flow.

Stage is the water level above a reference elevation, called a datum.
This is not the same as depth!
  Flow, also known as streamflow or discharge, is the rate of water moving past a location.
Graphic showing stage with the water surface being measured against an particular elevation or datum   Graphic showing flow with arrows indicating movement of water.


Almost all river observations used in river forecasts are provided by partner agencies such as the US Geological Survey and the US Army Corps of Engineers. Archived data from the US Geological Survey can be retrieved from the National Water Information System site. Archived data from the US Army Corps of Engineers can be retrieved from their site


Precipitation Gauges/Observers


Observations of precipitation can come from automated gauges which report by satellite or modem and also manual observations. Automated gauges are particularly helpful for determining the precipitation rate. A high rainfall rate can sometimes cause flooding which begins very rapidly, called flash flooding.


Complex or Simple


Observations may come from large stations with numerous pieces of sophisticated equipment sending information frequently over satellite, or may come from manual readings taken by a volunteer.


Picture of a weather station and a river gauge


Soil Moisture


Changes in soil moisture affect how precipitation behaves when it reaches the ground. In general, lower soil moisture increases how much precipitation can soak into the soil. In contrast, higher soil moisture decreases how much precipitation can soak into the soil and increases runoff. Soil moisture can be measured or modeled. Knowledge of soil moisture content tends to improve river forecasts.




Drought typically refers to deficiency in precipitation, relative to the local area's climate, which persists over a period of time such that it causes a water shortage. This water shortage may impact streams & rivers, local water supplies, and agriculture. Monitoring drought may include observations of precipitation over a longer period of time as well as information about impacts to agriculture and water supply.


Snow Cover


Snow and ice accumulation does not impact river conditions right away but is instead stored at the soil's surface. During warmer periods, snow cover melts, which may cause river rises similar to that caused by rainfall. Snow cover is modeled to help improve river forecasts.


River Ice


River ice may have very complicated effects on the behavior of a river. For example, flowing ice may become stuck, or jam, on the upstream side of bridges, near sharp river bends, or other locations where the river changes abruptly. Ice jams may cause flooding in local areas. The rapid freeze-up of smaller streams and rivers can also cause a sudden, temporary drop in water level downstream, a situation sometimes referred to as "ice bite."


Frost Depth


During very cold weather, soil near the surface may freeze. Depending on the thickness of this frozen layer, called frost depth, precipitation and snow melt may be blocked from soaking into the soil, increasing runoff.

Information From Other Sources


Not all information comes from official sources. A large portion of rainfall information used by the National Weather Service comes from volunteer observers. The NWS Chicago office also has a network of observers along area rivers who provide information about ice cover during the winter to help provide more lead time for ice jams.


Volunteers Needed!


Manual observations from volunteers help provide needed information in between official observing stations.


Interested in recording daily rain and snow totals where you live? Consider CoCoRaHS!
Interested in using a simple app to report precipitation type (rain, snow) you are experiencing? Check out mPING!
Live along an area river and have interest in keeping us informed about ice cover? Contact our office about the Volunteer River Ice Spotter Network.
Graphic showing CoCoRaHS logo Graphic showing mPING logo  


More Information

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