How is the flow of a river measured?

The rate of flow of a creek, river or flood is measured by quantity over time. This is often refered to as discharge: "the rate at which a volume of water passes a given point in a given amount of time. Common units are cubic feet per second (cfs), second-day feet (sdf), and cubic meter per second (cms)."

How much water is a cubic foot per second?

A cubic foot is like a box of water measuring one foot by one foot by one foot. The USGS defines cubic foot per second (cfs) as "the flow rate or discharge equal to one cubic foot of water per second or about 7.5 gallons per second."

The discharge of Boulder Creek near the Public Library can range from ten or twenty cfs (in the winter months,) to 200 cfs (when the surge of water from the hydroelectric generating plant comes downstream,) to 500 cfs in a year of higher than normal spring runoff (causing closure of the Creek Path in places) to an estimated 29,600 cfs in the event of a 500 year flood!

Side tributaries also vary due to seasonal changes or storm events; many tributaries will be dry much of the year but potentially brim with thousands of cfs of water and debris in the event of a major flood. These can be tracked in the form of a hydrograph, which provides a visual means of seeing changes of flow discharge over time.

BEYOND BASIN

For more hydrologic terms, see the online version of the USGS USGS Hydrologic Terms Glossary

Photograph of South Boulder Creek in Eldorado Canyon taken around 1920 by an unknown photographer. Copyright by the Denver Public Library. Go to BASIN Gallery for more information

Follow up questions:

• If one cubic foot of water containes around 7.5 gallons and a gallon of water weighs eight pounds, how much does a cubic foot of water weigh? How about 10 cfs? 100 cfs? 1000 cfs? 10,000 cfs?
• Find out how much water is anticipated for a tributary near you in the event of a 100 year flood. (See Watershed Address for more on how to find your local tributary.) How does this compare with Boulder Creek in a year of more or less normal runoff (400-500 cfs)?
• If 20 million gallons a day of drinking water are used, how many many cubic feet per second would that be?