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."
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!
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
For more hydrologic
terms, see the online version of the USGS USGS
Hydrologic Terms Glossary
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
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?