Streamflow

Why are we concerned?

Streamflow, or discharge, is the volume of water moving past a cross-section of a stream over a set period of time.  It is usually measured in cubic feet per second (cfs).  Streamflow is affected by the amount of water within a watershed, increasing with rainstorms or snowmelt, and decreasing during dry periods.  Flow is important because it defines the shape, size and course (i.e., amount of meandering) of the stream. 

Velocity is the speed of the water in the stream, measured in meters or feet per second.  Stream velocity is greatest in midstream near the surface and is slowest along the stream bed and banks due to friction.

Food sources, spawning areas and migration paths of fish and other wildlife are all affected and defined by streamflow and velocity.  Velocity and flow together determine the kinds of organisms that can live in the stream (some need fast-flowing areas; others need quiet, low-velocity pools). 

 

The background science

Streamflow is affected by both forces of nature and by humans.  In undeveloped watersheds, soil type, vegetation, and slope all play a role in how fast and how much water reaches a stream. In watersheds with high human impacts, water flow might be depleted by withdrawals for irrigation, domestic or industrial purposes. Dams used for electric power generation may affect flow, particularly during periods of peak need when streamflow is held back and later released in a surge. Drastically altering landscapes in a watershed, such as with development, can also change flow regimes, causing faster runoff with storm events and higher peak flows due to increased areas of impervious surface. These altered flows can negatively affect an entire ecosystem by upsetting habitats and organisms dependent on natural flow rates.

Tracking streamflow measurements over a period of time can give us baseline information about the stream’s natural flow rate.

 

Safety Considerations

You will need to enter the stream channel to make width and depth measurements and to calculate velocity.  Be aware of how fast the water is moving, water depth, and bottom conditions at your monitoring site.  Do not attempt to measure streamflow if water velocity appears to be fast enough to knock you down when you are working in the stream.  If you are unsure of water depth across the width of the stream, be sure to proceed with caution as you move across the stream, or choose an alternate point from which to measure streamflow.

 

More information on streamflow is available in the fact sheet "Streamflow:  Flow Speaks Volumes" Links to an external site..

 

Think you've got it?

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