Background – Why Evaluate the Physical Characteristics of a Stream?

A stream is the carpenter of its own edifice…

Stream channels and floodplains are constantly adjusting to the amount of water and sediment supplied by the watershed.  Four physical characteristics of a stream are in a dynamic state of equilibrium called Lane’s Balance.  These characteristics are streamflow, channel slope, sediment load, and particle size.  If one of these characteristics change in a stream, one or more of the other three must also change to accommodate and achieve equilibrium again.  The change of streambed texture (particle size) is the first thing to change in response to a disturbance to restore equilibrium and it is the most sensitive measures of change.  A Wolman Pebble Count can measure the texture of a stream.  From the pebble count and a subsequent measure of point bar deposits, a riffle stability index and log relative bed stability can be calculated.  These two values can provide insight into changes in sediment load carried by the stream.  In other words, they can tell us if sedimentation is occurring.

Pebble Count Method

The pebble count method is conducted on a portion of the stream to evaluate the streambed composition.  The composition of the streambed can tell you a lot about the characteristics of the stream.  It can illustrate the effects of flooding, sedimentation, and other physical impacts to a stream. 

What do the results of the Wolman Pebble Count mean?

The numbers generated from the Wolman Pebble Count are a unique characterization of the composition of the streambed at one particular point in time.  Some interpretation can be made about the results at this time, such as percent finer.  More useful interpretation of the data will come by comparing measurements over time or to a reference site (to a stream of similar classification).