Elk Creek Walk - "Pebble Count" EarthCache

This earthcache takes you to a popular public access fishing area along Elk Creek.  Here you will do a couple of tasks to show you how streambeds are often characterized based on the particle composition of the bed.

Please be courteous and respectful of any visitors to the area. The creek can get very busy during the steelhead run, so please do your best to not interfere with any of the fishermen.

Do not try to do this earthcache if the water level is too high or too fast.......... It could be dangerous.

The parking area is located near "American Legion Hole." This is a public access area along Elk Creek. It is accessed by taking a spur road which angles off to the south from Route 20 just west of Girard as you start down the hill toward Elk Creek. There are signs on Route 20 for several industries (including "Reddog Industries") that are located on this small road. Follow the road to the left, then past the industries to its end. The American Legion park is on the right, and the road continues straight down into a dirt parking lot along the creek.

Pebble Count refers to the characterization of a streambed based on the size particles that are present.  The particle composition can influence the form the channel takes, the erosion rates and other factors.  By repeating these measurements on a periodic basis it is possible to detect changes in erosion, etc. caused by man made changes to the stream.

Steep mountain streams often contain areas with a lot of boulders and/or cobbles.  Relatively flat or low gradient streams tend to contain smaller pebbles, sand and silt.  The particles in the stream can tell a lot about how the stream acts, and how it might act in the future.  Another important characteristic of a stream which is related to the composition of the streambed is the flora and fauna along the stream.  For example, trout require gravel to spawn on while certain insects require silt.  In order to document these difference a pebble count is done.

Examples of various streambeds:

There are several different techniques for making pebble counts.  Most of them rely on a systematic sampling of the particles followed by a statistical analysis of the data.  One of the most common techniques is called the Woman Pebble Count.  It was developed in 1954 by “Reds” Wolman.

 The general procedure for taking a Wolman Pebble Count uses a sizing system based on the intermediate axis dimension of a rock (dimension B in the diagram below).  This measurement (in mm.) is used to determine the size classification of the rock.

The procedure calls for a systematic, random sampling of the rocks in the stream.  Here is an abbreviated outline for this procedure: Start at one edge of the stream. Without looking, reach down and pick up the first particle you touch. Measure the length of the intermediate axis (dimension B in the diagram above). Record this information on the data sheet. Take one step across the stream and repeat the procedure. Continue until you have 100 or more samples.  You may have to cross the stream several times to reach this number. Do not stop part way across the stream.  If you reach 100 on your way across, continue until you have made the complete pass across the stream. There are several different methods for measuring the particles.  They include a ruler, a sieve and a special device known as a gravelometer.  In order to make comparisons over time, the same method must be used each time you make the count. Example of data analysis plot showing the cumulative percentage of particles smaller than a given size:       For example, this graph shows that the particles smaller than approximately 55 mm comprise roughly 50% of the particles in this sampling.   REQUIREMENTS FOR LOGGING THIS EARTHCACHE Equipment you will need – a ruler graduated in millimeters For this earthcache you will get a feel for what is involved in doing this sampling.  Following the procedure above: Randomly select a rock along the edge of the stream Take one step into the stream and select another rock Repeat this one more time for a total of three rocks Measure the size of the intermediate axis for each one Determine the size classification of each sample based on the chart above   In your log include your data (axis size and rock classification) for each of the three samples. Send an email to the owner describing how you would classify the streambed.  (Is it primarily boulders, cobblestones, gravel or sand?) As an optional requirement, include a photo of your three samples in your log.

FTF HONORS GO TO "bd's" !!!