For starters you won’t be able to do the earthcache observations if the Little Miami river level is too high.  Check here for the river level.

http://water.weather.gov/ahps2/hydrograph.php?wfo=iln&gage=mlgo1

The river was at 5.5 feet when I made my observations.

Next, there is no need to bushwhack to get to the GZ.  The earthcache is located in the Terrace Park Wilderness Preserve.  There is parking at N39 09.160 W84 18.826.

One last thing – it might be helpful to print this earthcache listing out (or study it beforehand) and take it with you.  If not, you might want to bring a ruler or tape measure along.  We don’t want you to have to take a pebble home to measure it!  You can take a picture of your pebble next to your GPS and scale it from the picture when you get home.  I try to do as much “paperless” earthcaching as I can.

When you get to the cache location you want to be standing in the middle of what is an occasion flow channel in the Little Miami River.  The coordinates could be off a little bit.  When the river level is low it flows south of the fluvial island you see to the south.  When the river level is high it spills through this channel.

Wolman Pebble Count

Developed by “Reds” Wolman in 1954 is this a statistical measurement of stream bed rock sizes.  This count is used to assess the erosion and sedimentation of streams and rivers.  These assessments are often done to study the effects of man made changes to a river – measuring the pebble count upstream and downstream of a dam,  and also to predict the future  behavior of a natural river.  The composition of the streambed and banks is an important facet of stream character, influencing channel form and hydraulics, erosion rates, sediment supply, and other parameters. Observations tell us that steep mountain streams with beds of boulders and cobbles act differently from low-gradient streams with beds of sand or silt. You can document this difference by collecting representative samples of the bed materials using this pebble count procedure.

Below is an example of the type of study on a point bar and the measurement points.

From the sample points a statistical make-up of the pebble bed can be made based on size and frequency.  The data is usually expressed in a chart as shown below.  A common statistic quoted for a study is the D50 or median pebble size, but the distribution around the median can also be used by those studying the behavior of rivers and streams.

Another way to depict the results is shown below.

For this earthcache we’ll just do the Wolman pebble sizing for one of rocks versus the 100 samples typical!

The diagram below shows how to make the pebble measurement.

The pebble size is based on the “B” dimension.

For you paperless cachers I’ll try to describe the dimensioning.  Almost all the rocks you’ll see around you are relatively flat on one side with an oval shaped face.   Dimension “C” is the thinnest dimension between the 2 flat faces.  If you look at the oval face of the rock “A” is the longest dimension and “B” is the direction perpendicular to “A”.  In the “B” direction you take the largest width.  Another way to think about it is if you tried to pass the rock through the smallest opening in a sieve what dimension would be limiting.

To log this earthcache please do the following and email me your observations and conclusions:

1)       Pick out the rock that you estimate is the median in size (half the rocks around it are smaller, half are larger) and measure its size.  If you printed this out I included a scale below for you to use.  Another method is to take a photo of your gps receiver next to the rock with the largest oval face up and scale it from the digital photo. 

2)      Based on the table below what would you classify your measured “pebble”,  gravel, cobble etc.

3)      Optional: post a picture of yourself at the site or of your pebbles and you gps.

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