#### Station #4: Pot holes / Profiles / Groundwater

At this location on the field trip, students observe potholes that have been carved in the streambed by the action of moving water.  Begin by viewing the pothole picture show. The first picture is a view of station 4 with potholes visible in the streambed as we approach it from downstream.  Pictures 2, 3, 4, and 5 are close ups of the potholes.

Water running downstream provides enough force to begin a whirling motion of rock fragments that fall into a small depression. As the rock fragments are swirled and bump into each other, they carve the bedrock of the streambed, making the depression deeper and larger. If you look carefully through the glare of picture 5 in the above link, you can see the rock fragments (called scouring stones) presently caught in the pothole. New rock fragments tumble into the pothole as older ones move on or are worn away, enabling the grinding process to continue. At this location, the water is only flowing fast enough for the scouring stones to be swirled when the water in the stream is very high.  In the pictures, the stream is not moving fast enough for pothole formation to be taking place.

## These activities attempt to mimic the process of pothole formation:

1) a) In the lower margin of this page, move your pen around in a 1/4" diameter circle for 1 minute and describe what happens below:

How many layers of paper did your pen tear through?

b) Get two pieces of rock – one pebble and one cobble.  This time rub the pebble in a small circle on the cobble  for 1 minute.

Describe the results below:

Water Sources at this station Examine picture 6 in the pothole picture show.  In this picture the stream is seen in the foreground. If you look at the far side of the stream there is a tree with a hollow at its base near the center of the photo.  Look carefully and you will see that water from an underground stream flows onto the surface from this small cave.

2)     Looking at picture 6 estimate the percentage of water entering the stream from the   spring.

_________ %

## Using the techniques learned at school, students determined the profile of the stream bed at this station.  They started approximately 5 meters before the pothole section up to about 5 meters past the point where they end.  Students took 4 readings: the starting point, the base of the pothole section, the top of the pothole section, and the finish point. (see diagram below).  Readings were taken in cm/m (centimeters per meter).   Actual student data has been recorded in the data chart on the following page for you to use.

3) a) Complete the table

 Flag   # Distance From Previous Flag (m) Total Distance         (m) Jakestick  Reading (cm) Change In Jake Stick Reading From Reference          (cm) Difference in Elevation from Last Reading        (cm) Elevation of this Location (cm) 1 0 0 166 Reference Level = Eye Height of Viewer N/A N/A 19,659 2 9.2 48 3 5.3 74 4 2.4 92

3) b) Draw the profile for the data represented by the table using an appropriate scale. Be sure to properly label the axes.

## Questions- Station #4

1) (a) What does the paper represent in the pothole activity?

(b) What does the pen represent?

2) Describe the process by which potholes are created.

3) What evidence do you observe for this process having taken place?

4)     From completing activity 1b, what might you infer about the length of time it takes to form a pothole?

Explain:

5)     (a) Calculate the gradient between flag 1 and flag 4 at Station 4 in the space below:

(Show all work and units)

(b)  Record the gradients calculated for the stream at Station 1 below.

Station 1, location A gradient ____________________

Station 1, location B gradient ____________________

(c) How do these differences in gradient between Stations 1 and 4 help explain the

presence of potholes at Station 4 and their absence at Station 1?

6)     (a) Does the surface water from the stream account for all of the water in the pothole area?

(b) Based on your observations of the running water at this station, where is this water coming from?