Printable Copy of the Field Study Guide (PDF)

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.
Pothole Activities
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.
Looking at picture 6 estimate the percentage of water entering the stream from the
spring.