The Formation and Growth of Streams EarthCache

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Hidden : 9/17/2014
Difficulty:
2 out of 5
Terrain:
2.5 out of 5

Size: Size: other (other)

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Geocache Description:

This is an EarthCache, so there is no cache container hidden at these coordinates. An EarthCache is meant to provide an earth science lesson by having you make observations and reporting these to the cache owner. For more information about EarthCaches, visit EarthCache.org

As you cruise through South Park, you will pass many scenic tree-covered hillsides. All seems peaceful, but, hidden beneath the trees, a battle is being waged. When it rains, or when the snow cover melts, any water that is not absorbed into the ground is flowing down the hillsides, picking up dirt, plant material, and even rocks, carrying this load downhill. The roots of trees and other plants are trying to hold the hillside together, but time favors the water in this battle. Take a good look at these hillsides - in a million years or so, they may be gone!!

The flowing water, or runoff, will tend to flow in the most downhill direction, not across the face of a hill. The steeper the hillside, the faster the runoff will flow. Faster runoff has more energy for carrying material down the hillside, and the energy of flowing water is a big part of what we will examine in this EarthCache.
Any irregularities in the hillside, like bumps or pits, will tend to focus the water in the more directly downhill direction, so the runoff tends to favor certain pathways. As more and more water is focused into these same pathways, the increased amount of water (the volume of water) can carry more and more of the hillside material along with it. This will result in a deepening of the pathway, and thus is a stream born!

On a steep hillside, the runoff has a higher energy, and the rate at which it can remove material from the hillside is increased. The channels formed by high-energy runoff on steep hillsides tend to be narrow and V-shaped. The steep banks of these narrow channels are not very stable, and material can collapse into the channel, where the runoff carries it away.
As the steepness of the slope is reduced, the energy of the runoff is also reduced. The stream no longer has enough energy to transport some of the heavier debris, like large rocks, so some of this debris stops being moved and is left behind. On these reduced slopes, the streams tend to become shallower and wider. The channel is no longer a narrow V-shape, but becomes a broader V or even more of a U-shape. The type of channel a stream cuts can change over and over as it goes down a hill, depending upon the slope of the hill at different places.
When a stream reaches the gently sloping valley floor, much of the energy is gone. The slowly flowing stream meanders back and forth across the valley floor, carving away material until the valley is a very shallow U-shape.

When studying streams, geologists spend a lot of time examaning the rocks making up the stream beds, especially their size and shape. The size of the rocks will give information about how much energy the stream has. (You've probably seen this without realizing it - streams high in the mountains tend to have very rocky beds, while the sluggish rivers in Pittsburgh have muddy or sandy beds.) The shape of the rocks is also of interest, because the shape can give information about how far a rock has travelled. When a rock first enters a stream, it probably has sharp edges and corners. (This is an angular rock.) The more the rock is moved, the more chance for collisions and breaking off the sharp edges, producing a rounded rock. These smaller, rounded rocks are found in many of the streams in our area. Again, this is something you've probably seen, and now you have some understanding of the science behind it. Of course, not all the rocks in one place are the same size or shape. They will vary depending upon how far they have been transported. Geologists will measure many rocks to get an average for that location. We will have to measure this average by eye.


The streams you will be looking at are ephemeral, meaning they usually only have water flowing during or after precipitation. The cache can be done at any time, except when the runoff is at its peak. Use caution at all times.
Parking is available near the trail head, at the Lost Spring pavilion. Please respect the hours of operation for South Park, which are sunrise to midnight.

To log this EarthCache, follow the instructions below, and email me the answers to the questions.

From the Trailhead, follow the path uphill to the coordinates for the cache icon. Here at Site 1, the path crosses the first stream. Look upstream and downstream. Question 1A: What is the shape of the stream channel here in each direction? Question 1B: Why do you think they are different?

Go the short distance to Site 2, where the path crosses the junction of two streams. Question 2A: What is the shape of the two channels coming together here? Question 2B: Look upstream to the left. How many other smaller streams feed into this stream? Question 2C: Again looking upstream, there is a rocky area. On average, are these rocks more angular or rounded? What is the average size? (Use a comparison like "deck of cards", "paperback book", "textbook", etc. Question 2D: Why do you think these rocks are here?

Go to Site 3. (I walked down the point of land between the two streams.) Here you see that the two streams at Sites 1 and 2 meet to form a larger stream. Question 3A: Looking downstream, what is the shape of the channel now? Question 3B: Go about 15-20 feet downstream of the junction. The streambed is rocky here. Why are the rocks here? Question 3C: How do the size and roundness of these rocks compare to those at Site 2?

Below this junction, there is an example of an attempt to change how this stream is eroding the hillside. A dam has been constructed to catch the runoff before it can wash through the field and parking lot. There is a drainage pipe on the right that will drain off very high waters, but mostly the water is allowed to slowly sink into the ground.

Return to the trailhead, then (very carefully!) cross the road to the final site. From the middle of the bridge, look into the stream bed. Question 4A: Look at the type of rocks that are most abundant. How do the rocks here compare to the rocks at Site 3 in average size and roundness? Why do you think they are different? Question 4B: Because the stream has been contained on one side by a wall, it may be hard to judge the channel shape. Ignore the area right around the stream and look at the slope leading uphill from the road. Look at the slope leading uphill from the far end of the bridge. What is the shape of this channel?

I hope you've gotten an appreciation for some of what goes into studying the many streams and rivers existing today, as well as how our world has been shaped into what we see around us. If you want to learn more, I found a lot of good references by searching "stream formation process".
As always, cache safely and have fun!

References:
Stream profile adapted from: Rogen, David L., "A Classification of Natural Rivers", Catena 22 (1994).
Roundness diagram: http://brian-mountainman.blogspot.com/2010/04/more-on-stoneshapes.html

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