Welcome! This site is one of five EarthCaches located on the Marias River between the Tiber Dam Recreation Area and Moffat Bridge at County Highway 223. While this series is intended for a younger audience, feel free to use this as a quick guide or learning tool no matter what your age.
For the next EarthCache in the trail, click here.
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Gaging Station: A Brief Introduction to River Mechanics
This EarthCache is located on the grassy bank of the Marias River just before a big right hand bend. Don’t worry about standing on the exact spot of the coordinates, as long as you have a good view of the river ahead you are in the right place.
Face downstream and take a look over at the left bank of the river. That strange looking white structure you see is called a gaging station. This is where we get information about how much water is going through the river. In places like this, a hydrologist (that means someone who studies water) can find out how high the river is, how fast it’s going, and even how much water is going through it. Usually these measurements are taken with equipment that’s been built into the river and recorded in the booth on land, but sometimes an unlucky (or lucky, depending on your point of view) scientist might have to go out on the wire you see strung across the river and dip a current meter in by hand. Yikes!
Since you’re going to be spending some serious time on the river over the next two days, let’s take a minute to find some cool features and see what they can tell us. If you really understand this stuff it should make your trip more interesting, and maybe even a little bit easier on your arms.
First, what do you notice about the shape of the river here? Is it straight or going around a bend? It’s taking a turn, isn’t it? In rivers, a turn is called a meander, and a channel with lots of curves in it is said to be meandering. The way these meanders work is part of what makes rivers so special. While a river may seem big and permanent, it is actually constantly moving and changing shape right in front of you. Meanders grow and then get cut off. Islands pop up and disappear again. The river can even seem to pack up and move somewhere else while you’re watching it. This all happens because as the water flows downstream, sediment (this means little bits of rock, like sand or mud) is constantly being moved around. Depending on where you are, you can see sediment being eroded (this means worn away) or deposited (this means put in place). The different ways that these things happen are responsible for the river’s ever shifting character.
This probably seems confusing, so let’s connect it to some of the things that you’re looking at. That way it will get much easier to understand.
Let’s learn about deposition:
Look down at your feet. What you are standing on is a feature called a point bar. Point bars are broad, flattish shapes made of sand or mud located on the inside parts of meanders. Over time they grow out into the river, and can even grow layers like a tree grows rings. Remember, point bars are always on the inside part of the meander.
So why do they form? It’s complicated, but basically, as water whips around a corner it pushes up against the outside bank. The force of this actually pushes the water up up than the water on this inside of the bend. This elevation difference causes the water to spiral downward and sweep the sediment off the side of the river and all the way across the channel, then back up the bank on the other side. At this point the water slows down and drops what it’s carrying. If you’ve ever made lemonade from powder you’ve seen this happen. Stir the glass and the powder gets lifted up and swirled around, but when the water stops moving the powder settles to the bottom. Sediment is put in place. This is what we mean by deposition.
From now on when you see a point bar, think deposition.
Let’s learn about Erosion:
Look across to the other side of the river. Notice how different the bank looks over there? It’s steep looking, right? Almost like a mini cliff. This feature is called a cut bank. Over time, cut banks can move back away from the river and eat away at the land. Don’t forget, cut banks are always on the outside part of the meander.
But what makes cut banks in the first place? They form when the spiraling water moves fast around the outside of the curve and cuts into the ground. Unlike at point bars where water is slowing down and depositing sediment, at cut banks the water is moving fast and taking sediment away. A good example of this when you spray a car with a hose to wash the dirt off. Sediment is taken way. This is what we mean when we say erosion.
From now on when you see a cut bank, think erosion.
Keep these ideas in mind as you continue down the river. See if you can spot any more cut banks and point bars as you go.
To claim this cache: Answer the following question and send the answers using Geocache's messaging tool.
Q: What is deposition? What is erosion?
Q: What river feature lets you know that deposition has taken place? What river feature lets you know that erosion has taken place?