Forest Stream Origins

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- Hi, Sanford Smith here with Penn State Extension.

Today we're going to talk about streams, streams and forests.

And I'm joined by John Duncan, a colleague of mine and a professor of forest hydrology at Penn State.

John is going to tell us a little bit about where streams originate and what they look like as they slowly develop and then become things that most of us then call a stream, but in the beginning they're a little different.

And John, can you tell us a little bit about that?

- Absolutely.

So a lot of people think of streams as the blue lines on topographic maps or your favorite hiking map, and those lines don't move.

They were drawn there in some cases back in the 1950s or sixties, but in reality, those blue lines, those streams are moving in the landscape.

Under really wet periods, they're going further uphill slopes, and in drier periods, they're contracting down farther into the landscape.

And so our area of research here is really trying to figure out when and where these streams are flowing and for how long.

- Fascinating.

So streams are only in that location on the topographical map when they make the map, and otherwise it's a moving fluid kind of thing, which makes sense.

Water's always wanting to go down, right?

And it's always seeking a level somewhere too.

So it's fascinating.

So what does it look like at the beginning of most streams?

I think people wouldn't recognize them as streams.

- Right.

And so a lot of times, especially if you're out hiking in the summer, you might come across something as a small depression that there's evidence of leaves that have moved or pine needles that have been moved by flowing water.

But you may not necessarily think about it as a stream.

But the fact is that in many instances, those locations are flowing time and time again every time it rains, and can be defined as streams.

- Yeah, very early on.

Now today, we're going to be following a stream from it's origin down to a point where it really looks more like a stream.

And this particular stream originates sort of at the base of a stone.

There's even a little bit of a bubbling going on there, which is unusual, isn't it?

- [John] It can be, but it's a groundwater seep or a spring.

You know, a spring is typically defined as a larger version of a groundwater seep.

And so this is a defined part of the landscape where the subsurface geology is such that it's allowing groundwater to come up to the forest floor.

And in this case, this little stream, it flows year round, day in and day out because of that groundwater source.

If we were to head a little bit to our left or to our right, what that stream might look like is, could be a little bit different and is a little bit different, but a large number of the seeps in Pennsylvania do feed into these streams.

- [Sanford] So what do they look like next?

You know, they originate, they're really tiny, as we've shown here.

What does it look like as it gets maybe a hundred yards, 200 yards downstream?

- Yeah, so in general, what it looks like up here where it's close to the forest floor, there's not a well-defined stream bed with cobbles or stream banks, but we'll see that start to develop as we head down the mountain.

So the stream bed will start to get a different substrate.

There will be small stones or some sandier material.

The banks will become a little bit steeper, even if they're only a couple of inches to start.

And that will just continue to progress the more often water is flowing and moving that streambed material during storm events.

- Finally, when we get down to the bottom of this small mountainside here, what's it going to look like then?

- So in this particular watershed, the streams are on the order of three-foot, four-foot size banks and a pretty wide streamed with large cobbles.

That is, I think, pretty common for lots of our Pennsylvania forested streams.

- [Sanford] When you say cobble, you mean big stones, right?

- [John] Big stones, that's right.

Yep.

- [Sanford] Okay.

So we've described this process here.

Let's just talk about the water cycle briefly because where do these little streams fit in the water cycle?

And also I want to ask a question kind of a double question here is, what about other little streams coming and joining into the small stream that we're going to walk down along today or that we have walked down?

What does it look like when you get another stream join in?

So let's start with the water cycle.

- Sure, so the water cycle is really just talking about how rain falls into a watershed or a unit of land.

And some of that will become stream flow.

In forests, most of that precipitation, whether it's in rainfall or in snow melt, will actually infiltrate into the ground, become soil water, and as that moves down through the soil, become groundwater, which then can feed streams.

The other big component of the water cycle here in the Forest of Pennsylvania is evapotranspiration.

So it's part evaporation, rain that fell on a leaf surface or on a stone that will evaporate back to the sky.

And the bulk of it is in transpiration, the trees that are transpiring liquid water out as water vapor.

And so almost 70% of the rain that falls in Pennsylvania forests is put back into the sky through evapotranspiration.

- [Sanford] Right.

And it's important to note that that is during the months when you have leaves on the tree.

So that 70% of the rainfall and water that comes down, actually goes back out through the leaves.

It goes down into the soil and up through the roots.

But when those leaves drop, the trees can no longer do that.

They're no longer pumping water up the trunk.

- That's right.

And that's starting to happen right now. - Yeah.

- As these leaves are changing.

- Right, we may be hit by an acorn here any moment.

So John, the last thing I asked you about is what happens when these little streams join?

Does it look? - [John] Yeah.

- [Sanford] Does it look dramatic or does it become a much bigger stream?

What happens there?

- [John] So, and it does depend on the stream junction or where those forks occur, it's almost an additive process.

And so those smaller banks and the stream bed that we were talking about earlier, when two of those meet, it allows for more water to flow.

And if they're of equal size, you're getting double the water.

And so there's double the force to create that stream bed in the stream bank.

- [Sanford] Well John, thank you very much for joining me today.

This has been fascinating.

We've studied or looked at forest hydrology today, which is the science of the movement of water on the landscape, and in this case, forests.

If you're a young person watching, perhaps you learn something completely new and maybe you've been inspired to study this topic.

And if you're an older person, I bet you learn some things about the streams and the waterways you've seen all your life.

So thank you very much, and we hope you'll join us for another video soon.