I?ll tell you the moment I realized I?m a raging ignoramus when it comes to rivers, and that I really needed to educate myself. It was when Lockwood and I were mooching about Avery Park.
We?d just had a nice dabble down by the Marys River.
Other rivers had compelled me with beauty, power, and drama, but those had been operas: you?re so sated by the performance that you don?t realize you didn?t understand a bloody word. The Marys River sang me a folk tune. Just a simple little ear-worm much like this one:
Fr?re Jacques, fr?re Jacques,
Dormez-vous? Dormez-vous?
Sonnez les matines! Sonnez les matines!
Din, dan, don. Din, dan, don.*
And it was just simple enough, just plain ordinary enough, that it started bothering me severely.
This is the tune she sang:
A slight rise in the ground at Avery Park.
I have walked up slight rises in the ground a million times. It?s nothing, it?s topography. In built-up areas, I don?t really think about them at all ? chances are, people were pushing dirt around. Or so I?ve always thought. It?s like hearing a song as only nonsense-words. No sense to it. No sense to be made. Hum it and move on.
But as we walked toward the rise, Lockwood pointed it out as a fine example of a fluvial terrace. ?Oh. Ah,? I said, and snapped a picture.
A slightly cropped view of the fluvial terrace.
Fr?re Jacques, fr?re Jacques ? Brother John. Yes, I know those words ? Brother John, just like I know the words fluvial terrace.
?We?re passing from the lower to the middle terrace,? Lockwood said as we climbed the gentle rise.
Dormez-vous? Dormez-vous? Are you sleeping? Yep, perfectly understanda ? wait. What does that even mean? The words are perfectly comprehensible ? but they don?t make sense in context.
I stopped for another look at the terrace. Lockwood said some things about it, such as the flood waters would frequently make it up over the lower terrace but not the middle. You don?t build your stuff on the lower, do it on the middle ? those few feet make all the difference. And this is stuff I?d always just bobbed my head at before, like singing the nonsense words of the song. Sure, there?s more than one terrace ? rivers make terraces, that?s what rivers do, it?s why they?re called fluvial terraces. Brother John sleeps. It?s his thing. But why? Why is the river making terraces?
The middle terrace, up close and personal.
If the flood waters don?t get up here, how the fuck is it making terraces? What are they? There?s a whole other part to this song, and I don?t understand the words, and they?re what will make sense of it. I will know why it?s important that Brother John is sleeping, know why he?s called brother, if I learn the rest of the words. I will understand this terrace if I learn just what the hell a fluvial terrace is.
What I?d been taught is this: the river rises, it deposits stuff, it recedes and downcuts, et voila: a terrace. It was like the next words of the song, which I?d been told said, ?Morning bells are ringing!? Okay, fine. Fluvial terraces are river stuff, and Brother John?s sleeping through the morning bells. And it had never occurred to me before to ask, ?How?? How does a river deposit so much sediment so far away from its course, build it up to that thickness, when floods don?t usually reach there? How does a man sleep through morning bells?
So let?s find out what things are really saying, then.
Sonnez ? ring the bell.
What do flooding rivers do?
All right. That doesn?t teach us specifically about terraces, but it shows how natural levees are created, and how floods start building up alluvium which the river may someday cut through in order to form a terrace. What, then, are terraces, when you get right down to it?
Hypothetical valley cross-section illustrating a complex sequence of aggradational (fill) and degradational (cut and strath) terraces. Note ct = cut terrace, ft = fill terrace, ft(b) = buried fill terrace, fp = active floodplain, and st = strath terrace. Image and caption courtesy Wikipedia.
All right, so the words fluvial terrace actually refer to very different things, which have some basic similarities. There?s the basic morphology: treads ? the flat bits ? separated by risers ? the slopes. So they have a common shape, and you can expect to find them right alongside the river channel, whether the new or old one. Fair enough. And there?s different kinds ? a fill terrace, which is formed by the valley filling with sediments, which the river then cuts through again. A cut-in-fill terrace, which are erosional bits that may form below the fill terrace when it?s been cut. A strath terrace, which is a fluvial terrace cut in bedrock. Right. But those terms don?t mean much on their own. They get us closer ? they don?t get us all the way there.
We now know bells aren?t ringing so much as we?re expecting Brother John to ring them. He?s apparently slept in. Which bells is he supposed to ring, though? None of this makes sense if we don?t know which bells.
Why might a river begin eroding a terrace? Let?s investigate:
Regional uplift: The river was flowing along happily in its valley, on a nice even keel, until the vagaries of plate tectonics hoisted it up. Bother. Now it?s got to cut down in order to find its happy place again. Voila ? a terrace!
Change from dry to wet climate: River wasn?t very big before, didn?t have a lot of cutting power, but then it started raining all the time and bam! It?s now big enough to slice through that landscape, baby, yeah!
Drop in base level: Sea level goes down, river has to go down with it. Such as what happened during the Ice Age. Yowsa!
Speaking of ice: Glaciers stop depositing outwash, and the valley fill they left behind gets dissected by a river.
All of these are possible reasons why the Marys River cut such nice terraces in its portion of the valley. Now it starts to make sense. Now I can grasp why a river was able to sculpt so much of the landscape in a place where its current floods don?t reach.
Les matines ? Matins. And din-dan-don ? that?s the sound bells make. Oy! Now I get it! He?s Brother ? friar ? John because he?s a Catholic dude, and he?s supposed to be ringing the bells for Matins, only he?s slept in. Aha! Lazy bastard! Let?s kick him out of bed with a little song:
Friar John, Friar John
Are you sleeping? Are you sleeping?
Ring the bells for Matins! Ring the bells for Matins!
Ding, dang, dong. Ding, dang, dong.
What this little stroll through fluvial terraces singing a nonsense tune is supposed to illustrate is that words alone have very little meaning. You can know the terms and not know a thing about what?s going on. It?s important to dig deeper, to understand not only the surface meaning of words, but what they represent. Then we?ll know that a fluvial terrace isn?t just a rise in the ground, but one that tells us about the changes the river has seen as it flowed through all those years. And if there are many terraces, we know that river has seen quite a bit of change in its equilibrium.
Much like our poor sleepy Friar John, whose equilibrium wouldn?t be half so disturbed in the mornings if he threw over this religion stuff and became a happy heathen.
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References:
DeWitt, L. (2012): OMN Willamette Valley Geology Presentation Part Two: Streams, Processes and Landforms. Outside the Interzone. Last accessed 10/16/2012.
Oberlander, T.M. and Muller, R.A. (1987): Essentials of Physical Geography Today, Second Edition. New York, NY: Random House.
Plummer, C.C. and McGeary, D. (1991): Physical Geology Fifth Edition. Dubuque, IA: Wm. C. Brown.
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*For one of the cutest children in the universe singing this song in four languages, go here. Brace yourself. Even those of us without a fully functional kids-are-adorable center in their brain shall melt. I am having to mop myself off the floor as we speak.
This post was originally published at ETEV.
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