… avulsed.

Dear LAR,

In reflection of my quite spectacular blowout of the established way I had been living my life, I was thinking in river terms of a flow of water bursting its banks to re-form a new path. Does this ever happen in real life?

— The Daughter of the Vineyard

My Dear Grape Girl!

ABSOLUTELY. When a stream abandons its channel to form a new one, we rather onomatopoetically call it avulsion. Here’s my neighbor, the Yellowstone River, and a note in which she describes her experience:

Dear Daughter,

See how my floodplain is dotted with semi-circular “oxbow ponds?” I used to flow slowly right through those big bends, but (gulp) I left each of those channels to jump along my journey more quickly.

It happens like this: I get HERE and I know I’m going right over THERE — I mean I can see the next destination, almost touch it — and this long, flat, winding slog in between is doing nothing for me, so… I just go there. NOW.

It’s wonderful for everyone in the long run – animals love the oxbows. In the short term, well… the whole thing gets a bit disheveled. But you can’t make wine without crushing a few grapes, am I right?!

TYR

If it’s so chaotic, WHY avulse?

Rivers flow downhill, pulled by the earth to meet the ocean. That’s what they do.

Picture yourself hiking down a mountain: the shorter path is always STEEPER. Take it, and gravity pulls you so strongly that you can’t help but barrel onward.

Just so with rivers. Slope determines a river’s power. The river will always pick the more direct way downstream because it has more energy.

I suspect you can identify, dear friend. I suspect that your “established way” felt NOTHING LIKE a bee line toward your best life – your oceanic bliss – and that your energy felt flat.

WHEN, then?

Avulsion-perfect conditions develop over a long period of time:

  • The river gets flatter, and then
  • it finally draws near some topographic feature that would provide a significantly better drop. (The Yellowstone’s enticing feature is usually another part of her own active channel. It can also be some other channel, an incline’s edge, a cliff.)

But healthy rivers and people are woefully tolerant, rarely avulsing UNLESS the new direction offers at least six times the gradient. Six times! Explains a lot of years, huh!?

And even when the circumstances are temptingly ideal, one last requirement prevents a shift: power.

Wait, you ask, isn’t the avulsion itself ALL ABOUT getting more power? Yes, but it takes a lot of energy to make the change, as you probably remember. A Catch 22.

Luckily, slope is only one of two variables that directly determine stream power. The other is flow. Avulsion happens during high flow, like spring runoff, when it seems the WHOLE WORLD is bearing down on the channel. It’s easy to think of the floodwaters as overwhelming the river. In fact, that deluge is the one thing that can and DOES gives the stream exactly what it WANTS: the energy to do what needs to be done!

I wonder, beloved Grape Girl, what kind of flood may have assisted your own “blowout.” And of course I wonder about what came next, because HOW a river like you typically carves her new channel surprises many people. We’ll cover that tomorrow, I promise. Until then, thank you so much for your letter and for your brave example. “Spectacular” indeed. XXX

One thought on “… avulsed.

  1. Wendy

    Wow. My whole dissertation on risk taking in one blog post about the Yellowstone. Thank you Betsy, and thank you TYR. I’ve just found my sister river.

    Reply

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