… bankfull.

“If you don’t know bankfull, you don’t know shit.”

Dave Rosgen

No, the rock star of Natural Channel Hydrology did not email Like a River with this insight. He told me in person, a few years ago – just as he does each student in every class he teaches.

You’re likely asking, dear reader, what is “bankfull” – a noun, a verb? That adorable cowboy from the bar outside Lolo where Dave took us after the last field class?

What It Is — The Storybook Version:

Picture a classic summer river – a wholesome child fishing from the grassy bank, a picnic spread beneath the nearby shade tree. Each of these archetypal elements illustrates a hydrologic feature:

  • The fisher-youth sits right where the upward-sloping bank flattens out.

That exact spot provides a bottom-beckoning horizontal seat AND space below for outstretched legs. Hydrologists refer to this spot as the point of imminent flooding. When rising water reaches this level, it is JUST ON THE VERGE of suddenly spreading wider.

  • The picnic blanket is close enough to see and hear the river, yet the ground is dry AND flattish.

This (mostly) horizontal space adjacent to a stream is the floodplain.

  • The tree grows on the floodplain…

… because its roots are irrigated from below by the river’s base level BUT its trunk is submerged only rarely (about one out of every three years) and not for long (just the few weeks of peak flow, usually in spring). That natural “flooding” actually SHAPES the floodplain and ENRICHES it by depositing soil, nutrients, and seeds.

When the stream level almost touches the youngster’s sit spot, then THAT is what hydrologists call bankfull flow (or bankfull discharge but that word is kind of icky.)

What It Is — The Geekily AMAZING Version:

Don’t run away. What follows is mind-blowingly elegant (and not just because I got to use SHARPIES with GRAPH PAPER but also because I snuck in a hot link to the most poignant picture book ever… hover over the graphs to find it!)

You remember that as a river’s flow increases, its power and ability to carry sediment also rise. This orange curve shows that very high flows move a lot of material:

But if floods are so powerful, then why aren’t channels carved to fit those HUGE flows?

Because high flows don’t happen very often, as the next graph shows:

The trouble here is that the most FREQUENT flow is not big enough to fill most channels – or powerful enough to form those channels. [Sorry the x-axis label got chopped.. it should of course read “Flow.” But you knew that.]

No, what we’re after is a river’s channel-forming flow — the one that moves the most sediment. We can calculate it by MULIPLYING frequency x sediment transport rate.

I have plotted the result in red on this last graph so you can really SEE the maximum value. Get ready because it blows my mind every time I use it in a PowerPoint (which is embarrassingly often) :

Notice I overlapped this graph with the first two? Notice the maximum, the most effective flow is where frequency meets power!? Ah. I told you it would be great.

Like a River…

… we mustn’t expect ourselves to be “at our most effective” in every single moment. Nor are we defined by our rare-but-epic episodes, no matter how great or devastating.

The times that shape us AND the times we shape the world lie somewhere in between — maybe during two-thirds of our peak moments — where our pretty darn strong power happens pretty darn often.

And guess what?

If you run that amount of energy down our storybook stream, it fills the channel exactly to the brim. When you know that, you know all kinds of #@!

PS — Because you were so attentive through graphs and lingo and even some multiplication, here’s a piece of river candy. My uncle took this photo of the Tuki Tuki River in New Zealand. He was always the whip smart one in the family and now he’s all creative too. It’s a little intimidating for me, but you guys luck out:

Thanks Uncle Conny! XXX

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