Sunday, October 27, 2013

Water Dragons

I love anime. I really enjoy my Trigun, my Bebop, and my Ranma, and I rewatch them from time to time. One of the better movies to come out in a long time was Spirited Away, by the ever talented Studio Ghibli. This is a fan-art image of one of the main characters, Haku the (SPOILER!) river spirit.

I've heard that some people have no idea why they chose a dragon to represent the spirit of a river. Why not a fish? Why not a bull/man creature like of ancient Greek legend? Is it arbitrary enough that it could as easily have been a unicorn or a griffin, or any creature off ?

How is a river like a dragon? Well, that's a complicated question. I know you come here for ecology reading, and I normally prefer to focus my academic reading in that direction, too, but the confluence (Hah!) of several things has resulted in me writing this piece about rivers and their morphology, and really, geological processes are a major part of how ecology functions, so here goes.

Lets talk about erosion for a minute first. Streams and creeks in high mountains often flow directly over bedrock, but as you move down toward the bottom of a watershed, you come across boulders, then cobbles, then smaller cobbles, gravel, then sand and silt. This happens as the the exposed rock of mountains get drenched in rainwater or snow, which later freezes and expands, enlarging cracks. Boulders crack off the side of a cliff face like a calving glacier, and water flows around them. Those boulders, too, crack and shatter, and over time the whole process turns mountains, the jagged, jutting bones of the earth, into rolling hills. Next time you're in an airplane, look out the window at the landscape and think about how the contours of every mound in the earth are shaped by epochs of wear and slumping caused by water. Our world is older than we can wrap our little heads around.

A river, in its natural state, jumps and writhes; it whips back and forth like an angry snake. This image is a drawing of the Mississippi River over geologic time. It changes like that both by its natural flow wearing new bends in its own bed, and also by the effects of periodic flooding and deposition of sediments, which can cause it to jump its own normal banks to a whole new part of the floodplain. How does that work?

When floods happen, a river swells to exceed its ordinary channel boundaries. It spills over into the floodplain and deposits silt everywhere. Rushing waters knock things around quite a bit, and sometimes an area that was a highly sinuous, meandering reach with slow flow might see a series of avulsions where those meanders get cut off, curves get abandoned, and the channel becomes a straighter, faster flowing reach along a steeper gradient. Imagine rolling a ball straight straight down a steep hill instead of carrying it down a trail with a ton of switchbacks. Rivers do that, and when they do, you get these abandoned channels in the floodplain, you get oxbow lakes, you get geological traces of the monstrosity of the river. Traces that can be mapped.

Now check out this sexy beast:

To the right, there, that's a segment of the Willamette River, baby sister to the Columbia, yet still the 13th largest river in the US, a more modern, computerized mapping project showing similar change to the image above of the Mississippi. In ancient times, glacial floods from the Columbia, which carved the great Columbia Gorge out of the continent and pushed enough sediment around to create rows of hills across Washington that look like the teensy rows of sandmounds left by retreating waves at the beach, deposited as much as 5 meters of sediment in the southern Willamette Valley, more than halfway across the state from where the Willamette River meets the Columbia. In the northern Willamette Valley, at a lower elevation, closer to where they meet, the sediment deposition was as much as 35 meters deep.

But even in more recent history, the beast has still been deadly. Few early European settlers in the 1850s tried to claim land anywhere near the river, preferring to settle on the upper margins of the densely forested floodplain where they still had access to all that lumber, but some measure of safety from floods. After several decades of almost continuous work by the Army Corps of Engineers and local interests to improve the channel and control flooding, the upper Willamette especially was still prone to frequent channel change. It took until the 1930's for things to stabilize enough that even 50% of the Willamette could be bordered by agriculture.

If you've read my blog before, you may have come across the post in which I talk about how water flows through the riverbed as much as over it, if not as quickly. One of the results of this is that an awful lot of gravel and sand gets moved around. As long as the river flows, more sand will always be brought down from up high. Fortunes have been made just mining the gravel and sand from rivers. But this also means that rivers require a certain amount of . . . maintenance . . . in order to be reliably usable for commerce.

When the Columbia River was first being used to access the continent, none of this work had been done. The mouth of the Columbia was very wide, and very shallow, and the navigation channel was different every year; sometimes there were four channels a ship could use, sometimes one. It's hard to overstate the danger this posed to early European explorers trying to get through. However well a river pilot thought he knew an area, the channel he'd used last time might not be there anymore, and the ship could run aground without any warning. A typical shipwreck in the 1850s could cost 40 human lives and a whole shipload of cargo, which represented the livelihoods of many tradesmen and businesspeople waiting at the port. It was 1879 before the US Army Corps of Engineers tried to narrow the mouth of the channel in an attempt to make the flow deeper.

Is it any wonder the spirit of a river can be represented as a dragon? Over the course of eons, the slender claws raked the landscape, gouging ice into the greatest of mountains to wear them down to hillocks, the body writhed across the floodplains and for centuries destroyed all we could hope to build, and the mouth could swallow entire ships whole.

Nowadays, we keep our dragons rather tightly chained, muzzled. We dredge and we build levees and seawalls to lock them down into beds we like for them. We chain their power with dams that suck the energy out of them, turning what was once a powerful flow into more a series of lakes.

But, I suppose . . . that's a post for another day.
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