The Evolving River

I just saw this fantastic picture of the Amazon river posted on this amazing website of aerial photos:



(Warning -- the site is enormous, if you're on dial-up, don't bother. I reduced the size on this particular image to make it less painful, but visit the link for the full resolution version.)

One of the things I like about the photo is how well it illustrates the growing, changing, constantly evolving nature of a healthy, natural river system.

Industrialized nations have largely tamed our rivers, constraining their wandering ways by building up levees and dikes, but wild ones are not naturally static -- they're dynamos of change, even if on our human time scale we tend not to notice. Take a closer look at that photo, with some arrows showing key features:



The water flowing downstream has tremendous force behind it. Powered by gravity and literally tons of mass (both from just the water itself and the various items carried in it), the current carves out the soil of the bank directly ahead of it. Blunted by running headlong into the solid mass, the water turns to one side and continues its journey, curving ever outward.

On the opposite side of the bank, though, the water eddies at a slower pace, and much of the soil, sand, and other particulates carried along in the flow get deposited. The river is carving out a path for itself, like a snake slithering across the dunes, constantly charging forward in one direction and leaving behind a new fresh bank in the other.

Eventually the giant, looping bows in the river get more and more exaggerated, to the point where two of them meet and ultimately break through the bank, forming a new, straighter channel for the river. In the image above you can see the pinch about to happen between the first two sets of red arrows.

Left behind are oxbow lakes, no longer supplied directly by the main waters themselves, orphaned remnants of an earlier time. Some of these oxbow lakes are fed by other sources and remain, but others eventually dry out and all that's left are dry, looping scars -- "meander scars" -- marking out the history of the system in the earth that's left behind. I've marked a few examples in the photo in green.

So the next time you fly or drive over a river, remember that this tame and consistent waterway was once a raging torrent, carving out new course for itself, leaving behind reminders of its former, wilder days until finally it was domesticated by humanity.