Monday, October 18, 2010

Question Of The Day, 74

Why aren't all rivers perfectly straight by now?

As a kid, I remember reading an article -- probably in Ranger Rick but maybe in National Geographic's World-- about how rivers that had curves in them would, over time, erode away the soil and straighten out their course, leaving little stranded lakes. I remember the diagram, too, which looked something like this:
With the original river on the left, the changes in between, and eventually the "new" river over on the right, with those little lakes left over. The changes would be caused by the force of the water coming up and hitting that curve and eventually just pounding through the dirt to create a new channel.

So if that's true -- and I assume it must be, because half-remembered Ranger Rick articles from childhood are always true (except the ones about tidal pools, which don't exist) -- then why are rivers curvy at all, anymore? Every river that exists has been around for something like 1 million years, time enough to carve the Grand Canyon. If the Colorado river can dig out that kind of canyon, why can't other rivers get through some mossy soil?

Lest you doubt me and my half-remembered Ranger Rick's, look at this photo. It's of Lake Marrangua,

In Africa, and Lake Marrangua is described as a stranded lake that once had an outlet to the sea. (Source.) Which is proof that it can occur, I suppose, but if it can happen, then why hasn't it happened all over the world? Are "stranded lakes" and rivers straightening their course another thing that scientists have made up, like velociraptors and the Boxing Dinosaur?

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