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?