In a sense, it’s hard to picture a river moving its path. After all, water is driven by gravity to find the lowest point available to it and what could be lower than the river’s bed? How could the river be elsewhere than where it is?
But it’s obvious that where rivers make bends, they erode the outer (concave) perimeter of the bank. Not so obvious is that the material so eroded is not all simply washed downstream. Since the flow around the river’s curve is a vortex, the river’s helicoidal flow also carries some of the sediment laterally to the inner (convex) bank where it is deposited. And as the river’s curve becomes more pronounced, this process intensifies. And so begins the creation of meanders, the feature so common to prairie rivers. In extreme cases, rivers can even cut meanders off, leaving behind ox-bow lakes, like these ones near my home.
I had a vague theoretical knowledge of all this, but it really struck me when viewing GPS tracks of my river kayak trips superimposed on 1:50000 topo sheets. Since the maps were twenty years old, the rhe river had carved a new path for itself and in some places my track was hundreds of metres outside the mapped riverbanks.
But nowhere is this more pronounced than the Mississippi River. Here’s a sample of some truly beuatiful mapping from 1944 show the extent of meandering in this powerful river. Though I’m not sure, I think these maps likely predate the widespread use of aerial photography. And this information is stunningly displayed.For more images see mouthtosource.org.