Cut Banks and Point Bars

It’s funny: water is nearly incapable of going in a straight line when it’s moving across the land. Think about it. Any little thing — or nothing — will divert the flow. I once had a student vividly demonstrate this truism by pouring water over different inclined surfaces: various woods and stones, Plexiglas, rubber, window glass (both scummy and polished), carpeting, steep angle, almost flat, whatever — the stream never ran directly to the bottom. Deb made one hell of a mess, but still got a solid ‘A’ on her GeoFantasy.

The 3rd Law of GeoFantasy tells us that water is compelled to flow towards the sea (and take anything not firmly attached to the land along with it). But it’s important to remember that water doesn’t really care how it gets to the beach. “Come on, guys! As long as we keep movin’ downhill we’ll get there sooner or later.” So the flow will take the path of least resistance, and that is rarely in a straight line for very long.

Sure, we can force the issue with pipes and concrete-lined ditches and such, but this makes the water really unhappy, and unhappy water can be a determined and difficult adversary. It will rightly do what it can to break out, and — adhering to the 4th Law of GeoFantasy — will always win.

If moving water can carve the canyonlands of the American Southwest, it can certainly defeat your plumbing.

So, for whatever reason (or lack thereof), all natural streams tend to wander, at all volumes of flow and at all scales of wandering — “Nature’s rule, Daniel-san, not Miyagi’s”. As dictated by fate and physics, this natural bending of a stream directly affects its velocity, and because the velocity is affected so will be the stream’s energy, and its ability to transport the weathered and eroded continental debris. (Feel free to refer to an earlier post regarding streamflow and kinetic energy.)

As always, it’s easier than it sounds. As a stream meanders, the water on the outside of the bend has to travel a longer distance than the water on inside of the bend, and therefore it has to move faster. If it didn’t, you’d end up with a hole in the water, and — as the Pharaoh’s charioteers learned the hard way — that just never seems to work out very well. (Actually, a hole is created, but the upstream water falls into it — increasing its gradient, and therefore its velocity. Cute, huh?)

This is a simple concept that even a little kid can understand: the game ‘Crack the Whip’ makes use of this physical and mathematical reality to launch the poor sucker at the end of the chain into the bushes.

In any event, the end result is that since the water on the outside of the bend is going faster it has more energy, allowing it to cut into the riverbank, and then pick up the pieces and transport them downstream. Cut banks, they’re called (yet another clever name from the fine folks at GeoSpeak), and they build upon themselves — the sharper the bend the greater the velocity increase, and therefore the greater the erosional capacity of the water.

So cut banks migrate to and fro across a landscape, eroding and redistributing the stream’s floodplain and leaving behind oxbow lakes (a.k.a. meander cut-offs) as the stream meanders its way to the beach. Safety Tip #37: Never put anything you want to keep on the outside bend of a river.

That’s good advice!

What happens to all the eroded sediments? Well, they pile up on the inside of the bends where the velocity is slower, forming an accumulation called a point bar — a pile of rubble deposited in the lower-energy slack water across from the cut bank.

Cut banks and point bars: outside of the bend and inside; faster water and slower; higher kinetic energy and lower… erosion and deposition. Their development is orchestrated by The Rules of Reality, and they probably leapfrog down every meandering watercourse in the universe.