Have you ever flown over a winding river and wondered why rivers move? Why do they wander across the landscape and evolve the way they do? The lower Mississippi River and its delta form a spectacular example. But others are all around us.

Take, for instance, the Trinity River where it enters Galveston Bay. Or look on any map. You will likely see landscapes carved by rivers that leave evidence of their former paths behind.

But why do rivers change course? The brilliantly simple YouTube Video by Practical Engineering below describes the basics of “fluvial geomorphology.” That fancy phrase describes the science behind the shape of rivers.

The Mathematics of Geological Change

Back in 1944, a geologist named Harold Fisk, Ph.D., then a professor at Louisiana State, produced a report for the Army Corps of Engineers called “Geological Investigation of the Alluvial Valley of the Lower Mississippi River.”

Fisk produced gorgeous historical maps of the river snaking across miles of river valley.

A decade later, Emory Lane, a civil engineer and hydrologist at Colorado State University, went on to develop a unified theory of sediment transport. His theory explains the movement and shape of such rivers in an “equation” that uses just four variables:

• Quantity of sediment carried by the water
• Median sediment size
• Quantity of water
• Slope of the landscape (length divided by elevation change)

Lane’s “equation” looks like this.

That funny symbol in the middle means ‘is proportional to.’ Scientists use it to show something that varies in relation to something else.

If you change one variable, one or more other variables change to bring the river back to its “normal” state. Scientists and engineers still use this formula today.

It means that in a stable stream, the flow of water multiplied by the slope is proportional to the amount of sediment being transported times the size of that sediment. (But don’t let that scare you!)

From Stream-Table Models to Real World

In the abstract, that may be a lot for average people to wrap their heads around. So the video uses a “stream table” and balance-scale model to illustrate what happens when you change each variable. They bring the formula to life and make it easy to understand. For example…

More water (say, in a flood) can move more and larger sediment. So, the banks of a river erode.

This can threaten roads, pipelines, and property. The eventual deposition of all that sediment can also choke a channel and contribute to flooding. Or fill up reservoirs and reduce water supply.

Sound familiar? All those things happened in the Lake Houston area.

To restore balance, the river changes its slope by increasing its length. This explains the meanders found in most rivers in this region. A meandering river wanders back and forth across the landscape like a snake instead of making a straight line through it.

Wherever slight bends occur, the river scours the outside of the curve (called the cut bank). That’s because the water moves faster on the outside of a curve. The river then deposits larger particles of sediment on inside curves farther downriver (called point bars) where water moves slower.

Eventually these curves in a river become so exaggerated, that they cut themselves off, leaving oxbow lakes behind.

This National Park Service page contains an excellent series of illustrations that show the evolution of meanders over time plus their migration across the landscape.

Lane’s equation predicts that there’s no such thing as a stable river. All rivers change all the time in response floods, drought, development, dams, sand mining, farming and more.

When Natural Systems Lose Balance…

At every point along a river or stream, erosion and deposition are constantly balancing each other.

But Lane’s equation can’t predict exactly where or when a river will move. Nor can it predict the rate of change. The Practical Engineering video points out that the rate and volume of change depend on other factors not in the equation, such as vegetation and the “pulsing” of flows as you might see downstream of a dam like the one on Lake Conroe.

The screen capture below shows what happened in models comparing a steady flow and a pulsed flow.

The pulsed flow creates much more erosion and faster movement of channels. And that has many real world implications.

Who Should Watch This Video?

This 16-minute video is a real eye opener for a variety of audiences. It’s suitable for students from late middle school and up. You don’t need to be a math or science whiz to understand it. Its power is its simplicity.

Among students, the video may stimulate curiosity in earth sciences, engineering, math, economics, history and urban planning. And for adults, it shows how four variables tie them all together.

It makes a great tutorial for policy makers struggling with issues such as setbacks from rivers for homes and businesses.

In addition, everyone who lives near or is considering buying property near a river, stream or channel should view this.

The producers say the next video in the Why-Rivers-Move series will show how human changes affect the flow of rivers. Can’t wait!

My thanks to Dr. Matthew Berg, CEO of Simfero Consultants for bringing this to my attention.

Posted by Bob Rehak on 3/24/23

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