We often think of rivers as mostly straight or gently bending waterways.

If they are, it’s because we’ve made them that way, guiding them with levees or dredging them for navigation.

When viewed from the air, natural rivers, especially across nearly flat landscapes, bend dramatically, sometimes folding back on themselves.

Rivers meander, constantly changing their paths.

Rivers have several sources: lakes, springs, drainage plains, and melting snow or glaciers.

When they originate in steep areas, gravity moves the water fast.

Here, rivers are mostly straight, as they have the power to cut their own v-shaped channels into the land.

As the slope becomes gentler, so does the river. It loses some of its cutting power and begins to bend around high points.

When the land flattens, the river begins to erode sideways, rather than down.

That’s because water on the outside of a bend moves faster than on the inside, cutting into the bank.

Over time, that pushes the bend farther and farther out. Dozens of wide bends, one after another, can give the river a shape like bunched ribbon.

Over the last 50 years, scientists have turned to math, physics, and satellite photos to study the natural paths of rivers and better predict where and how their bends will form -- and when they might meander into farmland or development.

That’s often when we give the river a little human guidance to conform to our desired landscape.