What if a river changed its course?

Look at a river from high above. It bends this way and that, like a wiggly snake.

Rivers seem to stay in one place. But do they? What if a river moved to a new path?

From an airplane, many rivers look like ribbons laid in loops across the land. A river with lots of loops is called a meandering river. Because of the loops, the distance along the river is much longer than the straight-line distance across its valley.

Rivers look fixed on a map, but given enough time they wander. This thought experiment asks what happens when a river changes its course, using what scientists see in real rivers. Choose a path to explore.

A meandering stream is a single channel that winds through its valley so that its length along the channel exceeds the straight-line distance. Its bends are not fixed: they migrate sideways and slightly downstream as the river reworks its banks.

This thought experiment follows that process from growing bends to cutoffs and oxbow lakes, then scales up to a whole river switching its route, using the lower Mississippi and the Atchafalaya as a real case.

When water flows around a bend, it goes faster on the outside.

The fast water nibbles the outside bank away. On the inside, the slow water drops sand.

When a river flows around a bend, the water on the outside of the curve moves faster than the water on the inside. The fast water wears away the outer bank, making a steep edge called a cut bank. On the inside, the slower water drops sand, building a beach-like point bar.

So the river eats away one side and builds up the other. That way it slides sideways without getting any wider, and its bends slowly grow and shift.

Around a bend, the fastest flow shifts toward the outside, eroding the outer bank into a cut bank, while reduced current on the inside deposits coarse sediment, especially sand, as a point bar. By eroding one bank and depositing on the other, the channel migrates sideways without changing size.

Because of the channel slope, erosion is more effective on the downstream side of a meander, so bends also migrate down-valley. Satellite studies of Amazon rivers found that the sharper the bend, the faster it migrates, and that the fastest migration occurs a little downstream of the sharpest curvature.

Sometimes two bends grow so close that they touch. The river takes the short way!

The old loop is left behind as a curvy lake, called an oxbow lake.

As bends grow, two of them can come so close that the river breaks through the narrow neck of land between them. The water takes the shortcut, and the old loop is left behind. When it fills with water and is cut off, it becomes a horseshoe-shaped oxbow lake. Over time it may fill in with sediment.

Satellites have watched this happen. Landsat images of the Ucayali River in Peru from 1985 to 2013 show its loops moving and new oxbow lakes forming.

Where two meander bends meet, the river bypasses the loop, creating an oxbow lake that may later be infilled with overwash sediment. Repeated cutoffs keep the channel's sinuosity in check while the river shifts across its floodplain.

Landsat time series show this clearly in the Amazon Basin, where high sediment loads drive active migration. On the Ucayali River in Peru, images from 1985 to 2013 capture cutoffs and new oxbow lakes as the channel changes course. On a bigger scale, a whole river can shift to a new route, which is what threatens to happen on the lower Mississippi.

Water always flows downhill. It likes the quickest way down to the sea.

If a new path is steeper and shorter, the river may try to go that way.

Water flows downhill, and a river carries water and sand toward the sea. If a river finds a path that is steeper and shorter, more and more water can flow that way, deepening the new channel until it takes over.

Near its end, the Mississippi River passes close to the Atchafalaya River, which is 20 feet lower and 150 miles closer to the Gulf of Mexico. By the 1940s, the Atchafalaya was already taking a third of the Mississippi's water. Over thousands of years, the Mississippi has shifted its course many times.

A river's route is a balance of slope, discharge and sediment. A competing channel with a steeper gradient can capture flow; as it carries more water it scours deeper, attracting still more, a positive feedback.

NASA's Earthdata feature describes the Atchafalaya as 20 feet lower and 150 miles closer to the Gulf than the Mississippi at their junction, with one-third of the Mississippi captured by the 1940s. Over millennia the Mississippi has made natural shifts, wandering hundreds of miles in grand arcs with ancient channels stretching into Texas; each shift may take hundreds to thousands of years.

Towns and bridges are built next to rivers. If a river moved, a bridge might cross dry land!

Engineers build strong walls and gates to help keep big rivers on their path.

Towns, farms, ports and bridges are built where rivers are today. If the Mississippi switched to the Atchafalaya, NASA says the change threatens to leave New Orleans dry, and cities such as Baton Rouge and New Orleans would need to be ready for it.

To prevent this, the U.S. Army Corps of Engineers built a set of dams and spillways at a place called Old River in 1963. Its gates can let up to 30 percent of the Mississippi's flow into the Atchafalaya, but no more. Engineers measure the water every day during floods.

In 1963 the U.S. Army Corps of Engineers built a seven-mile system of dams and spillways at Old River, where the two rivers are linked. The Old River Control Structure, with eleven 44-foot gates across a 566-foot span, is designed to channel as much as 30 percent of the Mississippi's flow into the Atchafalaya. In the 1973 flood it was nearly overwhelmed.

NASA notes the management has trade-offs: floods sent down the Atchafalaya can inundate farms and ecosystems, and levees steer sediment and nutrients away from wetlands. Many expect the Atchafalaya to capture the Mississippi eventually if left to nature.