When you stand next to a river, its path doesn't seem to move. But this series of satellite images of Peru's Ucayali River — featured in Time Magazine's Timelapse project — reveals something pretty remarkable.
Over the course of fewer than 20 years, its path crawls back and forth, carving out deeper and deeper curves before cutting them off and starting over.
All rivers naturally change their path over time, but this one forms meanders (the technical name for these curves) at an especially fast rate, due to the speed of the water, the amount of sediment in it, and the surrounding landscape.
Why rivers meander
Initially, when a slight curve is already present in the river, water travels around the outer edge of the curve faster than the inner edge, because it's covering a greater distance in the same amount of time.
As water moves faster, more of the sediment in it stays suspended, instead of settling to the bottom. So along the inside of the curve, sediment gradually gets deposited as it settles out of the slower-moving water. Along the outside of the curve, more sediment stays suspended, and some of it scrapes against the riverbank, carving out the curve further.
This process accelerates as the meander becomes more curvy, since there's a bigger and bigger difference in water speed. Eventually, though, when adjacent curves in the river become so curvy that they meet, water punches through, looking for the shortest path.
This cuts the curve off entirely, forming a separate body of water called an oxbow lake, which often dries up quickly. You can see this whole process happen in several places in the GIF at top (and in the diagram at right, by Wikipedia user Maksim).
We think of rivers as stable features of the landscape: something we can build towns and cities next to, and expect to stay in the same spot permanently. But in reality, they're constantly moving — and as much as we try it's impossible to keep them entirely under control.
*Hat tip to the How Things Work twitter account for spotting the time lapse.