Q How are waterfalls made?

— Maeve Gervasi, Madison, Wis.

A Faith Fitzpatrick, research hydrologist at the U.S. Geological Survey's Wisconsin Water Science Center:

It basically takes four things to make a waterfall.

The first one is, of course, water. Second, there needs to be a land surface that’s made out of different rock types or sediment. Third, that land surface needs to have a vertical drop. Lastly, gravity is really important because it’s what makes the water flow downhill.

You put all those things together, and you end up with a waterfall.

As water flows across the land surface, it starts to erode the land. The water flows and erodes the land over long periods of time and will encounter rocks of different erodibility. Some rock can withstand the force of the water working on it, and that rock stays in place and ends up forming the vertical drop.

Some waterfalls last for a really long time. It depends on how long that rock can withstand the force of the water on it. Other waterfalls basically work themselves upstream as more rock erodes.

The rate at which a waterfall erodes backward is called a retreat rate. The fastest retreat rates can move a waterfall back about a meter and a half per year. Retreat can happen pretty quickly because the rock ends up being plucked out by flowing water in large chunks. During periods with less water flow, the waterfall may not change much.

Blue Sky Science is a collaboration of the Wisconsin State Journal and the Morgridge Institute for Research.

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