The sign said it all: "On Labor Day, 1995, a bus-sized piece of basalt fell from behind the waterfall into the pool."
That's the sign that greets visitors right next to a 620-foot waterfall just outside Portland, Ore.
Standing on the stone bridge separating upper and lower Multnomah Falls, those visitors may have a hard time seeing the bus-size piece of rock underneath the water. But in fact, the waterfall continually erodes sediment – from the size of sand particles up to the size of a car or bigger. It all ends up in the pool.
Behind the waterfall, visitors can see large cracks in the face of Larch Mountain. During the cold winter months, freezing water from the falls expands and further cracks the rock. But it isn't the mountain face that experiences the most dramatic erosion; it's the area at the base of the waterfall.
As water falls and hits the pool, it swirls around, carving out a hole at the base of the waterfall. This is called undercutting. Soon the flat, rocky face begins to look as if a balloon is expanding behind the falls and under the pool. The rock above becomes heavy, and, without a bottom, it breaks off and plummets into the pool. It is here that large, noticeable erosion occurs.
So the next time you see a waterfall, check out what is happening behind the water. You might be able to see the dramatic effects of erosion at work.
Online, you can also visit these websites, where you can learn more about waterfall erosion: