Rebeccah Watts, age 10, of Toronto, Ontario, Canada, for her question:
How are waterfalls formed?
Most streams travel many miles through changing scenery, on their long journeys to meet the sea. All sorts of adventures happen along the way and usually a number of streams join to form a wide river. Sometimes a river has to form foamy rapids as it rushes fast down a rocky slope. Sometimes it has to plunge over the edge of a steep cliff and create a dashing white waterfall.
The water in streams and rivers flows freely over the ground but there is one rule it must obey. It must flow downhill all the way to the sea. Never may it flow up even the gentlest slope. Sometimes the slopes are too gradual for us to notice and only a flowing river knows which way is dowel.
As we know, our earth is famous for its bumpy ups and downs. Rivers tend to dawdle through valleys and fairly flat plains. But they spend ages digging their channels around the hills and humps. However the earth's crust is restless and always busy remodeling its wrinkles and ridges. Gradually old mountains wear down and new ones arise. The ups and downs keep changing.
Most of these changes are much too slow for us to notice, but the patient old rivers have to adjust to them. And one of the earth's remodeling jobs can force a river to dash down a waterfall. Often it happens where there is a very hard layer of rock on the surface and softer layers below it. In time, ground water helps the river to weaken and wash away the soft levels under the hard surface layer.
Then, perhaps quite suddenly, the top layer collapses and a great slab drops down to a lower level. This leaves a steep sided cliff 'standing in the path of the river. There is only one thing that this flowing stream can do. It must plunge headlong over the edge of the cliff in a waterfall.
Many of the world's waterfalls are formed when sections of old river beds sink to lower levels. Others form from springs born high in the mountains. There the mains and melting snows sink through porous rocks and form reservoirs of groundwater. Here and there this water finds a way to get outside again. When there is a deep crack on a high slope, it gushes forth in a bubbling mountain spring.
The spring becomes a stream and the only way it can go is downhill. The sides of a high peak are often steep slopes with ledges and sheer cliffs. This is the dashing path that the stream must take. On the way down the mountain it may have to take several flying leaps and form plunging waterfalls.
All waterfalls are formed by sudden dips in the ground. But not all o£ them are formed by the earth. A man made dam blocks the path of a river and its water backs up to fill a reservoir. This lake is higher than the old riverbed. After a rainy season, it may slop over the dam in a man made waterfall.