Misconceptions – Rivers Flow Passively over the Surface of the Earth

There are many misconceptions about geological concepts. There is a list here, developed by Kent Kirby of the University of Minnesota. This post is to debunk a few of those misconceptions. There will be others. Find them here.

Misconception: Streams are simply flowing water

Misconception: Rivers do not carve valleys, but only passively flow down them

Misconception: Although rivers can cut down over time, they do not cut to the sides

These are three different misconceptions, but they are all related in that in all three cases, the ability of a river to carry sediment and debris is neglected.

River valleys exist because of the action of the rivers flowing within them. The water itself has sufficient power to cause rock to move. If you’ve ever been knocked off your feet when standing in a stream, you are aware of this.

This power is enough to move sediment and debris. Depending on the magnitude of the flow and the steepness of the slope the river flows down, even whole boulders can be moved.

Ever notice how brown some rivers are? That’s silt and small particles being carried along. That material came from somewhere further upstream. Even though that silt might not seem like much, it is the result of the very slow cutting of the river into rock and sediment to carve its own valley.

While this carving is going on, the banks of the channel occasionally collapse into the stream, effectively widening the valley. The V-shape of river valleys is due to this process over immense periods of time. No matter how deep or wide a valley is, its depth and width are due to the erosive power of the river contained within. Valleys get deeper as the river cuts down, and grow wider as the sides collapse in and fall into the stream, to be carried away down stream.