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Earth Geology Erosion Valley and Stream Erosion

Valley and Stream Erosion

Valleys change and grow over millions of years. Stream erosion digs them deeper and wider. It also makes them longer by eating farther and farther into the uplands where they begin. Where the slope flattens out, the stream slows. Then its water can't carry as much sediment (soil, rocks and other material) and drops it. The sediment builds up and changes the way the stream flows. 
You can look at stream valleys in two main ways. One way is how the valleys of a stream and its tributaries look on a map. The Bryant watershed's valleys make a tree-like pattern. Each stream valley branches into smaller ones, and these then branch into still smaller ones. This pattern comes from very large cracks hidden in the bedrock lying underneath that direct the flow of water on the surface. 

Another way of looking at stream valleys is by their shape. At their headwaters, the valleys of Bryant tributaries are V-shaped. The streams cut right down into the soil and rock because they are flowing steeply downward, and so they flow fast. Fast water carries more sediment, which grinds the stream deeper and deeper into the bottom of the V. A V-shaped valley is a younger valley, because it is cutting back into the higher ground where it begins. ("Younger" means hundreds and thousands of years. "Older" means hundreds of thousands, even millions of years!) 

The Bryant's valley is older where it has already cut pretty far down through the bedrock. There it flows less steeply on its way to join the North Fork. Where it's older, it's broader, and you find the rich, flat bottomlands good for farming. There the stream "meanders," year by year slowly moving back and forth across the valley, one side losing, one side gaining land. There the land floods in high water times. That's why the bottoms are also called "flood plains". 

You may notice that it is the outside of a curve that eats into the stream bank. The water flows fastest on the outside of the curve. On the inside curve, the slower water drops its sediment, and the land builds up on that bank. That's where you find gravel bars. Then sycamores sprout. In a few years what was stream bottom has become bottomland! 

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