The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. Describe how streams and running water erode the Earth's surface. (B)

Erosion refers to the process of wearing down and carrying away rocks and soil from the Earth's surface. Running water, such as streams and rivers, is one of the primary agents of erosion. The erosive power of running water comes from the water's ability to pick up and transport sediment, as well as its ability to wear away the Earth's surface over time.

There are several mechanisms through which running water erodes the Earth's surface: 1. Hydraulice Action: The force of the water itself can break down and move sediment, as the flow of a stream or a river encounters obstacles on its path, it can exert pressure on the rocks and soil, causing them to crack and break apart. 2. Abrasion: Running water carries rocks, sand, and other sediment that can scrape against the bed and banks of a stream or river, much like sandpaper. This process of abrasion wears away and smooths the bedrock beneath, as well as the surrounding land. 3. Attrition: As sediment particles carried by running water collide with one another, they gradually become smaller and more rounded. This process of attrition further contributes to the erosion of the Earth's surface. 4. Solution: Some minerals and rocks are dissolved in water, especially in streams and rivers with a high concentration of dissolved chemicals. This process of solution can cause the Earth's surface to be slowly worn away as these materials are removed and carried downstream.

Several factors affect the erosive power of running water, including: 1. Discharge: The volume of water flowing through a stream or river affects its erosive power; the higher the discharge, the greater the potential for erosion. 2. Gradient: The slope or steepness of a stream or a river determines the speed of the water flow. Steeper gradients result in faster water flows and greater potential for erosion. 3. Stream Channel Shape: The shape and size of a stream channel can influence the velocity of the water and its erosive power; a narrow, deep channel leads to faster flow, whereas a wide, shallow channel results in slower flow. 4. Sediment Load: The amount of sediment carried by a stream or river impacts its erosive power. Higher sediment loads can increase the rate of abrasion and attrition, leading to greater erosion.

In summary, streams and running water erode the Earth's surface through mechanisms such as hydraulic action, abrasion, attrition, and solution. Factors such as discharge, gradient, stream channel shape, and sediment load influence the erosive power of running water, shaping the Earth's surface over time.