10.1 Water and Weathering

Target Language or Knowledge:

Weathering is the process by which rocks are broken down by the forces of nature. Rocks may also crack from repeated expansion and contraction as they are baked by the sun and cool at night. Rocks can also be weathered by mechanical stress. The main agent of chemical weathering is water. Chemical weathering changes the composition of the original mineral. The process of erosion often begins with rain. Runoff means excess water. Erosion can also scour away soil, sand, and vegetation, dramatically changing the face of the land. Large streams that carry water from the mountains to the sea are known as rivers. The material carried by a stream is called its load. The largest drainage basin of the US is drained by the Mississippi River system. A river system consists of all the streams that merge together and flow toward the sea as a large river. When a stream deposits its load, coarser sediments are generally deposited first. Most of the rain falling west of the Great Divide drains into the Pacific Ocean. Not all rainwater drains off into rivers and streams.