As we said in our last Instruction, nothing has more effect on the Earth's surface than erosion caused by running water. Much of the water that falls on land immediately returns to the atmosphere through evaporation, or it seeps into the soil or into cracks in rocks. If the soil or rocks are saturated, however, the water begins to flow along the surface of the Earth toward a lower elevation. This flow is called surface runoff. Water always flows downhill, and that's how streams and rivers begin.
 
As water flows, it picks up (erodes) loose soil or rock particles that were formed by physical or chemical weathering. This flowing water creates a channel that the water will follow during future runoffs. At the place where runoff begins, the channel may be only a few inches wide. These channels are called rills. Over time the rills grow and merge to form larger channels called streams. The streams then merge to form rivers.
 
 
The Importance of Rivers
http://www.shambles.net/pages/learning/primary/Rivers/
 
Rivers have always been very important to people, although more water is actually stored in oceans and glaciers. The earliest civilizations grew up along the banks of rivers, like the Tigris and Euphrates and the Nile. For an interesting exploration of the early River Civilizations, go to Grade 6 History Lesson 2. Some of the earliest human construction projects involved moving and storing water, like the building of dams and canals, and some of the earliest laws dealt with water rights, like the Code of Hammurabi in 1700 B.C.
 
Today we will deal with the physical properties of rivers and how they shape Earth's surface.
 
 
What Makes a River a River
 
The simplest definition of a river is "fresh water flowing across the surface of the land, usually to the sea."
 
A river flows in a channel. The bottom of the channel is called the bed. The sides of the channel are called the banks. The place where a river begins is called its source. The place where it empties into the sea or ocean is called its mouth.
 
A delta is the silt and sediment deposit that forms at the mouth of a river as it enters the sea. The Nile River delta is shaped like the Greek letter D ("delta"), which is how all deltas got their name. In reality, deltas come in many different shapes and sizes. Famous deltas include those of the Mississippi, Ganges, Rhone, Mekong and Danube Rivers.
  
Rivers make changes in the land through erosion, which carries earth and rock away from their original locations. The movement of earth and rock is done in two ways: by hydraulic action and by abrasion.

Hydraulic Action
http://www.tiscali.co.uk/reference/encyclopaedia/hutchinson/m0023618.html
 
Hydraulic Action is erosion by the force of the moving water. This force acts against the sides and the bottom of the channel. It can be strong enough to pick up and move anything from sand to giant boulders. And the steeper the slope, the faster the water rushes.
 
Erosion by hydraulic action is greater in mountain areas or other places where the slope of the channel is steep. This results in a deepening of the channel.  

Abrasion

Abrasion happens when one material scrapes or strikes against another material. This scraping or striking takes fragments away from both materials.

Flowing water transports rocks and other abrasive materials and when these materials scrape or strike the sides and bottom of a river, they change the size and shape of its channel.
 
 
The Grand Canyon
http://www.grand.canyon.national-park.com/
 
As we said in our last Instruction, one of the most dramatic examples of what river erosion can do is the Grand Canyon, although wind and ice also played a part in its formation. The river involved in the erosion is The Colorado River.

Some people find it hard to believe that a river could have such a massive effect, especially since the Grand Canyon is located in the desert, but being in the desert is exactly why the erosion happened.

The soil in the Grand Canyon is baked so hard that it can't absorb water. The plants there have very shallow roots that don't hold the soil in place, and when the rains come, they come in massive downpours. For centuries flash floods have roared down the steep sides of the channel, cutting an ever deepening and more spectacular Grand Canyon.

Experiments for Home and Classroom