Precipitation comes down, literally, everywhere—in various forms. It may rain, hail, snow or sleet. Whatever the form, once it reaches the ground some water is absorbed by trees and other plants. 

Water not absorbed by roots enters the ground water supply by infiltration. Most water runs off the surface, down a hill and eventually into a river. Ditches and small streams, which may be dry when it does not rain, can quickly fill up with water when it does rains.

Once the water finds its way into a stream or ditch, where does it go?

If you have ever studied a map in detail, you will notice how rivers and streams form a network of waterways across the countryside.

Little streams come together to form small rivers. Small rivers join together and become medium-sized rivers. All these rivers may be tributaries of a large river. such as the Mississippi. Collectively, the network of rivers and streams form a watershed which drains the land of excess water.

This type of pattern is repeated many places in nature. In the human circulatory system, for example, or in human lungs, like those represented to the left. When you exhale (breath out), your lungs behave just like a watershed. The air in your lungs (which is like the water in a watershed) starts out in the smallest vessels (which are like small streams), passing into larger vessels (that are like medium rivers) and finally into one large central air passage (which is like a large river). Ultimately, all the air in the lungs (which are like a large watershed) pass through this one large air passage, out your mouth, and to the atmosphere beyond, just as water in a watershed flows out the mouth of a large river and into the ocean! (Of course rivers don't breath in!).

It comes as no surprise that the widest rivers have the biggest watersheds. The Mississippi watershed drains about half the Midwest and eastern United States. 

Think about rivers as excess water disposal machines. Rivers transport water from wherever it is to the ocean. In places where it rains more, such as in the temperate deciduous forest and the rainforest, there are more rivers and streams to deal with the large and consistent annual rainfall. 

In the desert, however, there is significantly less rain. Logically, fewer rivers exist in the desert. When it does rain, the rains are very short and intense. Since there is no vegetation to absorb the water, almost all the water drains immediately into dry river basins that for most of the year look like flat plains. These rivers swell up very quickly and produce swift torrents.

Where rivers meet the ocean is called the mouth of the river. Soil and dirt carried by these rivers is deposited at the mouth, and new land is formed. The new, soil-rich land is known as a delta.

The Amazon River is the second longest river in the world. Like all large rivers, the Amazon deposits a lot of soil and sediment, forming a delta, as it enters the ocean. Its delta is located in Brazil. 

As rivers prepare to enter the ocean, they tend to get off course and branch into many directions, creating many small islands in the delta region. This is particularly true with the Amazon River, as you can see.

Scattered across the delta at the end of the Mississippi River is the city of New Orleans. By the time the mighty Mississippi winds its way south through America's center it becomes a force well over a mile wide. It's a very dramatic sight to watch as this mammoth river spills into the ocean.

Because the Mississippi watershed drains much agricultural land, it has high sediment content. The mud spilling from the river into the Gulf of Mexico is clearly visible in this picture.

The Nile River is the longest and probably the most famous river in the world. As it flows toward the Mediterranean Sea from the mountains of central Africa, it grows in size. The Nile Delta is a very large and impressive sight. It is home to some of the most productive agricultural land in the Middle East.