The Water Cycle
Scientists estimate that there is about the same amount of water on Earth today as when it was formed. This water is continually being recycled between the earth, the atmosphere, rivers, lakes, oceans, and living things in a complex process called the water cycle (or hydrologic cycle). Water enters the atmosphere as vapor (its gas form) in two primary ways:
When water evaporates, the vapor is moved up into the atmosphere by rising air currents. The cooler temperatures high in the atmosphere cause the vapor to condense into tiny droplets of water, forming clouds. These clouds are moved around the earth on various air currents, colliding with other clouds on the way. Eventually, water in these clouds falls out of the sky as precipitation - rain, snow, sleet, or hail.
When precipitation falls on land, some of it seeps into the ground and becomes part of the ground water, which feeds wells, springs, lakes, and rivers. Much of the precipitation flows over the ground as surface runoff and joins rivers flowing back to the ocean. Some of it is soaked up by plants and drunk by animals or humans and then put back in the atmosphere through transpiration.
This cycle occurs continually, transporting water from one side of the world to the other. The water you drank today could have fallen as rain on China last year!
Your younger kids might find the full-size PDF version of the water cycle chart helpful.
Rivers are important players in the water cycle. They collect run-off from precipitation and move it back toward the oceans. Rivers are also extremely important to our society, providing us with drinking water and irrigation water, helping produce electricity, and allowing us to transport material and food by water. These are some of the reasons that most major cities in the world were founded next to a major river - Rome is on the Tiber, Paris on the Seine, London on the Thames, Cairo on the Nile, and New York City on the Hudson.
Did you know that you are living in a watershed? No, that's not a type of building! A watershed is an area of land that has a related set of streams and rivers. All the water that falls on that land will eventually flow to the same place - a larger river, lake, or ocean. Some watersheds are small, and some are very large. The watershed of the mighty Mississippi, which drains into the Gulf of Mexico, covers about 40% of the lower 48 states of in the US!
What happens in a particular watershed will affect all the water flowing through the outflow point. (The outflow point is the place where all the watershed's water flows into a larger body of water.) In particular, a river's streamflow (the amount of water flowing in it) changes constantly because of what happens in its watershed. When it rains in one part of the watershed, rivers and streams will rise in lower parts, sometimes hundreds of miles from where it rained. This could cause flooding even in a place where it hasn't been raining!
Rivers generally run their course in three stages:
As the river flows into the sea it begins to deposit sediment that it has been carrying. Often this sediment builds up faster than the sea currents can wash it away, and it forms islands that force the river to split up into many channels. This flat area of islands and channels is called a delta (click here for a picture of the Mississippi delta). Deltas contain very rich soil that is ideal for farming. In many countries, such as Bangladesh, people live in the delta and farm there, even though the threat of flooding is always present.
Have you ever wondered why dams are sometimes built across rivers? Dams help control river flow and regulate flooding, but they also use the river to produce hydroelectric power. When a dam is built, water from the river is backed up to form a reservoir. This water is allowed to flow through the dam to join the river below, and as it does so, its force is used to turn a propeller-like piece of equipment called a turbine. The turbine turns a metal shaft in an electric generator. The electricity that is produced is carried away from the river in power lines. About ten percent of electricity in the U.S. is produced by hydroelectric dams.