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Teacher/Student Learning Packet
Water Resources

CONCEPTS:

The purpose of this unit is to give the student a general overview of water; how it relates to the earth and more specifically the Nevada-Arizona-California area. This will be achieved by:

INTRODUCTION - WATER IN THE DESERT:

One glance at Hoover Dam will reveal a most obvious fact - there's a lot of water behind the dam. You might think that there is enough water for thousands of years. Actually, there is enough to supply 29 million households (about equal to the population of California) for one year. In the regions of the Southwest where people are water wise, Lake Mead could supply twice this many families with water for one year.

The Colorado River is more than 1400 miles in length, making it the third longest river in the United States. The watershed covers more than 244,000 square miles, 1/12 of the area of the lower 48 states. Although the river begins in the Rocky Mountains, most of its length drains the arid Southwest region.

This river is an important source of water to seven western states - Colorado, Wyoming, New Mexico, Utah, Arizona, Nevada, and California, as well as a portion of Mexico. Along the way it is used for irrigation, domestic water, recreation, and hydroelectric power production. These uses would not be possible if not for the series of dams along the Colorado river and its tributaries.

Just after the turn of the century a succession of floods broke through levees just above the border with Mexico. The resulting flood waters created the Salton Sea in southern California and inundated much of the Imperial Valley. On the other hand, drought years reduced the river to a trickle. In order to supply water to the arid Southwest, a series of dams along the Colorado River was devised. These dams would act as flood control, silt control, and water storage units.

Hoover Dam was the first of the large reclamation projects along the Colorado River. The dam backs up the waters of the river to form Lake Mead. Lake Mead is the largest man-made lake (reservoir) in the United States, holding almost 29 million acre feet of water. An acre-foot of water is the amount of water that will cover an acre, one foot in depth. This water is stored behind Hoover Dam and is used throughout the Southwest. The dam also helps to settle out silt and reduce the amount of sediment sent down the river.

Water is a top priority at Hoover Dam. Without water, life in the desert would be impossible. Hoover Dam, as well as other dams along the Colorado River, make the storage of this precious water possible.

ACTIVITY #1 - H20 ON THE GO

Water is all around us, in many forms. Even in the middle of the Mojave desert, it may seem dry and forbidding, but if you know where to look, the water is there. It can be held in the clouds or fog, in the form of vapor. It can fall from the clouds as a liquid, like rain or a solid like hail and snow. Once it has fallen, it can lay in a lake, flow in a stream, sit in a snowpack, or travel through the soil (percolate) to an underground reservoir called an aquifer. The water on the surface may be evaporated (turned back into vapor), form a cloud and continue on its journey. This continuous recycling of water is known as the Hydrologic Cycle.

You can create your own Hydrologic Cycle. For this you'll need:

Cut the top off of the soda bottle, below the narrow neck. Pour the sand into the bottle, create your own hills, valley, and lake, you can even put in a small plant, if you like. Give your ecosystem a drink of water, about 1 cup. Next, cover the top of the bottle with the sheet of plastic wrap and keep it in place with the rubber band. Set your mini-earth in a sunny window and observe what happens.

It won't take long, especially if the day is warm, for things to start happening. Do you observe any water collecting on the underside of the plastic wrap? This is called condensation and is a result of water molecules warming up enough to evaporate and become vapor.

The vapor will naturally rise until it meets with the plastic wrap at the top of the bottle where the air is cooler. In the Earth system, this vapor will rise until it meets the impermeable layer in the stratosphere. Since the vapor can't move through the plastic it condenses or changes back to it's liquid form. When enough collects, it will drop back down to the sand, much as rain would. You may even see small lakes or streams form where the water drops back to your mini-earth.

There is as much water in the earth's system today as there was when the earth was formed. At any given time, more water may be locked up in icepacks or caught up in weather systems. That is why we have cycles of drought and flooding. During the last ice age, large quantities of water were contained in icepacks, which reduced the amount of water available to the oceans. Ocean levels dropped and more shoreline became exposed. When earth's temperatures increased by a few degrees and the icepacks melted, more water was freed up and the water levels in the ocean again rose. The water levels at Lake Mead can also be altered due to climatic changes.

At times there seems to be more water than we can use. At other times we can't seem to find a drop. But, as we found out, there is always some out there somewhere!

ACTIVITY #2 - SEDIMENTATION: THE NITTY GRITTY

Before Hoover Dam the Colorado River was a much different river then we see today. In the place of the warm, silt-laden stream of pre-dam days now lies a cool, clear river. In its journey down from the Rocky Mountains, through the arid southwest region, it would pick up a tremendous load of sand, silt and other debris, called sediment. This sediment load would then be carried down stream, only to be dropped out (deposited) when the water slowed down. This sediment would usually end up in the bottom of the river or along the sides.

To examine how this process, called sedimentation, works you'll need a few simple supplies:

Pour the water into the bottle. Carefully pour the dirt into the bottle, put on the cap, then shake it up! Wait a while for the dirt to settle. It might take about 5 to 15 minutes. Now, take a close look and examine what you see in the bottom of the bottle.

As long as the water kept moving, the dirt stayed mixed with the water (in suspension). As the water slowed down, it was no longer strong enough to hold the dirt up and the dirt settled to the bottom.

Depending on the type of dirt that you used, you will see various layers. These layers are comprised of the many types of soil particles which make up dirt or soil. The heaviest of particles, like rocks, will drop out first. Sand may be next, since it's grains are lighter than rocks, but heavier than most other soil. The lightest fragments, such as clay and small pieces of leaves and twigs , will stay in suspension longer and therefore be on top.

This is like how the river works. The river usually flows, or moves, fast enough that it can keep the dirt it picks up along the way in suspension. If the river slows down, as it's going around a bend or coming into a lake, then that sediment will settle to the bottom, much like it did in your bottle.

At Lake Mead, the sediment will usually drop out where the lake starts to form at the west end of the Grand Canyon. If the water coming through the canyon is moving fast enough, like after a heavy rain, then the sediment will be carried farther into the lake before it settles to the bottom. It is estimated that 60 to 90 feet of sediment (or silt) now lies behind Hoover Dam. Eventually, enough of this sediment may come down the river to fill up the whole lake, but we don't expect that to happen for hundreds of years.

For now, the dam does it's job of slowing down the river so water can be stored in Lake Mead. As we've seen, this also "cleans" the water by allowing the dirt to settle to the bottom of the lake.

BIBLIOGRAPHY

50 Simple Things Kids Can Do To Save The Earth; The EarthWorks Group; Andrews and McMeel, Kansas City; 1990.

HydroExplorer, The Colorado River Run; IBM and Mac versions; Water Education, Sacramento, CA.

Life Science Library, Luna B. Leopold, Kenneth S. Davis; Time, Inc, NY; 1969

The Magic School Bus at the Water Works; Joanna Cole; Scholastic, Inc, NY, NY; 1986.

Water: the Power, Promise, and Turmoil of North America's Fresh Water; National Geographic Society, Washington D.C., Nov. 1993.

What Makes It Rain? The Story Of A Raindrop; Keith Brandt; Troll Associates, Mahwah, NJ; 1982.

Where Does Water Come From?; C. Vance Cast; Barrons Educational Series, Inc., Hauppauge, NY; 1992.

Last Reviewed: 9/16/2004