Lesson: Fresh or Salty?

Contributed by: Integrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder

A colorful drawing of the water cycle showing the evaporation of water, precipitation, and run off in landscapes ranging from mountains to ocean.
Figure 1. The water cycle.
Copyright © U.S. Geological Survey, http://ga.water.usgs.gov/edu/watercycle.html


Between 70 and 75% of the Earth's surface is covered with water and there exists still more water in the atmosphere and underground in aquifers. In this lesson, students learn about water bodies on the planet Earth and their various uses and qualities. They will learn about several ways that engineers are working to maintain and conserve water sources. They will also think about their role in water conservation.

Engineering Connection

Water is an important resource that all forms of life need for survival. Engineers study the water cycle and the various water bodies that make up the water cycle in order to continually ensure clean water is available. Here in the United States we sometimes take for granted the clean water coming out of our taps, but engineers have worked hard to make this clean water available. They have designed water treatment plants and built distribution systems that bring this resource to our homes. To build a water treatment plant, engineers must thoroughly understand the properties of nearby water bodies, including whether they are a good continual source for water. Currently engineers are working on projects, such as tidal turbines, using ocean tides to create energy.

Educational Standards

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Learning Objectives

After this lesson, students should be able to:

  • List the major bodies of water on Earth.
  • Describe the Earth's water resources available for humans to use.
  • Explain several ways that engineers are working to maintain and conserve water sources.


Why is water important? For what purposes do we need water? Do you need water to survive? Does your food need water to grow? What would the world do without water? These are all questions we ask when learning about how important water is.

If you are thirsty and need a glass of water, do you want a glass of dirty water or clean water to drink? Probably clean — or fresh — water is what you would rather have. Finding fresh water, however, is becoming more and more difficult as the population in the world grows. With more people around, the demand for fresh drinking water is increasing while the amount of fresh and clean water is becoming more and more scarce. Where do we get our drinking water? We turn on our faucets at home, but how do we know where that water originates? Did it come from the river running through the field behind your backyard, was it pumped out of the ground somewhere, or is it water from the ocean that has had the salt taken out of it? Well, it can be from many of these sources. Environmental engineers are responsible for making sure we have clean drinking water. These engineers learn about the properties of water, the different sources of water and how to clean up dirty water. Environmental engineers develop water treatment systems that clean up water from natural sources, like streams and snow pack, and they even develop systems that clean up water we have used and flushed down our toilets and drains. Of course, once water has been used, it is dirtied; and it can never be completely cleaned again.

About 75% of the Earth is covered in water. Where is the Earth's water located? Well, water on the Earth can be found in ocean, rivers, lakes, atmosphere, and even more water can be found underground, in areas called aquifers. The oceans are the largest water source, containing 97% of the Earth's water. Unfortunately, we cannot normally use this water for drinking because it is too salty. Engineers are working on ways to get the salt out of ocean water and make it usable for drinking, but these methods are very expensive and not used very often. Very little of the Earth's water is suitable to be used for drinking water. In fact, less than one percent of our water is in rivers, lakes and groundwater. We get most of our drinking water from these types of surface waters. Another way to get drinking water is from the atmosphere, such as rain and snow. What happens if you count on snow for water and the weather becomes too warm? Well, snowmelt in the spring and summer actually provides a great source of fresh water for a lot of communities. It is estimated that 75% of water used in the western United States is from stored- up snowmelt.

We use a lot of water! Think of all the ways that you use water during the day. You most likely use water to drink, brush your teeth, take a shower or bath, cook, flush the toilet and wash your dishes. These are just a few ways we use water. Now think about this: do you leave the water running while you brush your teeth or hands? Well, most of us waste a lot of water somehow. The average person in the United States uses 125 gallons of water per day! We are using up our water fast. Engineers are working on ways to help us save some of this valuable fresh water. They have developed low-flush toilets and showers that use less water. Can you think of some things that you can do to ensure that there is water for the people/animals/plants of tomorrow?

Lesson Background and Concepts for Teachers

  • Seventy to seventy-five percent of planet Earth is covered with water, and even more exists underground, in aquifers. It is estimated that the total water supply of the world is 326 million cubic miles. That is a lot of water. Why should we feel the need to garner this resource then? In actuality, very little of the water on the planet Earth is available for human consumption. The oceans account for 97.24% of the water on Earth. This water is unfit for human consumption because of its high salt content. There are methods for desalinization, but to date, they are expensive and not widely used. Another 2.14% of the water is tied up in icecaps and glaciers. Of what is left .61% is ground water. Only .0091% of the water on planet Earth resides in rivers and lakes. Of the ground and surface water that is supposedly available for drinking, only .3% of this is suitable for human consumption.

Fresh Water Predictions and Usage

The Earth is a closed system, meaning that matter does not enter or escape. This means that the water we have here on Earth today has always been here. It cycles through in different forms, but at any given point in time, we have the same amount of water here as we have always had.

In the United States, in the year 2000, we used about 323 billion gallons per day of surface water, and about 85 billion gallons per day of ground water. This is water that now has to be cycled through a water treatment plant where it is never quite as clean again as it originally was. The average individual person in the United States uses 125 gallons per day.

Ocean Water Make Up and Process of Desalinization

The ocean moderates the temperature by absorbing incoming solar radiation. Until the year 2000, there were four oceans: Pacific, Atlantic, Indian and Arctic. In the spring of 2000, the International Hydrographic Organization delimited a new ocean, the Southern Ocean, which surrounds Antarctica and extends to 60 degrees latitude.

The salinity of the oceans and seas vary throughout the world, but the average salt content is 2.2 pounds of salt for every cubic foot of water. The saltiest seawater is in the Persian Gulf, which is 40% salt; the least saline-rich water is in the polar regions, where the source water is melting ice and heavy precipitation.

Snow Melt

Photograph of a rapid river.
Copyright © U.S. Geological Survey, http://ga.water.usgs.gov/edu/watercyclesnowmelt.html. Photograph by Cagan Hakki Sekerioglu, naturalphotos.com.

It is estimated that 75% of water used in the western U.S. is from snowmelt. During the winter season, snow falls in rural and mountainous areas, leaving solid, packed snow to freeze in these higher altitudes (oftentimes, the snow that falls in rural areas melts within days or even hours). This hard, often very deep, snow freezes month after month, developing a good base for skiing and other winter recreational activities. But, more importantly, this snow pack is a major source of water for our planet's water cycle.

Obviously, residents of warmer regions (e.g., Florida) do not directly benefit from the snowmelt in colder regions, but they are indirectly impacted since the water eventually enters the Earth's water cycle. Residents of colder regions, where the snow builds up, directly benefit from snow melt, as upon melting, the water immediately enter streams, rivers and reservoirs in their local communities and is oftentimes their only water source.

Engineers need to know a great deal about snowmelt in order to manage this vital water source for our planet. Engineers at the U.S. Geological Survey monitor snow runoff in order to prevent dangerous flooding of areas that are at lower elevations — a very real threat during spring runoff.


aquifer: An underground bed or layer of earth, gravel, or porous stone that yields water.

ground water: Water that exists underground.

salinity: Containing salt.

Associated Activities

Lesson Closure

Who can name a major source of water? Our water comes from oceans, rivers, lakes, the atmosphere and groundwater/aquifers. Which water body has the greatest amount of water? (Answer: The oceans contain 97% of the Earth's water.) Where do we get most of our drinking water? (Answer: We get most of our drinking water from surface water.) Why is ocean/sea water not used for us to drink? (Answer: It is too salty.) What are some things you think we should do to ensure that there is water for the people/animals/plants of tomorrow? (Answer: We can conserve water by being cautious about how much we use daily: turn off faucets, take shorter showers, water lawns less and when the weather is cooler [can help prevent loss due to evaporation], etc.) How do engineers help us get clean drinking water? (Answer: Environmental engineers design water treatment and wastewater treatment plants to clean and help conserve water for daily use.)


Pre-Lesson Assessment

Discussion Question: Ask a discussion question to get students thinking about the upcoming lesson. After soliciting answers, explain that these questions will be answered during the lesson.

  • What are the major types of water on Earth? (Answer: oceans/seas, lakes, rivers, groundwater/aquifers, glaciers/snow). Have the students rank the bodies of water in order of greatest to least amounts (quantity of water) on Earth.

Post-Introduction Assessment

Question/Answer: Ask students questions and have them raise their hands to respond. Write their answers on the chalkboard.

  • Where does our drinking water come from? (Answer: Most of it comes from surface water.)
  • Why are water engineers concerned with snow pack? (Answer: It can provide drinking water.)
  • Can you drink water from the ocean/sea? (Answer: No; it is too salty.)

Lesson Summary Assessment

Drawing: List these percentages on the board. Oceans - 97.24%; Icecaps/Glaciers - 2.14%; Ground water - 0.61%; Rivers and Lakes - 0.0091%. Have students make a drawing of each of these major water bodies and label each one with its respective water percentage of the Earth's total available water. Have them identify where their drinking water comes from on their drawing by circling it. For some students, this may be groundwater/aquifer, while for others, it may be snow from the mountains that melts and makes a reservoir near their home.

Journal Reflection: Ask the students to write a few sentences in their science journal or on a sheet of paper about how water is used in their daily lives and what things they can do to conserve water. Have them include a sentence on how engineers work to conserve and clean water as well.

Lesson Extension Activities

Environmental engineers are the ones who design water and wastewater treatment plants so that we can have clean water. Have students draw a picture of the water cycle using their source of drinking water and major water bodies in their state of which they are aware. They should be able to show with words and pictures how these water bodies contribute to the water cycle and how they do or do not provide water for their house/school, etc. Students can also integrate a water treatment plant if desired. This shows them that water has to go through a water treatment plant before it comes out of the tap in their home or school.

Using the same idea, students can also integrate a wastewater treatment plant showing that the water coming out of their house must be cleaned.


U.S. Department of the Interior, U.S. Geological Survey, Water Science for Schools, "The Water Cycle," May 10, 2006, accessed June 24, 2006. http://ga.water.usgs.gov/edu/watercycle.html
U.S. Department of the Interior, U.S. Geological Survey, Water Science for Schools, "The water cycle: Snowmelt runoff to streams," May 10, 2006, accessed June 24, 2006. http://ga.water.usgs.gov/edu/watercyclesnowmelt.html


Sara Born; Malinda Schaefer Zarske; Janet Yowell


© 2006 by Regents of the University of Colorado.

Supporting Program

Integrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder


The contents of this digital library curriculum were developed under a grant from the Fund for the Improvement of Postsecondary Education (FIPSE), U.S. Department of Education and National Science Foundation GK-12 grant no. 0338326. However, these contents do not necessarily represent the policies of the Department of Education or National Science Foundation, and you should not assume endorsement by the federal government.