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How Watersheds Work

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HealthWiki > A Community Guide to Environmental Health > Chapter 9: Protecting Watersheds > How Watersheds Work

Everyone’s health is affected if the watershed is damaged. To understand how important watersheds are to the environment, it helps to think of rivers and streams as the veins of the earth. They carry and move water through the land the way our veins carry blood through our bodies. Just as we depend on blood for life, the environment depends on water for life.

Illustration of the below: Streams from villages in the mountains join a river flowing by a city.

The boundaries of any watershed are the peaks and ridges of the hills.
These small watersheds...
...are parts of the larger watershed.

The water cycle

Water is always moving. Sometimes it moves by flowing along, like a river. Sometimes it moves by changing from a liquid (water) to a gas (steam or water vapor) or to a solid (ice or snow). But the total amount of water in the world never changes. All the water there is moves from the sky to the earth, soaking into the ground, flowing into rivers, lakes, and oceans, and then evaporates back into the sky. This movement of water is called the water cycle.

Illustration of the below: Arrows show the path of water from rain on hills, to lake, to groundwater, and back up to rainclouds.
Heat from the sun causes water to evaporate from the surface of lakes, rivers and oceans into the sky. Water is also released from plants and from the soil.
Clouds carry water and release it back to the earth as rain and snow.
Water runs across the earth's surface into streams, lakes and rivers.
Water seeps into the soil where it nourishes plants and trees. It sinks underground where it is stored as groundwater, the source of water in wells and springs.

How watersheds protect water and soil

Most of the water in a watershed is not in the rivers and lakes, but in the soil itself. A healthy watershed has a supply of clean water and rich soil. Trees and plants, especially grasses, in the higher parts of the watershed and along the banks of rivers and streams, improve the quality and quantity of groundwater.

By protecting and conserving water, plants, and soil, we protect the watershed.

Illustration of the below: In a rainy landscape with a river and wetlands, arrows show where water will enter the ground.
Plants and trees slow the rainwater, help it spread and sink into the ground, and prevent it from carrying off too much soil.
Flowing rivers and streams
support people, fish, and animals.
When water enters rivers slowly through the ground, there is more water in the river between rainfalls and less flooding during storms.
Wetlands, often at the bottom of a watershed, filter and clean water as it moves along. Wetlands can even filter out some toxic contamination.

Make a watershed

This activity helps people understand how a watershed works and how all things within a watershed are important to the health of all the people living in the area.
Time: 30 to 45 minutes
Materials: For each group a large sheet of paper, a basin or pan, colored pencils or water-based colored pens, and water

  1. Divide into groups of 3 to 5 people.
  2. Each group takes their large sheet of paper, crumples it up, and then partly smoothes it out, being sure to leave some ridges and raised areas.
  3. The group colors different features of the watershed on the paper, showing ridges in brown, valleys in green, rivers and waterways in blue. Then different colors can be added to show what people have added to the watershed: red for waste dumps, black for pesticides, gasoline, and other chemicals, and so on.
  4. Place the paper in the pan or basin and fix the shape so that it resembles a watershed, with creased lines to show ridges and depressions to show valleys.
  5. People in the group wet their fingers with water and gently flick water on top of the watershed until the colors begin to run on the paper. Within each group, discuss what happens to the colors as they run down into the lowest parts of the watershed.
  6. Bring the groups together to discuss how what they have seen represents what happens in a real watershed. Note the distance that things can travel and the way different elements mix within the watershed.

Questions for discussion:

A woman gesturing.
  • What health problems can runoff from waste dumps (red color) and pesticides (black color) cause for people living downstream?
  • What changes do you think your community would see if the watershed were damaged?
  • What actions could your community take to protect or restore the watershed?

Watershed damage in the Aguan River Valley

40 years ago the hills above the Aguan River were forested. The valley was one of the most fertile regions in all of Honduras, and provided a good livelihood for people in many villages and farms. Many small, clear streams flowed down from the hills into the blue Aguan River. The river flowed through the heart of the valley and into the Caribbean Sea.

Then people started cutting down trees to use more land for farming and cattle grazing. Big fruit companies came in and cut down more trees to make banana plantations. Families started moving into the hills because the best valley land had been taken by rich landowners. Finally, most of the trees were cut down and there were many more people living on the hillsides. There was less water in the river and streams, and the water was no longer clear.

The people of the Aguan Valley knew things had changed, but it took a hurricane to make them understand how much their watershed had been damaged. Heavy rains caused landslides in the hills. Many homes and entire villages were washed away. Many people died and many more became ill.

As they worked together to recover from the storm, people began to see that the loss of trees on the hillsides, the landslides, and their health problems were all related. Cattle polluted their drinking water, causing diarrhea and other illness in their children.
Houses sit at the edge of an eroded cliff, and one house has fallen to the base.
Harvests got worse. Because the soil no longer held water from the rainy season, the fields dried out quickly. Then when the winter rain came, it washed the soil away. Harvests were so poor that people were always hungry, and hunger made their health problems worse.

The villagers began to understand that to improve their health, they had to protect their watershed.

After the discussion of the "Health effects of damaged watersheds," the Aguan River Valley story continues...

Illustration of the below: A deforested, overbuilt city and a factory sit on the banks of a river.
Water contamination from industry, oil, mining, and industrial farming pollutes water.
Deforestation makes soil erosion and flooding worse.
Destroying wetlands by too much building or paving over land causes more flooding and water contamination.

Health effects of damaged watersheds

When land is cleared of trees and plants (deforestation), soil holds less water, drying up wells and springs. Dry periods may become longer or more frequent, causing all the health problems of not having enough water. Deforestation also causes loss of soil (erosion) which makes growing food more difficult, leading to hunger and migration.

When wetlands are destroyed, they cannot filter toxic pollution out of the water, leading to greater contamination. Damage to wetlands and deforestation both cause flooding, which leads to injury, death, and increases in diarrhea diseases.

Changes in a watershed increase illnesses from mosquitoes

Mosquitoes breed in slow-moving and standing water. When large or sudden changes are made in how land is used and how water flows through the watershed, they often create conditions for mosquitoes to breed. Changes from:

A tree stump and puddle are surrounded by mosquitoes.

If you can keep the water moving, changes to the watershed do not have to lead to more mosquito-borne illnesses such as dengue, malaria, and yellow fever. For more about preventing problems from mosquitoes, see Chapter 8.