We all live in a watershed! Animals and plants live there with us. Everyone affects what happens in a watershed by how we treat the natural resources in the watershed. So, what is a watershed? A watershed is the land area that drains water to a stream, river, lake or ocean. Water travels over the Earth's surface across forest land, farm land, pasture land, suburban land, neighborhood front yards and back yards, city streets and parking lots. Or, the water can seep into the soil to an aquifer and make its way to a stream via groundwater. Watersheds come in many different shapes and sizes. Some watersheds contain mountains and hills and some are nearly flat. A watershed can be affected by many different human activities and natural events. The routine of our everyday lives can affect the quantity and quality of water flowing in a watershed. Construction work, farming, logging, and the application and disposal of many garden and household chemicals can affect the health of a watershed.
Floods are a major natural event that occurs in watershed. Floods occur when the volume of water exceeds the ability of a stream, creek or river to hold the water within its normal banks. Any stream, creek, lake or river can flood. The size or magnitude of a flood is called 'recurrence interval'. By studying long periods of flow records for a stream or river, it is possible to estimate the size of a flood that would, for example, have a 100-year recurrence interval, thus being called a hundred year flood. This means that on the average, a particular stream or river would flood every 100 years. However, there is a chance that a 100-year flood could happen any time. The severity of a flood is often determined by loss of human life or property, which is usually directionally proportionate to the amount of human development in the flood plain surrounding the river or stream. A flood plain is a strip of fairly flat land bordering a stream, river or lake that usually holds or carries the overflow of flood waters. Flood plains are Nature's way of taking care of flood waters. Because they are flat areas, they are often ideal sites for construction and development.
Watershed come in many shapes and sizes. Larger watersheds are composed of many smaller watersheds. This watershed is a sub watershed of a larger watershed that has unplanned development. A watershed is determined by connecting the highest topographic points on a map between two adjacent areas. These points form a watershed boundary, similar to the edge of a bowl.
The quantity and quality of water draining from a watershed are dependent upon the climate, vegetation, soils, geology, and development within that watershed. Activities that change the vegetation, soils, geology, and surface characteristics of some watersheds will affect the quantity and quality of water contributed to a stream. For example, a greater volume of water, perhaps of poorer quality, will flow from a parking lot than from a forest or pasture, which may result in increased flooding in a watershed because the greater volume exceeds the natural ability of the stream to transport the water.
Underdeveloped watersheds are drainage basins that have no development affecting the quality or quantity of water in that watershed. These watersheds are primarily on public-owned lands in national forests, national parks, and wilderness areas. Underdeveloped watersheds provide scientists with areas to study the natural processes of a watershed and the movement of water within a watershed.
Planning the development within a watershed requires consideration of the entire drainage basin. Planned actions that consider the effect on the natural resources of the watershed will help preserve the quality and quantity of water flowing from a watershed. Actions such as controlling surface runoff from streets, providing recycling centers, farming along the contours, and logging practices that include controlling runoff and protecting stream channels help preserve the quality and quantity of water flowing from a watershed. Limiting the number and type of structures on a flood plain is one method of preventing loss of property from floods. Placing parks, golf courses, or farmland on a floodplain can reduce property loss caused by floods.
Unplanned development within a watershed has the potential for degradation of water quality and increased loss of property from flooding. Runoff from city streets, improper farming and logging techniques, and poor residential and industrial chemical-disposal practices all can affect water quality. Locating homes and businesses on flood plains greatly increase the chance of damage from flooding. In some places, flood-control structures such as dams and levees are required to protect development already located on the flood plain.
Watersheds, Floods, and Flood Plains
A watershed is an area of land that drains to a stream, river, lake or ocean. It is a land surface feature that can be identified by connecting the highest elevations between two areas. For example, a pitched roof of a house or a building is usually divided by a ridge. The back part of the roof is a separate watershed from the front part of the roof. During rainstorms, some water runs off both parts of the roof but meets in the street. Rain water from other houses in the neighborhood also flow into the street. Water from the street flows to a drain, ditch, or stream. Thus, the street is a larger watershed consisting of several smaller watersheds.
Floods occur when the level of a stream, river, or lake exceeds its normal height. Any stream or river can flood. During floods, water flows over the banks of a stream and into the surrounding low-lying areas called flood plains. During flooding, the threat to life and property damage most often occurs in the flood plain. Limiting the development within the flood plain is the best way to reduce damage associated with flooding.
The following activity is designed to demonstrate a watershed and the connection between small watersheds and larger watersheds. The activity also demonstrates property damage control during flooding through the placement of buildings in a flood plain.
Objectives - Students Will:
Identify a watershed.
Observe how water flows from higher elevations to lower elevations in a watershed.
Observe the interconnection between watersheds.
Observe the importance of locating buildings within a watershed.
Experience a flood in a model watershed.
Materials - Each Group Will Need:
One container at least 22 cm wide, 33 cm long, and 6 cm deep. One possible container is a metal baking pan.
Two sheets of newspaper.
One sheet of thin (0.5 mils) plastic at least 30 cm larger in all dimensions than the container.
One waterproof marker.
On spray bottle.
Colored water to fill a spray bottle.
Two Styrofoam packing peanuts.
This activity is designed for students to work in groups of three.
Display a copy of the poster titled "Watersheds: Where We Live" on the classroom wall several days prior to conducting this activity.
Fill the spray bottles full of water and add several drops of blue food coloring so that the water can be easily identified.
Assemble one of the models as an example for the students.
Divide the class into groups of three. Provide each group with a container, two sheets of newspaper, one sheet of plastic, one waterproof marker, one book, and one spray bottle filled with blue water.
Have one student in each group crumple both sheets of newspaper separately and place them next to each other at one end of the container. Drape the sheet of plastic over the crumpled newspaper, causing it to form hills over the high places and valleys in low places. Put a book under one end of the container to allow water to flow down the valleys and pool at the front of the container. Place the sides of the plastic sheet down into the container to prevent water from overflowing the container.
Explain that the plastic sheet represents the ground surface covering the hills and valleys. Using the markers, have the students draw where they believe the main rivers will flow in their models. Have each student spray several pumps of water, using the spray bottle, on the model. Point out to students how water runs down one side or the other of the ridged and forms rivers in the valleys. The ridges divide individual watersheds. All the areas from which water flows into a river is that river's watershed. Have the students count the number of small watersheds that drain into the main river they drew with the marker. All the watersheds should drain into the lake at the lower end of the container.
Discuss with the students that each small stream is formed by its own watershed. As streams join together, their watersheds and stream flow form larger watersheds and rivers.
Have each student place two of the Styrofoam peanuts (representing houses) on a flat location in the watershed. Have each student rapidly spray nine pumps of water on the upper portion of the watershed. Explain that rapidly spraying more water creates a flood in the watershed. Observe the houses during the rainstorm. Did the flood cause different amounts of damage (cause some to move) to the houses based on their location in the watershed?
Have students examine other groups' models. How are they alike and how are they different?
- How many watersheds are above the lake that forms at the bottom of model?
Answer: The answer will vary from model to model, but students should be able to identify at least four. Have students look carefully because some of the watersheds may be hard to see.
- What happens to the size of the stream as the watersheds get larger?
Answer: The streams get larger.
- What happens to the houses that the students placed on the level locations of the watershed? Were any houses washed away by the flood?
Answer: The houses that were the closest to the river were the ones that were washed away be the flood.
- Have students write a short essay discussing what they learned about watersheds and floods. As part of the essay, have them draw a picture of a watershed including the stream and associated flood plains. Also have students discuss where the best places to build homes within their watershed would be in order to avoid flooding.
Aquifer - An underground body of porous sand, gravel, or fractured rock filled with water and capable of supplying useful quantities of water to a well or a spring.
Drainage basin - Land area drained by a river.
Flood - Any relatively high flow of water that overflows natural or artificial banks of a stream, river, lake, or body of water.
Flood plain - A strip of relatively flat land bordering a stream, river or lake that conveys the overflow of flood waters.
Ground water - Water found in pores or cracks in sand, gravel, and rock beneath the land surface.
Precipitation - Rain, snow, hail. or sleet.
Recurrence interval - The average interval of time within which the magnitude of a given event, such as a flood, will be equaled or exceeded one time.
Runoff - That part of precipitation that appears in surface-water bodies.
Watershed - The land area that drains water to a stream, river, lake, or ocean.