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Great Rivers 2: The Ups and Downs of River Flooding

What You Need

 
Great Rivers 2: The Ups and Downs of River Flooding

Purpose

To help students understand that rivers flood, but not on a schedule. To introduce students to the difference between seasonal floods and great floods.


Context

This lesson was developed by Dr. Penny Firth, a scientist, Dr. Richard Sparks, of the University of Illinois Urbana-Champaign, and Milton Muldrow, Jr., a scientist, as part of a set of interdisciplinary Science NetLinks lessons aimed at improved understanding of environmental phenomena and events. Some of the lessons integrate topics that cross biological, ecological, and physical concepts. Others involve elements of economics, history, anthropology, and art. Each lesson is framed by plain-language background information for the teacher.

This is the second of a strand of three lessons entitled Great Rivers: Where Ecology Meets History. The lessons address the concept of large rivers as dynamic ecological systems that have had vital influences on human history. In Great Rivers 1: The Nature of Great Rivers, students are shown the basics of the water cycle. They are introduced to the concept that flowing water carries materials such as silt, plankton, leaves, and wood. They also learn how rivers and the materials that they carry define how they look and act.

Great Rivers 2: The Ups and Downs of River Flooding focuses on flooding, both because of its importance to human history, and because it is often misunderstood. Key to this lesson is the concept that while some floods may be predictable in a general way (i.e., in the springtime), floods do not occur on a schedule.

Great Rivers 3: Great Rivers, Great Givers consists of a variety of brief sketches that illustrate for students how rivers have influenced human history from the dawn of civilization.

Students should know that precipitation is responsible for flooding. When the amount of water entering the river exceeds what it can carry in the channel, the river rises up onto its floodplain. The floodplain is the land on either side of the river banks.

Students may need to be reminded that the precipitation that causes flooding might be distant in either time or space from the flood itself. For example, precipitation that falls as snow might cause flooding when it melts, long after the snowstorm. And precipitation may cause flooding far downstream from where it fell.

Contact Dr. Firth at pfirth@nsf.gov.


Planning Ahead

A good background book is: World Disasters! Flood, by Brian Knapp (Steck-Vaughn, Austin, Texas. 1989). This book includes personal stories from around the world, good explanations of floods from rivers, hurricanes, and tsunamis, as well as a glossary.

The set of three Great Rivers lessons will take approximately six periods, depending on how many of the exercises are selected.


Motivation

The Amazon is truly one of the world's Great Rivers. The word Amazon comes from Greek mythology and refers to a race of female warriors. At Amazon Basin Facts, you and your class can read some gee whiz statistics about the river basin. In the Amazon, the flood lasts nine months of the year, and river levels may rise 30 to 50 feet above normal. After you have looked at this resource, ask students these questions:

  • How long is the Amazon River basin?
        (The Amazon River basin is ranked as the largest in the world in terms of watershed area, number of tributaries, and volume of water discharged. It is about 4,195 miles long, covering about 2,720,000 square miles in area, including its 15,000 tributaries and subtributaries (four of which are in excess of 1,000 miles long).
  • In the Amazon Basin, flooding often occurs between June and October. If the flood lasts so much of the year, is it considered the "normal" condition for the river?
      (Answers will vary.)
  • How wide does the Amazon get?
      (It can get to be as wide as 35 miles.)

Development

In this section, students will learn about the patterns and processes of flooding, how rivers may have "flashy" floods versus longer-duration floods, and how seasonal flooding influences the life of the river and floodplain.

To start, have your students use River Flooding student esheet to go to The Ups and Downs of River Flooding where they should read the "What is a Flood?" section. Once students have finished with this section, they should use their esheet to go to the San Diego River annual hydrograph for an example of a river with very flashy flooding behavior.

After the students go through this material, they should be able to discuss the questions found on the esheet as a class. They can record their answers to these questions on the River Flooding student sheet. You can find answers on the River Flooding teacher sheet.

Once you have finished this discussion, students should go on to read the "What is a Flash Flood?" section of The Ups and Downs of River Flooding resource. Once they are done, divide the class into small research teams and have them refer to their esheet and search the Web for information on well-known flash floods. They may use the examples below, or others that they have heard of or can find in a Web search.

Example Flash Floods:

  • Cheyenne, Wyoming; August 1, 1985
  • Shadyside, Ohio; June 14, 1990
  • New Braunfels, Texas; August 2002
  • Madison County, Virginia; June 25-27, 1995
  • Fort Collins, Colorado; July 28, 1997
  • Rapid City, South Dakota; June 9, 1972
  • Johnstown, Pennsylvania; May 31, 1889
  • Big Thompson Canyon, Colorado; July 31, 1976

Ask each research team to find the answers to the questions on the Flash Floods student sheet. Each research team should prepare a brief report on "their" flash flood. It might be illustrated with maps, photos or drawings, and bulleted points.

The next exercise involves truly awesome powers of nature: great floods. Ask students to read the "What is a Great Flood?" section of the student resource. Once they have finished that section, they should take a visual tour of the Grand Forks Flood of 1997 and visit the PBS Journey to Planet Earth site and read about the great flood of 1993 and how it affected Grafton, Illinois. Once they are done, discuss the questions from the River Flooding student sheet.

If you have time, and especially if your school or many of the students' homes are near a river, try the Great Flood Prediction exercise. Hand out the student sheet to your class and divide students into three teams. Students should use the student sheet to help them calculate the probability that a flood of a given magnitude will occur in a given year using local river gauge information.

Before students do this, you should go to the USGS Waterwatch site and click on the state and then the gauging station that you choose. Then follow these directions:

  1. When you click on the gauging station, a pop-up will open that shows two tabs: Summary and Hydrograph. Click on the Hydrograph tab then click on the graph you see. This will open a new window for the gauging station.
  2. On the pulldown menu "Available data for this site," select "Surface-water: Peak streamflow."
  3. Scroll down the page a little to find the "Summary of all available data for this site" link and click on that. Then click on the "Peak streamflow" link again.
  4. The table will indicate how many years back the record extends. Students will need the "years of record" for the exercise so they should record it.
  5. Have students order the peak flows from high to low.
  6. Now number the peak flows by rank: the highest peak flow is given the rank of "1," the next highest peak flow will be rank "2," and so on. Students will need these ranks for the exercise.
  7. One group, Team One, should be assigned the high peak flow. The second group, Team Two, should be assigned the mid-range peak flow, and the third group, Team Three, should be assigned the relatively low peak flow.

Now students are ready to do the calculations described on the student sheet.

Once students have finished the great flood prediction exercise, ask them to use their esheet to go to the "How are Floods Good?" section. Once they have read it, they should use their esheet to go to Watersheds of the World: Africa - Nile Watershed. Ask them to study the map and the map description statistics below it and be prepared to answer the questions from the student sheet.

Now ask your students to use their esheet to go to the "How are Floods Bad?" section and read it. Then have the class go to the Fatal Flood site and work their way through some of the available resources describing how the Mississippi Flood of 1927 affected people. The transcript of the film and some of the primary sources used in making the film are particularly interesting.

Discuss the questions from the student sheet with your students.


Assessment

Students should now be familiar with what a flood is, and what some of the causes and contributing factors are. They should understand the difference between seasonal floods and great floods, and that floods do not occur on a schedule. They should appreciate how floods have been considered both good and bad from a human perspective through history, and they should understand how human actions may cause or exacerbate floods.

In order to bring this lesson to closure, have your class—working in small teams—develop a "recipe book" for floods. Randomly assign seasonal floods, flash floods, and great floods to different teams, one flood per team, and have the students determine what the "ingredients" are (i.e., spring snowmelt, heavy late summer precipitation, dam collapse, etc.). Then have them describe the circumstances for "mixing and combining"—that might include recent extensive paving in the catchment, poor agricultural or forestry practices, loss of wetlands that might soak up flood waters, channel dredging or filling, or anything else they can think of. Have them discuss whether the recipe they are developing will be applied to a river with a flashy hydrograph, or perhaps a Great River with a more stable hydrograph. Be sure they include how long different events, including the flood itself, will last. They also can include "yields" much as food recipes do, but emphasizing the effects of the flood on the river itself, the people who live along it, and floodplain ecosystems. 


Extensions

Flood Patterns and Processes
Look up the annual hydrograph (water pattern) of your local river or a river of choice:

  • Go to the USGS Waterwatch site and click on the state and then the gauging station that you choose.
  • When you click on the gauging station, a pop-up will open that shows two tabs: Summary and Hydrograph. Click on the Hydrograph tab then click on the graph you see. This will open a new window for the gauging station.
  • On the pulldown menu "Available Data for this Site" select Time Series: Daily data.
  • In the box in the middle of the page, select the dates that you want for the hydrograph. Note: Using several years instead of just one can work well.
  • Submit the request and you will shortly be seeing the hydrograph.

The catchment is just what you might think, it is the area that catches the precipitation that ultimately moves downhill to the stream or river. Another term for catchment is watershed. Catchments that are hilly and steep deliver water to the river very quickly, and can be prone to flash flooding. Catchments with a lot of impervious surfaces (i.e., paved areas) do the same thing, because the water is not able to soak into the ground. Forested catchments slow the movement of water, and allow more of it to enter the groundwater. This is why flooding is often a problem in catchments that have been logged off.

What can you find out about the catchment of your local river? Has the amount of impervious surface changed much in the last decade? Why? How about the amount of land that is in forest or other vegetation? How might these influence the hydrograph of the river? Would you expect more flooding, or less, from the changes that have taken place?


Superfloods

The Superfloods Continue to Generate Heat in Earth Science article introduces superfloods as follows:

"Around 15,000 years ago, as the last ice age was melting down, you didn't want to be standing in Idaho's Clark Fork River Valley. Giant glacial Lake Missoula stretched 174 miles behind an ice dam ready to burst. When the 2,000-foot-high ice dam did go, the lake surged downstream with a flow ten times greater than the flow from all the world's present rivers combined. The massive flood cut deep canyons, stripped away soils, and carried boulders all the way from Idaho to what is now Portland, Ore. In 48 hours, it transformed the landscape of the Pacific Northwest."

Geologists call this a superflood. Several occurred at the end of the last ice age. Others have happened in Iceland within human history when volcanoes have heated glaciers. Still other superfloods occurred on Mars, according to recent analysis of spacecraft photos.

Have your students explore superfloods and produce a superflood report that describes one of them, discusses how and where it happened, explores its main effects, and highlights a present geomorphological feature of the Earth or Mars that resulted from the superflood.


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