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Unit 4: Hazards from Flooding

Kyle Gray, University of Northern Iowa (

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These materials have been reviewed for their alignment with the Next Generation Science Standards as detailed below. Visit InTeGrate and the NGSS to learn more.


This unit examines detailed hydrologic data from a river to identify ways in which precipitation and stream discharge influence flooding which often impacts nearby human societies.

Science and Engineering Practices

Developing and Using Models: Use and/or develop a model of simple systems with uncertain and less predictable factors. MS-P2.3:

Constructing Explanations and Designing Solutions: Apply scientific ideas, principles, and/or evidence to construct, revise and/or use an explanation for real- world phenomena, examples, or events. MS-P6.4:

Planning and Carrying Out Investigations: Select appropriate tools to collect, record, analyze, and evaluate data. HS-P3.4:

Cross Cutting Concepts

Patterns: Patterns can be used to identify cause and effect relationships. MS-C1.3:

Patterns: Mathematical representations are needed to identify some patterns HS-C1.4:

Disciplinary Core Ideas

The Roles of Water in Earth's Surface Processes: Variations in density due to variations in temperature and salinity drive a global pattern of interconnected ocean currents. MS-ESS2.C4:

Structure and Properties of Matter: In a liquid, the molecules are constantly in contact with others; in a gas, they are widely spaced except when they happen to collide. In a solid, atoms are closely spaced and may vibrate in position but do not change relative locations. MS-PS1.A4:

Natural Resources: Humans depend on Earth’s land, ocean, atmosphere, and biosphere for many different resources. Minerals, fresh water, and biosphere resources are limited, and many are not renewable or replaceable over human lifetimes. These resources are distributed unevenly around the planet as a result of past geologic processes. MS-ESS3.A1:

Earth’s Materials and Systems: All Earth processes are the result of energy flowing and matter cycling within and among the planet’s systems. This energy is derived from the sun and Earth’s hot interior. The energy that flows and matter that cycles produce chemical and physical changes in Earth’s materials and living organisms. MS-ESS2.A1:

  1. This material was developed and reviewed through the InTeGrate curricular materials development process. This rigorous, structured process includes:

    • team-based development to ensure materials are appropriate across multiple educational settings.
    • multiple iterative reviews and feedback cycles through the course of material development with input to the authoring team from both project editors and an external assessment team.
    • real in-class testing of materials in at least 3 institutions with external review of student assessment data.
    • multiple reviews to ensure the materials meet the InTeGrate materials rubric which codifies best practices in curricular development, student assessment and pedagogic techniques.
    • review by external experts for accuracy of the science content.

  2. This activity was selected for the On the Cutting Edge Reviewed Teaching Collection

    This activity has received positive reviews in a peer review process involving five review categories. The five categories included in the process are

    • Scientific Accuracy
    • Alignment of Learning Goals, Activities, and Assessments
    • Pedagogic Effectiveness
    • Robustness (usability and dependability of all components)
    • Completeness of the ActivitySheet web page

    For more information about the peer review process itself, please see

This page first made public: Jan 22, 2015


In this unit, students examine detailed hydrologic data from one river to identify ways in which precipitation and stream discharge influence flooding which often impacts nearby human societies. They also research a local river and determine the hazard associated with flooding, describe historic flooding, and assess ways a local community mitigates the risks associated with flooding.

Learning Goals

Unit 4 Learning Goal

By the end of this unit, students will be able to:

  • Describe that flooding is periodic and probabilistic, caused by short-term and annual meteorological factors, and can have profound impacts on humans living along a river system.

Unit 4 Learning Objectives

In order to achieve that learning goal, students will meet the following objectives:

  • Objective 4-1. Students will interpret hydrographic and meteorological data to draw conclusions regarding the interaction between precipitation, discharge, and flooding.
  • Objective 4-2. Students will calculate recurrence intervals of major flooding for one river system using stream gauge data.
  • Objective 4-3. Students will define a "100-year flood" and explain why floods of that magnitude can occur in successive years.
  • Objective 4-4. Students will describe hazards associated with a river system and evaluate their impact on ecosystems and human society.

Context for Use

Unit 4 is an activity designed for an introductory geoscience content course that is aimed primarily at pre-service teachers. It may be used as part of the Interactions between Water, Earth's Surface and Human Activity module, or as a stand-alone activity. The curriculum is designed to build a strong foundation of pedagogical content knowledge for teaching Earth science. This type of course is common at state and regional schools with large teacher preparation programs. Activities are designed to foster group collaboration as students work in small groups (ideally in groups of 3–4) with a faculty member acting as the facilitator.

Unit 4 is designed to take two hours in a lab setting. It is not recommended for implementation in a large lecture class.

The content in this unit aligns well with Science and Engineering Practices, Disciplinary Core Ideas and Crosscutting Concepts in the Next Generation Science Standards (NGSS):

Developing and Using Models

    • Calculated recurrence intervals are a mathematical model that describes the probability that a flood of a given discharge will occur. Performance expectation (5-ESS2-1)
Analyzing and interpreting data
    • Interpreting precipitation and discharge data. Performance expectation (K-ESS2-1), (4-ESS2-2), (MS-ESS3-2)

Constructing explanations

    • Student explanations for when rivers flood and what factors influence flooding. Performance expectations (2-ESS2-1), (4-ESS3-2), (MS-ESS2-2), (HS-ESS3-1)

Obtaining, evaluating, and communicating information

    • Interpreting discharge data to infer climatic factors that influence flooding. Performance expectations (K-ESS3-3), (2-ESS2-3), (3-ESS3-1), (5-ESS3-1)

ESS2.A: Earth Materials and Systems
    • Kindergarten. Use and share observations of local weather conditions to describe patterns over time. Performance Expectation (K-ESS2-1)
    • Grade 2. Wind and water can change the shape of the land. Performance expectation (2-ESS1-1), (2- ESS2-1)
    • Grade 5. Develop a model using an example to describe ways the geosphere, biosphere, hydrosphere, and/or atmosphere interact. Performance Expectation (5-ESS2-1)
    • High School. Analyze geoscience data to make the claim that one change to Earth's surface can create feedbacks that cause changes to other Earth systems. Performance Expectation (HS-ESS2-2)

ESS2.B: Plate Tectonics and Large-Scale System Interactions

    • Grade 2. Maps show where things are located. One can map the shapes and kinds of land and water in any area. Performance expectation (2-ESS2- 2)
    • Grade 4. Analyze and interpret data from maps to describe patterns of Earth's features. Performance Expectation (4-ESS2-2)

ESS2.C: The Roles of Water in Earth's Surface Processes

    • Grade 2. Water is found in the ocean, rivers, lakes, and ponds. Water exists as solid ice and in liquid form. Performance expectation (2-ESS2-3)
    • Grade 5. Human activities in agriculture, industry, and everyday life have had major effects on the land, vegetation, streams, ocean, air, and even outer space. But individuals and communities are doing things to help protect Earth's resources and environments. (5-ESS3-1)
    • Middle School. Water's movements — both on land and underground — cause weathering and erosion, which change the land's surface features and create underground formations. Performance expectation (MS-ESS2-2)
    • Middle School. Analyze and interpret data on natural hazards to forecast future catastrophic events and inform the development of technologies to mitigate their effects. Performance Expectation (MS-ESS3-2)

ESS2.D. Weather and Climate

  • Grade 3. Represent data in tables and graphical displays to describe typical weather conditions expected during a particular season. Performance Expectation (3-ESS2-1)

ESS3.B. Natural Hazards

  • Grade 3. Make a claim about the merit of a design solution that reduces the impacts of a weather-related hazard. Performance Expectation (3-ESS3-1)
  • Grade 4. Generate and compare multiple solutions to reduce the impacts of natural Earth processes on humans. Performance Expectation (4-ESS3-2)

ESS3.C: Human Impacts on Earth Systems

    • Grade 5. Human activities in agriculture, industry, and everyday life have had major effects on the land, vegetation, streams, ocean, air, and even outer space. But individuals and communities are doing things to help protect Earth's resources and environments. Performance expectation (5-ESS3-1)

    • Patterns in rates of change and other numerical relationships can provide information about natural systems. Performance expectations (MS-ESS3-2)

Cause and Effect

  • Events have causes that are sometimes simple and sometimes multifaceted. Performance expectations (K-ESS3-3), (4-ESS3-2), (HS-ESS3-1)
Scale Proportion and Quantity
  • Time, space, and energy phenomena can be observed at various scales using models to study systems that are too large or too small. Performance expectations (MS-ESS2-2)

Systems and System Models

    • Models can be used to represent systems and their interactions — such as inputs, processes and outputs — and energy, matter, and information flow within systems. Performance expectations (5-ESS2-1), (HS-ESS3-6)
Stability and Change
  • Stability, rates of change, and evolution of a system are critical for natural and built systems. Performance expectations (2-ESS1-1), (2-ESS2-1)

Description and Teaching Materials

In this unit, students use data from the United States Geological Survey (USGS) and the Federal Emergency Management Agency (FEMA) to identify both short-term (hours or days) and long-term (months to years) factors that influence flooding along a river. The Cedar River in Iowa is the initial focus of their explorations of annual variations in discharge and the connection between precipitation and discharge. Then students analyze the floods along the Mississippi River in 1993 and 2008 to calculate recurrence intervals. Finally, they apply these skills to analyze a river system near them and develop an informational brochure for the community.

Student Handouts and Materials

Required Materials

Students ideally work in small groups of three or four; each group will need:

  • A workspace for group work.
  • A whiteboard and markers (recommended as a way to facilitate group discussions and presentations to the larger class).
  • Access to a spreadsheet program like Microsoft Excel, if students download discharge data from the USGS website.

Initial ideas

To begin this unit, elicit students' ideas about flooding and its impacts. These questions are included in the student handout for this unit (Microsoft Word 2007 (.docx) 1.2MB Jan9 15):

  • Where does the water come from when a river floods?
  • What is meant by the phrase "a 100-year flood?" How often would one occur?
  • Brainstorm a list of ways that a river can impact a home or a city.

This can be done in class or as homework prior to class. Students should have time to write down their own ideas first, then share them in small groups and with the class.

Eliciting initial ideas effectively

Students write down their own ideas first, then share their thinking in small groups. The small groups create displays to share their ideas with the rest of the class — these displays can be on small, portable whiteboards, poster-sized Post-it notes, or on whiteboards/chalkboards around the room. This is a sharing of ideas only, no trying to "convince" anyone that their ideas are right or wrong. Students will revisit their initial ideas at the end of the module to analyze how their ideas have changed.

After the small groups share their ideas with the rest of the class, they are prompted to write down ideas that were different from their own.

Students should hang on to handouts with their initial ideas, as they will refer to these at the end of the module to assess their learning.

It is very important not to correct any misconceptions during the sharing of initial ideas. This should be a safe environment to get all ideas on the table. Students should know that their ideas are meant to change during the course of the activity.


All of this information is also included in the student handout.

In 1993, much of the Midwest experienced record flooding due to a wet spring combined with a persistent rain. Cities along the Mississippi and Missouri Rivers experienced record flooding. Des Moines, Iowa, is located between those two rivers and was without power for several days. This flood displaced tens of thousands of people, destroyed hundreds of homes, and affected an area 1,200 km long and 700 km wide. This flood was one of the largest and costliest floods in US history. Scientists who study streams called it a "500-year flood." These links describe the floods and show some photos from the event:

In 2008, many parts of the Midwest were again under water. In Cedar Rapids, Iowa, over 5,200 homes were flooded. See these links for a description of the flood as well as some photos of the floodwaters:

Scientists calculated the size of the 2008 flood and determined that it was also a "500-year flood." This meant that Iowa had experienced two "500-year" floods in just 15 years! In this unit, students will explore why rivers flood and how they impact the people who live along their banks. Students will also learn how floods are rated and how scientists calculate a 500-year flood. Finally, students will investigate flooding for a city in your area.

Part 1: Annual Changes in River Level

In this part of the unit, students examine data from the Cedar River in Iowa to learn about the concepts of annual variability and shorter-term changes in discharge. Students work through questions in small groups, then discuss their answers with the whole class.


For any given location, a river's "flood stage" refers to the height of the water's surface when it first begins to inundate areas that are not normally covered by water. It also relates to a specific discharge of water passing that location (volume of water in the stream, measured in cubic meters per second, m3/s). So once a river reaches a designated height (and discharge), the river is said to be experiencing a flood.

Streams of all sizes experience floods. For this activity we will use the city of Cedar Falls, Iowa, as a case study for flooding. Cedar Falls is a city of approximately 35,000 people located along the banks of the Cedar River; it has experienced numerous floods since its founding in 1845. The United States Geological Survey maintains a gauging station at Cedar Falls to measure the height of the Cedar River and reports the data on an hourly basis. When the Cedar River is at flood stage at this location, it has a discharge equal to 660 m3/s. Imagine a box one meter long, wide, and tall filled with water. Now imagine a stack of 660 boxes flowing past you every second. That is how much water is in the Cedar River when it begins to flood.

The graph below shows the daily discharge of the Cedar River for all of 2007. Every year, the discharge for the river is slightly different, but 2007 was a typical year for this river system. (Remember that the height of the water is related to discharge, so when discharge rises, the river level also rises.) Study this graph and answer these questions.


  • Pretend you own a house near the river. Describe how the discharge changed over the course of the year.
  • What causes the spikes in the discharge?
  • Discuss your answers with someone and summarize what happens to the discharge of the Cedar River over the course of a year. Explain what caused the sudden increases (spikes) in river discharge. Be prepared to discuss with the rest of the class.

Part 2: Connections with the Hydrologic Cycle

In this part, students connect river discharge to the hydrologic cycle by relating annual and daily variations in discharge to precipitation data. The data they interpret come from the USGS stream gauge and weather station at Waterloo, Iowa, and can be downloaded from the National Water Information System (NWIS) web interface for the Cedar River at Waterloo, IA stream gauge site. The graphs are provided here, but you might also want students to download the data and graph it themselves.

Here are two graphs comparing the discharge of the Cedar River with the area's daily precipitation. The first graph shows the entire year and the second graph zooms in on April and May of that year. Study these graphs and answer these questions on your own.


  • Describe the connection between precipitation and river discharge. What might cause this relationship?
  • Were there any times when precipitation and river discharge were not related? Can you think of a reason why this might occur?
  • Discuss what happens to the river's discharge after a rain event — and why the highest discharge is not on the same day as the largest rain event. Be prepared to share your ideas with the class.
  • The graph to the right shows a hypothetical rain event and the discharge for the Cedar River. The line showing discharge is shown through May 10. Draw in a likely discharge graph for the rest of the month and explain why you chose that shape.

Part 3: Yearly variation and what it means

Students examine graphs of stream discharge from four different years (1977, 1983, 1997, and 2010). In comparing these graphs, they infer the relative amount of rainfall in each year and look for patterns and cycles that are consistent year to year.

Possible extension: Students can look up newspaper stories from this region for that year or investigate the discharge from another stream in the area such as a smaller tributary river or creek.

Part 4: Predicting future floods

In this part, students calculate the recurrence interval (RI) for floods of a given discharge for the Cedar River. Using their calculations, they estimate the RI for the Great Flood of 2008. Then they explore the meaning of the terms "100-year flood" and "500-year flood," and how the recurrence interval is really an indication of the probability of a flood that size occurring in a given year, rather than a prediction.

Predicting future floods

It would be nice if people living along a river had a crystal ball and could predict the maximum discharge for the coming year. Unfortunately, we cannot predict the future and cannot travel into the future to see when the next big flood will occur. Instead, all we can do is use the data from past floods to calculate the probability that a flood of a given size will occur. Small floods happen almost every year, but the really big floods rarely happen. These are the ones that people are most concerned about.

Scientists have defined the term "a 100-year flood" to mean a flood of that has a 1% probability of occurring in any given year. The term "100-year flood" does not mean that a flood of that size occurs only once in a hundred years, just that the probability of a flood that size is very low.

The USGS and other organizations monitor the discharge (volume of water in the stream) at gauging stations along many rivers and creeks and use it to determine the frequency of flooding along the stream. Estimates of flood frequency are more accurate with a long record (many years) of discharge records. The flood frequency is typically expressed as a recurrence interval. This is the average number of years expected between floods of a given magnitude, and is calculated by listing all of the floods that have ever occurred and ranking them from the largest to the smallest.

The equation for recurrence interval is . . .

Recurrence Interval = (n+1)/Rank

Where n = the number of years on record (in this case 72)* and Rank = the position within that list.

Part 5: Consequences of living on a floodplain

The Federal Emergency Management Agency (FEMA) publishes maps that define the 100-year and 500-year floodplains for participating communities. People who live within a floodplain are required to obtain supplemental flood insurance to cover the costs incurred in a flood. Cities such as Cedar Falls often use these flood maps to regulate construction within the floodplain. In this part of the unit, students examine FEMA's floodplain map of Cedar Falls, Iowa, and discuss what it means for people living in the area.

Homework and summative assessment

The final component of this unit is a homework project in which students work in pairs (or individually) to develop an informational brochure for a community about the flooding hazards of a given river. The prompt is: Imagine that you and your research partner are writing an informational brochure that a riverfront city like Cedar Falls, Iowa, could use to educate its citizens on the impact of living near a river. All information should be written so a member of the general population could understand it. This means defining ALL technical language. You will use online and any other additional resources that you may find to gather your information.

Teaching Notes and Tips

All of the data and graphs used in this module were obtained from (or generated from) the US Geological Survey's WaterWatch website. Instructors can easily download data for a nearby stream to make this unit more relevant for their students.

Students must be encouraged often to read what is in the activity sheets and not look to the teacher to tell them what to do. Students must also be encouraged to write down their answers whenever a prompt is encountered. Skipping answers may lead to misconceptions or misunderstandings. Skipping answers also denies students the opportunity to later reflect on their thought processes as they learn the material.

Download these

on implementing Unit 4 plus expected outcomes and answers to questions in the unit.


Objective 4-1: Student worksheets: For each graph, infer the relative amount of rainfall for each year in Cedar Falls and describe your observations that support your conclusion.

Objective 4-2: Use the graph on the previous page to estimate the recurrence interval for that flood and determine the probability that a flood of that size could happen again. Explain below how you arrived at those answers.

Objective 4-3: Two students are discussing what that term means and whether it is safe to live near the river . . . Who is right? Explain why. (Question 4-8)

Objective 4-4: Research flood hazards for a city on a nearby river or on a river of choice. Students will create a brochure outlining the following points: Location and size of the river, largest recorded flood, cost of that flood, any measures that the city has done to protect itself from flooding (levees, flood walls, etc.).

References and Resources

General Resources

  • Flood Recurrence Interval activity modeled after Checkpoint 11.19 from McConnell, D., Steer, D. (2015). The Good Earth, 3rd Edition. McGraw-Hill, New York.
  • Allen, Jesse, 1993, NASA, Satellite Images of the Great Flood of 1993 - Earth Observatory using data provided courtesy of the Landsat Project Science Office.
  • Photograph of a house threatened by floodwaters near St. Louis, Missouri. Flooded House Near St. Louis, Missouri — Photograph by Sam Leone and used by permission from the St. Louis Dispatch.
  • U.S. Geological Survey Podcast - Podcast describing the definition of a 100-year flood
  • Graphing and Best-Fit Lines from The Math You Need When You Need It project

Information and Data on Flooding (USGS)

Water Data (including stream discharge)

Annual Peak Stream Discharge Data

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These materials are part of a collection of classroom-tested modules and courses developed by InTeGrate. The materials engage students in understanding the earth system as it intertwines with key societal issues. The collection is freely available and ready to be adapted by undergraduate educators across a range of courses including: general education or majors courses in Earth-focused disciplines such as geoscience or environmental science, social science, engineering, and other sciences, as well as courses for interdisciplinary programs.
Explore the Collection »