Two streams, two stories... How Humans Alter Floods and Streams

Eric Baer from Wittington and Baer (2004), adapted from Foley et. al (1999) and using data from U.S.G.S. Fact sheet 229-96.
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This is a class activity in which students determine the discharge of a "100-year flood" for two human-impacted streams. They determine the discharge for two time periods on both streams. Students discover that for one stream the discharge of a predicted "100-year flood" has dropped significantly, and on the other it has increased remarkably. Students are lead to a discussion of the 100-year flood and human activities as well as some calculations of probability of exceedence. This activity supports the quantitative concepts of recurrence intervals, floods and flooding, and probability. It is appropriate for a smaller class (under 40 students). This assignment uses real data, asks students to graph and interpret data, examines the errors associated with that data and its analysis (see model limitations) and requires the students to look at societal impacts.

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Learning Goals

Upon completing this assignment, students should: floodingman
  • Be able to determine recurrence intervals for a set of discharge data
  • examine the idea of a "100-year flood" and discuss the limitations of this concept
  • discuss human impacts on streams and flooding
  • calculate the probability of exceedence from a recurrence interval

Context for Use

I use this as a group activity for undergraduate students in an introductory level class. Each group should have a calculator and ruler. Groups of 2 seem to work well. You will need at least 4 groups (or multiples of 4), one for each data set of each river. This exercise typically takes about an hour and a half, but can be extended by developing a discussion of impacts and implications or reduced by assigning parts of the assignment as homework.

Description and Teaching Materials

Here are the files for word documents that you can use for this assignment:

Teaching Notes and Tips

This typically takes 90 minutes or so but can be reduced or extended as noted above. I use this in an introductory level geohazards class, but it would be appropriate for any introductory class where flooding is taught. You will need to find appropriate graph paper. There are several choices discussed in the floods and flooding section. I do not see significant differences between these choices.
The most problematic part I have experienced is that my students often do not know how to graph a best-fit line, which requires some supplemental instruction either to the entire class or on a one-on-one basis. Some students also have trouble choosing an appropriate scale for their graph's y-axis. I have them experiment to discover a satisfactory one, even though it may take a few attempts.


I use traditional assessment on the submitted work.

References and Resources

Whittington, C. and E.M. Baer, 2005. "Workbook in Geologic Catastrophes" Kendall/Hunt Publishing, Dubuque IA 154 pp.

Foley, D., G McKenzie,and R.Utgard, 1999. "Investigations in Environmental Geology. Prentice Hall, New Jersey. 303 pp.

Dinicola, K. 1996 "The "1OO-Year Flood"" U.S.G.S. Fact Sheet 229-96 discusses the common misconception that 100-year floods occur only once in a 100-year period. In reality, a 100-year-flood may occur more than once or not at all during any particular 100-year period. The fact sheet also explains the need to update the estimated flow expected in a 100-year flood as more and more data becomes available for a particular river. This shows the importance of continued river monitoring. It is the source of the data for this exercise.