Post-Settlement Erosion and Deposition

This exercise includes the following pedagogical strategies:

• working in groups
• collecting new data in the field
• collecting and working with quantitative data
• close reading of a professional article and associated commentary and debate
• writing a lab report in the format of a scientific article

A version of this exercise can be constructed for any area of the country where agriculture, grazing and/or urbanization has altered the landscape. See references at the end of the .pdf file to resources for the Pacific Northwest and desert Southwest.

Look for the following characteristics in choosing a field site:

• An area that can be covered on foot in a single lab period (less than one km2)
• A stream valley that has been partially filled with sediment and later incised, so that sedimentary features of the fill are visible
• Permission from all landowners for access

Students should bring to lab:

• Metric ruler
• Notebook and sharp hard pencil for taking notes
• Field clothes (long pants, long sleeves, boots)

Instructors should provide:

• 50 m tapes
• Two meter sticks
• Base maps of field area

Full lab handout in .doc form (Microsoft Word 41kB Sep6 06)

Teaching tips for this lab:

• Many students will not read the Trimble paper carefully until after they return from the field. Thus, it is important to take time in the field to explain how their field measurements can be translated into volumes of material deposited and eroded.
• You may want to divide the field site and have each small group of three students measure part of it. Each group's results could then be posted to the entire class.
• Make a base map for the students from an air photo or topographic map. [Link to "How to find an air photo of your area?]

From Geology 120 syllabus: "When reading written work, Mary looks for well-defined questions, good understanding of the subject, careful and complete observations of geologic processes, sound logic connecting the observations to the conclusions, clearly stated conclusions and complete discussion of the implications of the conclusions. All sources of information, including web sites, should be cited completely."
For this assignment specifically:

• Does the paper have appropriate sections and subheadings?
• Do each of the sections contain the appropriate content?
• Are all three questions addressed in the discussion?
• Are results reported accurately and clearly? Were enough measurements taken?
• Does the paper show a reasonable level of understanding of the geological history and how humans may have influenced this history?
• Can I tell from the paper that the students have read the Trimble article and its relatives?

References:

1. Trimble, Stanley W., 1999, Decreased Rates of Alluvial Sediment Storage in the Coon Creek Basin, Wisconsin, 1975-93: Science, v. 285, p. 1244-1246.
2. Glanz, James, 1999, Sharp Drop seen in Soil Erosion Rates: Science, v. 285, p.1187-1189

(Both of the previous are in the August 20 1999 issue).
3. Pimentel, David, Skidmore, Edward L., Trimble, Stanley W., 1999, Rates of Soil Erosion: Science, v. 286: p. 1477. (19 November issue)

(This is a comment and response to Trimble's original article)
4. Trimble, Stanley W. and Pierre Crosson, 2000, U. S. Soil Erosion Rates - Myth and Reality: Science, v. 289, p. 248-250. (14 July 2000 issue)
5. M. A. Nearing, M. J. M. Romkens, L. D. Norton, D. E. Stott, F. E. Rhoton, J. M. Laflen, D. C. Flanagan, C. V. Alonso, R. L. Binger, S. M. Dabney, O. C. Doering, C. H. Huang, K. C. McGregor, A. Simon; Stanley W. Trimble, and Pierre Crosson, 2000, Measurements and Models of Soil Loss Rates: Science, v. 290, p. 1300-1301 (17 November issue)

(This is a letter and response to the Trimble and Crosson article).