Floodplains in the field

Goals

• Observing fluvial landscapes in detail
• Connecting landscape observations to map and cross-section representations
• Relating earth materials to landscape position and geomorphic process
• Moving between cross-sectional and planar views of landscapes
• Reconstructing history of a river system
• Understanding rivers as transporters and depositors of sediment

Skills

• Surveying
• Coring and describing alluvial material
• Understanding vertical exaggeration in cross-sections
• Making a sketch map
• Observation, note-taking, and sketching
• Describing color and texture of soil materials

This lab can take from one to three hours, depending on the detail and methods of the surveying, drafting of the cross-section and maps, and number of soil cores dug.

Physical geology or geography course: Use during units on rivers, fluvial landscapes, or sedimentary processes
Earth History course: Use during unit on how geologic landscapes evolve
Earth systems science course: Use during discussion of storage and transport of water and sediment in river systems

Useful background information before lab: Distinguishing sand, silt, clay (although this can be taught during this lab).

Field site: a stream or river with a floodplain, ideally undisturbed by humans. It is helpful, though not essential, to have good visibility across the floodplain.

Equipment needed:

• 50-meter tapes
• Augers or other soil-coring devices (a meter in length is helpful)
• Automatic level, tripod and survey rod - or a handlevel in the hands of two students of equal eye height
• Graph paper (five to a cm) for each student
• Oversize sketchpad and markers
• Compass
• Soil color book, texture triangle and tips on distinguishing sand, silt, and clay (optional)


• Students should bring field notebooks and pencils

Scout the area ahead of time to locate a place for the topographic profile and soil cores that has these characteristics: approximately perpendicular to the river and floodplain margin, reasonable visibility along the line, typical of the area.

Students will expect that the local slope across the floodplain will be from the floodplain margins toward the river. On unmodified floodplains, with streams that overtop their banks every other year or so, the topographic profile will contain a higher area adjacent to the stream channel (the levee). It's common for the lowest part of the floodplain to be near the margins, not near the river. You can start the lab indoors by asking students to draw their conception of what the topographic cross-section will look like, and then return to those sketches at the end of the lab period (or a subsequent class period) to compare their conceptions with reality.

For a self-contained lab (no homework required): Use oral reports from groups to assess how well they interpreted their part(s) of the assignments.
At the start of the next class period, ask students to sketch the topographic cross-section and briefly describe the materials in each section and how they formed as an informal writing assignment and/or quiz.

Assessable homework after the lab: Drafted explanatory cross-sections and maps; soil stratigraphic columns, short paper about the relationship between river evolution and the stratigraphy and topography of the floodplain.

Also see the following print and web resources:

Under construction