Sediment Study - Background

Teacher Background
See Erosion Patterns Background for information about erosion and typical sediment deposition patterns.

In this project, I wanted students to grapple with the notion that science is rarely as simple as the textbooks. Usually, in a river, stream or creek bed, a fast-flowing midstream section will have lots of gravel and larger rocks and the mouth will have lots of silt and clay as the current slows. Usually, the riverbed at source will have whatever type of soil the surrounding hillsides are made of. However, in real life, things are rarely so simple. There are culverts that direct creeks underground, man-made concrete channels, and dams. There are polluters, dogs and gardeners to contend with, all of whom affect the types of sediments one will find in any given spot in an urban creek.

The beauty of this project is that the ability of fast-moving water to carry more sediment of larger sizes is so robust that irrespective of the other variations, if there is sediment at all to be observed, it will almost certainly follow this trend. Most importantly, students are able to look for patterns in real world data and consolidating everything they have learned about watersheds and erosion. Surprises come from changing students’ expectations of what a creek looks like at different places and from discovering the variability of data, even at a single study site.

In this project, different classes of students go to one of 3 sediment study sites to collect information. Students lay out a 4 meter transect line to create 5 imaginary lines stretching across the creek. Students do the following:

  • Draw a careful map of their study area
  • Measure the current velocity in meters/second
  • Approximate the width of the creek
  • Measure the depth of the creek 30 cm (approximately 1 foot) from the shore at each of their 5 imaginary lines
  • Collect a sediment sample from each of their 5 imaginary lines approximately 30 cm out from shore