Lag to peak with a stream table

Amanda Schmidt, Oberlin College
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Initial Publication Date: May 27, 2014 | Reviewed: June 25, 2014


Students use a small stream table in groups to investigate how channel form determines the shape of the hydrograph (including lag to peak). They use three channels: no channel (just sediment filling the table), a "concrete" channel (straight, no sediment on bottom), and a meandering channel. All channels get the same water input and students measure the amount of water that comes out to make a hydrograph. This gives students a chance to model a flood pulse moving through different channels to see how anthropogenic changes to rivers, such as channelizing them, alters flood hydrology. It specifically models an outburst flood from a dam release or burst, but is basically modeling any pulse of water moving through a system.

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This is used as a lab exercise the week we study hydrology in a required Earth Surface Processes class where the only pre-requisite is Physical Geology. Students range from first to fourth year and have a widely varying set of skills and background in classes. I try to encourage them to be in groups where they mix ages and experience, but I don't assign lab groups. There is a page describing this class here.

Skills and concepts that students must have mastered

Students should know what hydrographs are and what lag to peak is. My students have worked with the stream table before, so they know the basics of turning the water on and off and generally using it.

How the activity is situated in the course

This is the weekly lab assignment for hydrology week. By the time they are doing this lab, they have read the hydrology chapter of their textbook (Key Concepts in Geomorphology), had a lecture on hydrology, and done a computer exercise working with discharge data to make flow frequency curves.


Content/concepts goals for this activity

  • Differences between channel forms on hydrologic processes such as water movement.

Higher order thinking skills goals for this activity

  • Analyzing often messy lab data
  • Synthesizing data collected into a lab report

Other skills goals for this activity

  • Working in a group to collect the data (definitely requires a group effort)

Description and Teaching Materials

Students have played with the stream table before in my class, but if they haven't, it is worth giving them some time to do that. I am generally working with two lab groups of 3-4 students. In each group, one person will write the lab report, one will edit it, and one or two will do a concept sketch using the results of the lab. Ideally you'd have an hour to an hour and half to do all of this, particularly if they haven't used the stream table before. With a longer lab, they could make the hydrographs in lab.

Students arrive and I give them a brief introduction about outburst floods and dam releases, which is essentially what they are modeling in this lab. I expect that they have come to lab reading the lab handout, but that is probably a bit optimistic. I remind them what lag to peak is and how you can measure it. We make predictions about how different channel forms will affect water moving through the system. It is probably worth talking with students about how these different river channels aren't just modeling channels in different places but could be modeling how water moves through channels differently and that can be changed by human activity, such as channelizing rivers through cities. This can lead into a discussion with them about how you generalize this idea and introducing the idea of friction along channel boundaries. If they have knowledge of local rivers, extend this discussion to different reaches of local rivers with different channel boundary friction and think about how floods may move differently through those rivers.

After the introduction I have them determine a flow rate for the water coming in to the stream table. They decide on an input volume, rate, and time to aim for. It doesn't matter much what this is, as long as it is consistent for all trials. They also should determine how frequently they will measure water coming out of the table (we did 20 seconds into a 1 L graduated cylinder for 4 L of water coming in in 20 seconds). They should practice collecting the water and reading volumes out at the intervals determined. It takes a little coordination between the student with the stop watch, the recorder, and two with graduated cylinders (so they can switch back and forth).

They then make one of the three channel forms (either no channel - just sediment, a "concrete channel" (straight and with sediment pushed to the sides), or a meandering channel with sediment on the bottom). They wait for water to stop dripping out the end of the table.

After the table is ready, they turn on the water for their pre-determined input volume and time. Students at the end of the table collect water and read out volumes to the recorder. Keep collecting data until very little (or no) water is coming out of the table.

This is repeated for each of the three channel types. You could add more channel types or different input hydrographs.

Students make hydrographs showing the volume of water that came out in each interval of time (say 20 seconds). They then complete their assignment for the week (write the lab report, edit the report, or make a concept sketch).

This basically models a pulse flood from upstream (say a dam burst) and how different channels respond differently.

Lab handout (Acrobat (PDF) 142kB Oct2 15)

Rubrics for grading lab assignments (Acrobat (PDF) 357kB Oct2 15)

Teaching Notes and Tips

  • We happen to have an EmRiver Em2 stream table, but any would work. We only have one so I have the students come in groups of 6-7 (8 would be ok) for a 45 min session to work with the stream table.
  • I try very hard in lab to have students figure things out on their own. Often there are specific skills that I need to teach them (like surveying) that limit my ability to do this. However, with the stream table, I really can have them drive the experiment a bit more. This puts them into an uncomfortable position, but they are usually able to work with it. For this reason, I try not to dictate too much what they should do during the lab and instead give them a bit more chance to experiment. The actual data collection doesn't take much time, so it's ok if they have a hard time figuring it out.
  • Possible problems (logistical):
    • Students broke a graduated cylinder one year (oops)
    • If you add the water too quickly it comes out faster than you can measure
    • It's probably better to add water at a slower rate even though the flood pulse is less dramatic.
    • Students sometimes want to wait for all the groundwater to drain from the system, but then they have problems with the groundwater filling back up before any water comes out. Basically you want to wait for as much water as you put in to come out, if you can keep track of it.
  • Student misconceptions
    • Students have a hard time turning model world into real world. A good, guided discussion at the beginning of lab (or during the previous class period) will help significantly with this. It's useful to have them make predictions and think about channel boundary friction before they dive in to measuring things and lose the forest for the trees.
    • It's helpful to remind students what a hydrograph is and what the lag to peak is. The data processing was hard for them even though it seems really straight forward. Walking through the lab handout helps.


I grade their lab reports, edits to the lab report, and concept sketches using the same rubric I use all semester. I have uploaded these rubrics. They are generic rather than specific to this assignment because they are used for all labs throughout the semester. This includes the lab rubric (for lab report writing), peer review questions and rubric (for how to do a good peer review), concept sketch FAQs (on making concept sketches), and making effective concept sketches (which includes the rubric).

Specific content things I am looking for from this lab are as follows.

For the concept sketch:

  • Identification should give the results of the lab and show the figure with the different hydrographs.
  • Process should explain why the hydrographs are different.
  • Implications should make connections to real life examples (dams, concrete channels through cities, and so on).
  • Predictions should think about what happens when you change something to the system. Ideally this is in the real world connection, but it could be just the stream table.

For the lab report:

  • Content for all the lab report sections should directly relate back to what students did in the lab.
  • There must be explicit, reasonable, and clear connections from the model to the real world. The lab cannot totally live in model world.

References and Resources