FIELD EXERCISE: STREAM
FLOW DYNAMICS & SEDIMENTATION
Daniel J. Bisaccio
18 School Street
Troy, NH 03465
|Donald L. Woodrow|
Department of Geoscience
Hobart & William Smith Colleges
Geneva, NY 14456
Level: Senior high
Anticipated Learning Outcomes
- Students will produce a vertical profile of a stream
and be able to map as well as define thalweg, point bar, and cut bank.
- Students will measure flow velocity differences across
a stream transect and equate flow velocity with size of substrate sediment
- Students will be able to quantify discharge values for
Streams are the great shapers of landscape as they work
to erode, transport, and deposit sediment. In this exercise, we are going
to examine how one kind of stream does some of its work.
A meandering stream is one which has a curving or sinuous
path. Its curves are called meanders after the characteristic features of
a stream made famous in classical times. That stream, the Menderes River,
is located in the southwest part of modern Turkey. Meandering streams are
often muddy because they transport a lot of fine sediment in suspension.
They also transport sediment along the stream floor, mainly fine gravel
and sand. Each meander displays a cut bank and point bar (see figure 1).
At the cut bank the stream is eroding, at its deepest part (the thalweg)
the stream is transporting sediment and it is depositing sediment on the
We are going to measure the profile of a stream, determine
its current velocity at various points and depths and examine the sediment
on the stream bottom. Our purposes are to determine the relationships between
stream profile and flow velocity and between sediment grain-size and flow
- Meter stick
- Nylon rope marked at 0.5 meter intervals and long enough
to span stream
- Cork float
- "Flower Garden Pinwheel" with one blade painted
black to be used to determine stream velocities
- Dip container to collect substrate sediment and containers
to store sediments collected
- Stop watch to be used to determine stream velocity
- Calibrate your pinwheel to determine stream velocity.
- Determine the time it takes a cork float to drift 10
meters downstream WHILE a student counts the revolutions of the pinwheel
which has been positioned below the stream surface. (See figure 2)
- Calculate the stream flow velocity (meters/second) based
on the number of pinwheel revolutions as shown in this example. Cork float
travels 1 meter in one second and in that second the pinwheel revolves
ten times. Therefore, 10 pinwheel revolutions/ second=1 meter/second stream
flow (It is easier to count revolutions for 10 seconds and divide the number
of revolutions by 10 to obtain a velocity in revolutions per second.)
- Collect the stream data.
- Set up a nylon rope across the stream and select at least
four data-collecting points across the stream as shown in Figure 1 (points
A, B, C, D).
- At each data-collecting point, measure the stream depth
and construct the cross-profile of the stream as shown in the sketch.
- Students should identify the kind of sediment found on
the stream bottom at each data-collection point. Look for mud, sand, gravel,
or mixtures and note on the cross profile what you saw. If you plan to
carry away sediment samples, put them in containers labelled with the proper
- Using the pinwheel, determine the stream flow velocity
at each data-collecting point. At each point the velocity should be determined
at the stream surface, near the bottom and at a middle depth of the stream.
- Calculate the stream flow velocities and note them on
the stream profile.
- Calculate the stream discharge (Q) using this formula:
Discharge (Q) = width x
depth x velocity
Q = (meters) x (meters)
- One side of the stream profile is steep (cut bank) and
the other side has a gentle slope (point bar). The deepest part of the
stream is the thalweg. Label the thalweg.
- The stream flow velocity varies both across the stream
and with depth. The greatest velocities are near the thalweg.
- The coarsest sediments are located on the stream floor
where the stream flow velocity is greatest.
Students may want to note any plants and/or animals
found in the stream sediment and consider whether the plants and/or animals
live there. Do any of those creatures appear to live in only one kind of
exercise can be done by teams of students. Each team might include: one
to handle the stop watch and record data, one to wade in the stream and
take measurements, and one or two to keep the nylon rope taut and generally
encourage the others. If multiple sets of equipment are not available, then
one team can make the observations for the class or the jobs can be shared
around. Plotting the profile and doing the calculations should be a group
effort, regardless of how the teacher chooses to have the class do it.
|Fig. 1. Example of stream profile showing location of cut
bank (A), thalweg (B) and point bar (C and D).
|| Fig. 2. Pinwheel flowmeter|