(Written by Beth Troeger, Norwich, CT, with some modifications and additions
(Be prepared to get your feet wet!)
In Connecticut, most people live near at least one stream and most of us live not far from a major river. All of the water from these streams eventually flows into Long Island Sound. Whether we realize it or not, rivers and streams are affected by what we do in our daily lives. It is important that we understand the dynamics of streams and rivers. Then we can make well-informed decisions about such issues as building on a floodplain, minimizing erosion near bridges and dams, predicting the effects of flooding in an area, and preventing water pollution.
Even though you will look at a small local stream, its processes and behavior are identical to those of a much larger stream or river. Within a small area, you will be able to observe and measure a wide variety of stream features that will help you understand how larger systems work.
To study the structures that form in and around streams and rivers.
To measure a stream bottom profile, velocity, and discharge.
To study how streams change their course.
Before beginning your teamwork assignments, walk the length of the stream in the study area and make a sketch map of the stream. Include not only the shape of the stream, but also where there are flat areas such as sand bars, where the banks are steep, where there are rocks, and any other features of the stream you see. Indicate areas where erosion is occurring and where deposition is occurring.
Your class will be divided into four study teams that have from four to six members each. The study sites are:
1. Stream Shape (width and depth)
2. Velocity and Discharge
3. Texture and loction of sediments
4. Gradient of stream
Once you are assigned to a team, gather the equipment you need and report to your study site. When you finish your assignment, return the equipment to its original site, get the equipment for the next site, and begin work there.
Problem 1. Stream Size and Shape
This site will investigate the width and depth of the stream.
So depending on the water temperature, put on your wading boots, or your aqua-socks, or any footwear you don’t mind getting wet and muddy, and prepare to get acquainted with the stream.
Tape Measure (metric)
A. Determining the width of a straight stretch of stream
A meander is a bend or curve in a stream. Locate two meanders that are relatively close to each other.
Using a metric measuring line, record at least three width measurements along the straight stretch between the meanders. Calculate and record the average of your measurements.
B. Depth of straight stretch of stream
1. Find the depth of the channel at the same stretch of the stream where you found the width, using the following procedure.
2. Position a tape measure across the brook with one student holding each end.
3. Another student will use a meter stick to measure the depth in centimeters at the nearest edge of the stream, 20 cm into the channel, and continue measuring every 20 cm until reaching the opposite shoreline of the channel. (Note: If you will be using inch-square graph paper, measure every 25 cm, to make graphing easier later in the activity.)
4. Graph the channel cross-section on the straight stretch of the stream using a scale that will fill, or nearly fill the top half of your graph paper. (you will graph the cross-section of the channel at a meander on the lower half of the paper later.) In other words, make the cross-section large enough so it is easy to make and read. Does it seem that the channel is symmetrical? That is, does it get deep and then shallow at about the same distances from each edge?
5. Extra credit: Using a meter stick, measure the height of the banks on the sides of the channel along the straight stretch of the stream. Also measure the width between the two banks. During times of increased discharge, water can rise to the level you just measured. That is a maximum volume of water the stream channel can hold. Volumes greater than that flow out of the channel onto the surrounding floodplain. Collect data needed to include the part of the streambed that does not currently contain water on the cross-section you drew. Shade this part of the cross-section to indicate it does not currently contain water.
C. Stream Depth at a Meander: How deep is the stream where it curves?
1. Choose one of the places where the stream bends, and repeat Procedure B to determine if the shape of the streambed changes at meanders.
2. Is the stream at this meander symmetrical or asymmetrical?
3. If the streambed is asymmetrical at the meander, which side (near the inside curve or the outside curve) of the stream is shallower? Which part is deeper?
4. Using the same sheet of graph paper on which you drew the first profile of the stream, graph a cross-section of the meander below the first cross-section.
5. Label the outside and inside bends of your cross-section.
Problem 2: Velocity and Discharge
Here the velocity of the water will be measured in both straight and curved stretches of the stream. Stream discharge (the amount of water flowing past a given point in a given amount of time) can then be calculated.
2 Flags on sticks
Ping-pong or tennis ball
Metric measuring tape
A. Stream Velocity: How fast is the water moving?
1. Since velocity is a function of both distance and time, you will need to measure both of those properties as they relate to at least two stream sites.
2. You are going to measure the time it takes the water to flow a distance of 4 meters.
3. Begin with the straight stretch of the stream.
Measure a four-meter distance along the stream. Place flags on sticks (or other markers) at the beginning and end of the four-meter distance.
Place an object that floats, such as a ping-pong or tennis ball, in the water at the higher end of the four-meter stretch.
Drop the ball above the starting flag and start timing when it reaches the first flag; stop when it reaches the second flag. Record the time in seconds.
Repeat at least two more times, and then average your results.
Calculate the velocity of the stream in meters per second by dividing the average time in seconds into the distance of four meters.
4. Now determine the stream’s velocity around a meander by repeating step three. Measure the velocity near the inside curve and near the outside curve to determine if the stream travels at the same speed in each of theses locations.
a. Examine the differences in the velocities you computed along the straight stretch and both the inside and the outside of the meander.
b. What do you think those differences might mean about the future shape of the stream? Explain.
c. On what part of the stream do you think erosion might take place?
d. How will erosion change the shape of the stream over time?
B. Stream Discharge: How much water goes by every second?
1. The discharge of a stream is the amount of water moving through the channel. Since you have found out the average width, average depth, and average velocity of the stream, you can calculate discharge.
2. Following is an equation that determines the discharge:
Q = wdv
Q is the discharge
w is the average width in meters
d is the average depth in meters
v is the average velocity in meters per second
Using the data you collected along the straight stretch of the brook, calculate the discharge of the stream.
4. CHALLENGE: What unit do you think stream discharge is measured in? If you used units with your numbers as you worked the above problem, this will be easy to determine!
5. EXTRA CREDIT: Calculate the maximum discharge of the stream before it will overflow its banks.
Problem 3: Sediment Texture and Location
This team will take a closer look at the sand and other sediments under their feet. They will consider the size of the sediment, and by putting their data together with that of the Water Team, will try to understand the relationship between the size of sediment and the energy required to pick it up and drop it back down.
Hand lenses (one for each team member)
Plastic graduated cylinders (same number as sieves in the stack)
Funnels (one for each cylinder)
1. Each team member should first take a close look at the sediments that have been deposited along the sides of the brook. Sprinkle a small amount of the sand into a petri dish and examine it with a hand lens. Empty that sample and examine another one that has a different grain size. Now you are a bit more familiar with the material you’re going to be measuring.
2. Take the map of the stream that you sketched at the beginning of these activities and mark the location of each sample as you take it.
3. From a location near the stream use a plastic beaker to scoop up some sand. The sand must be dry to sieve it. If you can't collect dry sand, spread it out in a thin layer in a metal pan in the sun until dry. Record the volume of the sand (cubic centimeters).
4. Assemble the stack of sieves in descending order with the largest mesh size on the top and the smallest mesh size on the bottom.
5. Pour the sand from the beaker into the top sieve. Gently shake the apparatus for several minutes.
6. Take the sieve stack apart and empty the contents of each section into a separate graduated cylinder, using the funnels.
7. Record the amount of each grain size (cubic centimeters).
8. Calculate the percent of the total volume for each grain size.
9. If you have time, repeat this procedure at another location where the sediment size appears different from your first sample.
Problem 4: Gradient: How steep is the stream?
Rivers flow because the surface of Earth’s crust is irregular in shape. Gravity pulls water downward. Rivers that flow along steep paths produce different kinds of characteristics from those that flow along nearly horizontal paths. In this exercise, you will determine how steep a path the river you are studying travels. It is true that the river is flowing over the land, but because the land’s surface can be quite irregular, it is easier, and more accurate, to measure the steepness of the surface of the stream from a starting point to a lower point. The stream’s surface is actually an average of the irregularities of the land’s surface. Steepness, which is also called a stream’s slope or gradient, is determined by dividing a set horizontal distance the stream travels into the vertical distance the water dropped over that set horizontal distance. In this exercise, you will determine how many meters the stream drops over a horizontal distance of 10 meters.
Two 10-meter ropes connected at one end with a ball and a ring
Two meter sticks
Small lightweight level attached to center of one of the two ropes
1. Choose a stretch along the stream that represents a typical steepness as you see it.
2. Choose a starting point, called the benchmark, at the upper end of the stretch. Position one of the meter sticks inside the ring that is attached to the ropes and hold it in a vertical position at the benchmark. The ball will help to keep the rope afloat.
A second team member should hold the 10-meter end of the rope with the level. A third team member should level the rope by giving instructions to the team members at either end of the rope.
How steep is the stream?
Once the upper rope is in place and leveled, a fourth team member can pull the lower rope taut along the surface of the water, and, using a vertically-positioned meter stick to measure the distance between the ropes, determine how many centimeters the stream dropped over the 10-meter horizontal distance.
Now determine the stream’s slope (gradient) by dividing the 10-meter horizontal distance into the stream’s drop in elevation. Record the drop in elevation in centimeters. So the units for slope will be centimeters per meter. That is, on the average, the stream drops a certain number of centimeters every meter.
Can you diagram the shape of the slope you measured?
Using a scale that works on a sheet of graph paper, draw the slope of this stream. If possible, draw the slope without vertical exaggeration. That is, change the centimeter value for the stream’s elevation change to meters, and use meters for both elevation change and horizontal distance on the graph paper.
How would you describe this stream’s gradient?
Final Analysis of the Four Problems
Answer the following questions in complete sentences:
1. Do you see evidence that the stream is sometimes wider than it is now? If so, what is the evidence?
2. Is there evidence that the velocity is sometimes faster? What is it?
3. The chart below shows the relationship between the velocity of water and the size of the sediment it can carry. Looking at the data you have collected, decide whether the velocities you calculated are powerful enough to pick up and move the sediment sizes present. If not, what velocities must be reached to move the largest sediment?
|Velocity (m/sec)||Grain size moved|
|1.2||Fist-sized or larger|
|0.9||Between gravel and fist sized|
4. Look at the sediments you collected in the stream. Are they distributed evenly along the streambed? If not, describe their distribution. From what you know of the stream characteristics, why do you think the sediments are distributed as they are?
5. EXTRA CREDIT: If the stream were bank full, what would be its cross-sectional area?