Some of the water that contributes to the volume of water in the Great Lakes enters from streams, channelized flow of water. Normally the discharge of a stream (the volume of water that passes a given point in a given unit of time, typically given in cubic feet per second or cubic meters per second) increases with increasing precipitation, although some lag time occurs between peak precipitation and peak discharge. Discharge can be calculated by multiplying the cross sectional area of the stream channel by stream velocity. (The latter can be measured using a current meter, or by measuring the time it takes a stick tossed in the stream to flow a certain distance and then calculating distance/time.)
In order to obtain data for flood control and water use, the United States Geological Survey (USGS) calculates the stream discharge for thousands of locations across the United States. Also at these gaging stations, the elevation of the water surface, or stage, is measured relative to sea level. At many locations an arbitrary reference datum is used. For example, if the datum at a gaging station has been set at 3,538.5 ft, a stage of 7 ft indicates the surface water is at 3,545.5 ft above sea level. Flood stage for a stream is that level at which the water overtops the stream banks.
A stream hydrograph is a plot of stream discharge or stage versus time. Hydrographs are particularly useful to study the behavior of a stream during a flood. For example, the time lag between rainfall or snow melt and peak discharge or stage can be easily noted. That helps to evaluate the relative danger of flooding to people and property at that particular location on a stream. Indeed, by examining hydrographs, the response of stream discharge to rainfall can also be seen to vary along the length of a stream. For example, discharge or stage is likely to increase much more rapidly after a rainfall upstream than downstream. As you might suspect, time is needed for floodwaters of numerous small tributaries to reach a downstream, larger river.
This activity uses real-time, discharge data made available on the web by the USGS. As the data constantly changes, this lesson plan can only give general, suggested questions to ask your students. The questions may need to be altered as the data available changes. The example activity below is for real-time data for the Grand River, which empties into Lake Michigan at Grand Haven, Michigan. If you live outside the Grand River basin, using data for a river that has more significance to your students will probably be more meaningful to them.
Go to the USGS web site at http://water.usgs.gov/realtime.html. Go to the map and click on “Michigan”. On the map of Michigan, click on “Southern Lower Peninsula”. You will see a table with "Gage Height" in feet, "Discharge" given in cubic feet per second, and “Long-term, median flow.”
1. Just by looking at the “Long-term, median flow”, without regards to geography, can you predict which locations for the Grand River are near the headwaters of the stream and which are near the mouth? Explain your reasoning.
2. Compare the present “Discharge" to the “Long-term, median flow”. Are present discharge values for the Grand River currently relatively high or relatively low? Why might that be the case?
3. Click on “Grand River at Jackson” and observe the hydrographs shown there. What time period is shown by the hydrograph? What is the range of the discharge shown on the hydrograph? (Be sure to include the units.)
4. Go back to the previous page and click on “Grand River at Grand Rapids”. What time period is shown by that hydrograph? What is the range of the discharge shown on that hydrograph? Are the discharge amounts greater or less than those at Jackson? Explain why that might be the case.
5. Which hydrograph shows more percent variation in discharge from day to day, that for Jackson or Grand Rapids? Explain why that might be the case.
6. Go to the "Grand River at Grand Rapids" page and go to "Daily discharge statistics, in cfs, for 'x' based on 'x' years of record: and click on "more." What is the precise location of the gage for the Grand River at Grand Rapids? What is the datum (reference elevation) for the gage in Grand Rapids? (Be sure to include units.)
7. What is the elevation (relative to sea level) of the surface water for the last day of the record shown on the hydrograph? Show how you got that number.
8. The elevation of the surface of Lake Michigan is about 580 ft above sea level. Using a road map of Michigan and a string estimate the distance from Grand Rapids to the mouth of the Grand River at Grand Haven. _________ miles. Now calculate the mean gradient (vertical drop over horizontal distance) for the Grand River between Grand Rapids and Grand Haven. (You will need to use the elevation you calculated in #7 above.) The units for your answer should be in feet/mile. Show your work.
9. Below is an idealized longitudinal profile (line drawn from the head
of a stream to its mouth) for a typical stream. Based on what you
know about the drainage basin of the Grand River and the geographic location
of Grand Rapids (Grand River Watershed Map),
place an “x” about where Grand Rapids would lie on such a longitudinal
profile for the Grand River. Do you suspect the gradient of the Grand
River at Jackson would be higher or lower than that you calculated for
the Grand River at Grand Rapids in #8 above? Explain your reasoning.
10. Using the available record ("x" years of data), determine the range of flow that has been observed for the Grand River at Grand Rapids ______________ How can such a large range in discharge be possible?
11. What do think the effects of urbanization might be on the discharge of a stream (i.e., would it tend to increase or decrease)? Is that good or bad? Explain your reasoning for both of your answers.
12. Go back to http://water.usgs.gov/realtime.html.
Using the map of the U.S.A. describe the general stream flow conditions
for Michigan (dry to wet). _________________________ Describe
the general flow conditions for the entire Great Lakes basin. ______________________________
Finally, which parts of the U.S.A. currently have relatively high flow
conditions? ___________________________ Low? _______________________
Should people living in the Great Lakes region be concerned about water
shortages in other parts of the country? Explain your reasoning.
http://www.gvsu.edu/mattoxs/flood%20act1.html – lesson plan on flooding in Grand Rapids, Michigan
http://www.gvsu.edu/mattoxs/1904%20flood.html – photographs of the 1904 flood in Grand Rapids, Michigan