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Rollin' Down the River

This lesson introduces how streamflow is measured by streamgages and how various factors can change streamflow. Students use USGS data to make their own hydrographs for the Colorado River and for a river or stream of their choice. This lesson is intended for grades 9-12 and is aligned to NGSS standards.


Classroom Lesson: Rollin' Down the River: Streamflow in the Colorado River and in a river near you!

Grades: 9-12

Topics: Streamflow, discharge, streamgages, dams

Length: 4 lessons, 30-45 minutes each


  • To understand what streamflow is, how streamflow is measured, and the factors that affect streamflow.
  • To understand that real-time streamflow data are available and important to a variety of stakeholders.
  • To create hydrographs for the Colorado River and a local river and interpret how streamflow changes over time and how the presence of hydroelectric dams alters streamflow.

NGSS Alignment:

  • HS-ESS2-2. Analyze geoscience data to make the claim that one change to Earth's surface can create feedbacks that cause changes to other Earth systems. 
  • HS-ESS3-3. Create a computational simulation to illustrate the relationships among the management of natural resources, the sustainability of human populations, and biodiversity.
  • HS-ESS3-6. Use a computational representation to illustrate the relationships among Earth systems and how those relationships are being modified due to human activity.

Materials Needed: Internet access, spreadsheet/graphing software, Attachment 1_Rollin' Down the River (.xls dataset file)


  • Lesson 1: Be a Hydrologist for a Day! In this lesson, students investigate streamflow of the Colorado River: how streamflow is measured, what factors affect changes in streamflow, and why streamflow is important.
  • Lesson 2: What is a hydrograph and how do I make one? In this lesson, students make their own hydrograph from USGS data for the Colorado River to understand the form and function of the Colorado River and how streamflow changes are affected by the operation of hydroelectric dams and high-flow experiments. 
  • Lesson 3: Get to know a streamgage near you! In this lesson, students graph streamflow (make a hydrograph) from a USGS streamgage station in a location of their choice to investigate how streamflow changes over time.
  • Lesson 4: Exploring hydroelectric dams in your local watershed. In this lesson, students explore the presence or absence of hydroelectric dams on a nearby river and determine what affects the dam may have on streamflow.

Background Information: 

What is streamflow and why is it important?

The amount of water flowing in a river is called streamflow. When hydrologists (scientists who study water) talk about streamflow in a river they are not only interested in the amount of water in the river, but also the timing of the water in the river. Some streams may have more water or higher streamflow in the spring when snow is melting in the watershed. Some rivers have low flows in the late summer when the weather is hot and there is less rainfall. While rivers across the country vary in the amount and timing of streamflow, most rivers have some type of a normal cycle such as high streamflow during one season, low streamflow during another season, or stable flows throughout the year. This normal cycle is what defines the form and function of a river. 

The normal cycle of streamflow of a river can change over time due to changes in climate leading to more or less water running off into the river. Sometimes the streamflow of a river changes due to human impacts, such as removing water for agricultural or industrial/household uses. The amount and timing of streamflow is also changed when humans build dams or other structures on rivers.

These changes in streamflow change the form and function of the river. The form of the river can be described by telling someone how deep and how wide the river is. Form can also be described by what material makes up the bottom or bed of the river, such as sand, gravel, or bedrock. The type of sediment along the banks and the bed of the river (mud, sand, gravel, rocks), the gradient, and the types of plants also help to describe the form of the river. Even how straight or curved the river is (called sinuosity) helps scientist to understand the form of a river. Changes in streamflow lead to changes in the form of the river over time and these changes can affect how the river functions. 

The function of a river is how plants, animals, and humans use a river. There are many geographic examples of how changes in streamflow change the form and function of a river. One example is the Platte River which is a wide, shallow, sandy-bed river. Many species of animals rely on the large bare sand bars for nesting and raising young, such as the endangered interior least tern, whooping cranes, pallid sturgeon, and piping plover. However, irrigation and human-made reservoirs have reduced the streamflow in the river. Without large floods to build more sand bars and clean the vegetation off existing sand bars, the stable sand bars have developed into islands with established trees. These new islands force the flow of the river into narrow, deeper channels instead of the very wide, shallow channels that were previously typical of the Platte River. 

Hydrologists monitor streamflow at streamgaging stations (streamgages). Learn how hydrologists measures stream flow here and learn how streamgages work here. Streamflow is typically measured in the unit cubic feet per second (cfs). This means that if the stream has a measured streamflow (discharge) of 200 cfs, that is equal to the amount of water that it takes to fill 200 1 ft x 1 ft x 1 ft cubes flowing through the cross section of the river every second. This is approximately 1,500 gallons (can you picture 1,500 gallons of milk?) or 19 bathtubs full of water traveling past the gage each second.

The United States Geological Survey (USGS) is the federal science bureau within the U.S. Department of the Interior. The USGS plays an important role in the monitoring and understanding of the streamflow of our nation’s rivers by operating over 8500 streamgages. These streamgages monitor streamflow and/or gage height in real time, meaning that the data are automatically collected by instruments placed in the streams and transmitted by satellite to be available online every 15-60 minutes. USGS scientists ensure the data are accurate by regularly checking the data and instruments. In addition, all USGS scientist use the same methods and quality-checked instruments to collect the data across the entire country and all data follow the same format for consistency.

The Colorado River and the Glen Canyon Dam

The Colorado River flows from the Southern Rocky Mountains in North Central Colorado to the southwest through Colorado, Utah, Arizona, Nevada, California, and Mexico to the Gulf of California. This river carved (and continues to carve) the Grand Canyon, one of the largest and most unique canyons on Earth and was aided by the tectonic uplift of the Colorado Plateau. The Grand Canyon is 277 miles long, up to 18 miles wide, and in some areas over one mile deep. The Grand Canyon is geologically significant because it is a good record of the early geologic history of the North American continent. Today very little water within the Colorado River makes it to the Gulf of California, because the water is diverted for other uses.

Colorado River Basin map

The USGS operates numerous streamflow gaging stations on the Colorado River and its major tributaries, including through Grand Canyon National Park. At these streamgage stations, the USGS measures stage (the elevation of the river), streamflow (discharge), water temperature, salinity, turbidity, dissolved oxygen, and suspended sediment. Data are collected every 15 minutes and transmitted via satellite to USGS office for online access. These sites are also visited routinely by hydrologic technicians who take direct measurements of these water quality parameters and who often collect additional data that cannot be collected automatically. Through this network of streamgages and hydrologic technicians, managers, park rangers, and the public can quickly access real-time data. 

Several hydroelectric dams were constructed on the Colorado River in the 20th century. In 1963, the construction of the Glen Canyon Dam was completed on the Colorado River near the town of Page, Arizona, to provide power and a stable water supply. Upon completion of the Glen Canyon Dam, the diversion tunnels around the dam were closed and water began backing up to form Lake Powell. The Colorado River that had flowed south freely through Glen Canyon, and Grand Canyon downstream, for millions of years was dramatically changed by the large, concrete physical barrier that stopped the natural flow of water and the sediments carried within the water.

Before and after photos of the Glen Canyon Dam site, 1889 and 1992
Side-by-side repeat photographic images of the Glen Canyon Dam site on the Colorado River. The first is a historic image taken on December 23, 1889, at River Mile Mile -14.7. The view is taken upstream from river left in 1889 by Franklin A. Nims during the Stanton Expedition. Original image courtesy of The National Archives, #57-RS-235, public domain. The second image was taken by the USGS on October 29, 1992, by Robert Webb. Stake (camera point) s2638 in the SBSC’s Southwest Repeat Photography Collection.

When the people of the West need more electricity, water is released from the dam, which turns hydroelectric turbines and generates electricity. When demand decreases, less water is released. The amount of water released from the dam fluctuates as the powerplant responds to power system load changes, peak load demands, regulation of the power system, and power system emergencies. This means that the flow of the river below the dam can change significantly throughout the day and throughout the year. 

The river that runs in the same riverbed downstream of the dam bears little resemblance to its predecessor. The streamflow of the pre-dam river varied tremendously over an annual cycle, from late spring floods that frequently topped 100,000 cfs to late summer flows of only a few thousand cfs. Today the Colorado River streamflow below Glen Canyon Dam typically ranges from 8,000 to 25,000 cfs with greatly reduced seasonal variation.

Aspects other than streamflow of the river also changed after the construction of the dam. The pre-dam Colorado River carried a heavy load of sediment within the water and along the bottom of the channel (bedload), which was why it was named "Colorado,” Spanish for red. Instead of moving freely downstream, sediments are now trapped in Lake Powell and the river water flowing from the dam is clear.  Water temperature that once ranged from near freezing to 80° F (27° C) now runs year-round within a few degrees of 46° F (8° C), because water for power generation is drawn from the deep, cooler water of Lake Powell, not from the sunlit, warmer surface. 

Glen Canyon Dam has had many effects on the Colorado River through the Grand Canyon, downstream of Glen Canyon. The very large size of Lake Powell allows storage of the spring snowmelt, preventing natural floods from reaching the area below the dam. The reservoir also physically traps the incoming fine sediment from the Colorado River watershed behind the dam. Other downstream effects due to changes in streamflow and sediment include:  

  • Many normally barren sandbars (sandbars with no plants growing on them) have become vegetated because floods no longer wash away plants that take root. This creates a different habitat that is beneficial to some riparian (river-edge) species and challenging to other riparian species.
  • As downstream areas naturally erode, they are not naturally replenished with new sediment from upstream. Beaches dependent on annual replenishment during spring floods are eroded away. Rocks and boulders washed into the main river channel from side canyons are not redistributed during high water, making some rapids more hazardous.
  • Archaeological sites have been drowned, exposed, and/or damaged as the annual natural replenishment of beach sand is now limited to tributary inputs, a small fraction of pre-dam levels. The potential loss of historical artifacts continues to be of great concern to park managers.
  • Water released by the dam is drawn from the deep, cooler layers of the reservoir instead of the river surface, so areas immediately below the dam experience consistently cool water temperatures. Introduced sportfish like trout are thriving in those areas, while many native fishes that have evolved to prefer cycles of cool and warm water have been lost. Other non-native species such as bass, carp, and catfish also thrive in the post-dam river. However, there are still areas further downstream in Grand Canyon National Park where water temperature is warmer and variable that support healthy populations of native fish.
  • With changes in streamflow and sediment the number and quality of spawning grounds for native fish has declined.
  • Variation in daily streamflow caused by the operation of the powerplant can impact species that use the shallow water areas of the channel including aquatic insects and juvenile fish. Aquatic insects and juvenile fish may become trapped in an area that is left without water during parts of the day or may use shallow areas to lay their eggs which become dry in a few hours, decreasing egg survival rates. 
  • The variation in daily streamflow caused by the operation of the powerplant also affects humans using the river. For instance, if you were on a rafting trip and tied your raft up at the edge of water in the evening your raft may be high and dry the next morning. Or perhaps people wading onto a mid-river gravel bar in the morning to fish may find themselves precariously stranded a few hours later.

The water is clearer and cooler, the temperature is less variable, and streamflow changes throughout the day. All rivers change over time and the Colorado River is no exception, but construction of Glen Canyon Dam has been responsible for many of the changes. The USGS and U.S. Bureau of Reclamation work together to implement and study periodic high-flow experiments (HFEs) to release large amounts of water from the Glen Canyon Dam at specific times of the year to understand how varying amounts of water affect streamflow, sediment transport, and the riparian (river-edge) ecosystem.



Lesson 1: Be a hydrologist for a day

Teacher Recommendations: Explore the rotating carousel links at the top of this page for background reading about streamflow.

Phenomenon: Not all streams or rivers flow the same. Depending on the size, the distance from the source of the water, the topography, and weather and climate, there is a large amount of variability in streamflow across the United States.

Inquiry/Guiding Questions:

  • Why do you think it is important to understanding streamflow?
  • How is streamflow measured?
  • How do streamgages work?


  • Carefully read the background information above to learn about streamflow in the Colorado River.

Activity Questions:

  1. In your own words, summarize how streamflow is measured and how streamgages work.
  2. Why is understanding streamflow important?
  3. How does streamflow impact people and ecosystems?
  4. Explain at least three ways that the Glen Canyon Dam has changed the form and function of the Colorado River since its completion in the 1960s.
  5. What are HFEs and why are they important?

Lesson 2: What is a hydrograph and how do I make one?

Teacher Recommendations: Complete this exercise before assigning it to your students. Explore additional datasets and other hydrography information here. Streamflow varies due to a variety of factors and scientists create a specific type of graph, known as a hydrograph, to record properties of water over time. Streamflow data are publicly available and can be used to generate hydrographs. Learning how to read hydrographs can be important for understanding streamflow related to boating, fishing, agriculture, and other activities.

Procedure: A hydrograph is a graph that shows a property of water (in this case, discharge) with respect to time. In this lesson, you will use USGS streamgage data to create your own hydrograph. The data provided contain daily discharge measurements at midnight for water years (October 1 through September 30 of the next year) 1951-52 and 2012-13 for the USGS streamgage on the Colorado River at Lees Ferry, just downstream from Glen Canyon Dam. This streamgage is located just upstream from the Grand Canyon and has been operated continuously by the USGS since May 8, 1921.

Using the data table in Attachment 1_Rollin' Down the River, create a scatter plot (PLOT ONE) with straight lines and markers that represents discharge (cubic feet/second, cfs, or ft3/sec) for water years 1951-52 (before Glen Canyon Dam was built). The X-axis should show dates with a range from September 15, 1951 through October 15, 1952, and the Y-axis should show discharge (ft3/sec) with a range from 0 to 140,000 cfs.

1. Describe the flow pattern in 1951-52. When did the peaks occur?

2. What do you think may have caused these large peaks? 

Using the data table in Attachment 1_Rollin' Down the River, create a scatter plot (PLOT TWO) with straight lines and markers that represents discharge (cubic feet/second, cfs, or ft3/sec) for water years 2012-13 (long after Glen Canyon Dam was built). The X-axis should provide dates from September 15, 2012 through October 15, 2013, and the Y-axis should show discharge (ft3/sec) with the same range as the first plot (0 to 140,000 cfs).  

3. What happened to the streamflow between Nov. 19-23, 2012?

4. Why do you think this may have happened? 

Look up recent streamgage data at USGS station 09380000 Colorado River at Lees Ferry, AZ at:

5.For each plot discuss daily and seasonal flow changes.   

6.Do you see any variability in daily streamflow that may have been caused by the operation of the dam?

Next, go to and scroll down to the discharge and gage height graphs. 

7.Do discharge and gage height change daily? Why or why not?

8.Are there any other dams other than the Glen Canyon Dam on the Colorado River? If so, how many?


Lesson 3: Get to know a streamgage near you!

Now that you understand a bit about streamflow, you will investigate a river or stream near you. 

  1. Go to
  2. On the left side of the screen, enter a place name in the ‘Search by Place Name’ box.
  3. Enter your city (Example Lincoln, NE) and click the green arrow.
  4. Use your mouse to scroll in and out or to move around to view the streamgage station marks, which are gray with black triangles.
  5. Select a marker that is close to your home or school or on a river you are interested in by left clicking on it.
  6. This will show the Site Information with the Site Number, Name, and Type with a link to Access Data.
  7. Select Access Data.
  8. This will take you to the page for your streamgage.
  9. Select Daily Data to access a hydrograph of streamflow for your streamgage.
  10. Next, find the boxes that read ‘begin’ and ‘end’ dates. Enter the begin and end dates to include one full year (example 2021-1-1 begin date and 2021-12-31 end date).
  11. Click Go and scroll down the page. 
  12. You will now see a hydrograph that shows the full year of streamflow (discharge in cubic feet per second). Use Ctrl+ to zoom in or select the “Create presentation-quality graph” from below the graph displayed.

Now that you have a hydrograph from a stream near you, answer the questions below.

  1. During what time of year do you see peaks in discharge at the stream you selected and what do you think may be causing these peaks (snowmelt, heavy rains, dam releases, etc.)?
  2. Is there any indication that your stream has ice cover in the winter? Explain your answer.
  3. What is the peak discharge for the year, in cfs? 
  4. Has the stream been above flood stage during the year? Hint: The header information for the site typically lists National Weather Service Flood Stage so you can compare this value to the gage height/feet graph.
  5. Select a begin date and end date for a different year. Is the hydrograph similar? Why or why not? 
  6. Complete the exercise again for another nearby streamgage. Do you see similarities in the streamflow hydrographs? Why or why not?


Lesson 4: Exploring hydroelectric dams in your local watershed

In Lesson 2, you learned how the construction of the Glen Canyon Dam influenced streamflow. In this lesson, you will discover whether or not hydroelectric dam influences the streamflow of a river in your watershed.

  1. Go to
  2. Zoom to your city and/or your nearest streamgage station.  
  3. Select the Layer List. Most maps are made up of several layers. Each layer contains different data to display on the map. You may have a layer of streamgage stations or a layer of dam sites or a layer of roads. The layer icon is in the upper left corner and looks like a stack of three white squares.
  4. To turn on or off the visible layers that you will need to find dams in your selected watershed: 
  • From the Layer List select the drop-down arrow by the Geographic Names (GNIS) layer.
  • Select the drop-down arrow by the Cultural Points.
  • Click in the square to turn on the Dams.
  • Scroll back up and click on the square to turn on the Geographic Names (GNIS) layer.
  • Click in the square next to the Political Points to turn it off (this will help reduce clutter).
  • Scroll down and click on the square next to the Watershed Boundary Dataset to turn it on so you can see if nearby dams are in the same watershed as your selected streamgage station. 
  • Now you should be able to zoom into the location of your streamgage station. Hint: The zoom level is shown above the scale bar in the lower left corner of the map. To see streamgage stations you must be at a zoom level of 14 or higher.
  • Once you find the streamgage station, represented by a black and white shaded circle, you can zoom out to see the watershed (which is represented by purple lines or bright blue lines if selected) and any dams (green circles). Hint: You must be at a zoom level of 11 or higher for the dam layer to plot. If you do not see any green dots representing dams near your site or within the purple watershed boundary than you can assume there are no major dams affecting your hydrograph.  
  1. Is there a dam on the river you selected?
  2. What changes to streamflow should you expect to see in the hydrographs for this river and why?