Homework Assignment: Downstream Hydrological and Ecological Effects of Dams

Global Water Concerns

  • Overview

In this homework exercise you will examine the downstream effects of large dams on river hydrology. In particular, you will use data from the U.S. Geological Survey to assess dam impacts on annual peak discharge (Qpk), the largest discharge observed in a given year. You will start by analyzing the impact of Glen Canyon Dam on the Colorado River. You will then explore issues related to the ethical use of hydrological data, and how such data can be used to inform important water management decisions.

This file contains the homework assignment instructions. See the second file, Dam Impacts Answer Sheet, which you will complete and upload to Canvas.

  • Background: Glen Canyon Dam

The Glen Canyon Dam is a concrete arch dam on the Colorado River, 16 miles upstream from Lee’s Ferry, Arizona. Built between 1956 and 1966, its construction led to the creation of Lake Powell. At 250 miles2, Lake Powell is the second largest artificial reservoir in the United States (after Lake Mead, located several hundred miles downstream). Lake Powell is the focal point of the popular Glen Canyon National Recreation Area. Initially, the Glen Canyon Dam was built to moderate flow of the Colorado River, reducing both floods and droughts in the region, and to ensure water availability to the Upper Basin of the Colorado River.

However, the Glen Canyon Dam is a contentious issue, pitting environmental, social, and financial concerns against one another. Proposals to decommission the dam and drain the reservoir have gained support from mainstream environmental organizations (e.g., the Sierra Club Board of Directors endorsed the idea in 1996), while stakeholder support to maintain the dam and lake abounds as well. The dam regulates streamflow in the Grand Canyon, a popular whitewater rafting and recreational destination.

Since the mid-1990s, dam managers have occasionally released above-average volumes of water (termed High Flow Experimental Releases, HFE’s) in order to temporarily alter river hydrology and attempt to restore geological and ecological characteristics of the river below the dam. These flows are designed to mimic historic natural floods, though their magnitude is constrained by the capacity of the dam. Without using the spillways, the dam can release a maximum of 45,000 cfs through the hydropower system and jet tubes.

  • Impact of Glen Canyon Dam on Colorado River Peak Flows

The US Geological Survey (USGS) collects streamflow (discharge) data at >25,000 sites in the USA. For the first part of the exercise, you will work with streamflow data for the Colorado River at Lee’s Ferry, Arizona. Lee’s Ferry is located just downstream of the present day Glen Canyon Dam, which was completed in 1966. Glen Canyon Dam is a 710-foot high concrete arch dam. It provides hydropower, and also regulates Colorado River flows to the downstream Lake Mead (the largest reservoir in the USA, impounded by Hoover Dam). Prior to dam construction and river regulation, the natural Colorado River had a snowmelt-dominated annual hydrograph, and high inter-annual flow variability (large differences in annual flow volume and peak discharge from year to year).

  • Download the Colorado River data series from the USGS website

You will first download the streamflow data for the river from the USGS water data website:

This is the web page for the National Water Information System Web Interface. On this page, select ‘Surface Water.’ On the next page, scroll down and select the ‘Peak-Flow Data’ button.  On the next page, click ‘Site Number’ as the Site Identifier you will use to find your data, and then click ‘Submit.’ Next, type in ‘09380000’ and click ‘Submit.’ Once you click on the site number on the next page, you will be taken to the main webpage for the gage. The gauge you are using is: USGS 09380000 COLORADO RIVER AT LEES FERRY, AZ.

  • Once you are at the web site for the gage, click on the menu for ‘Available data for this site’ and choose “Surface water: Peak streamflow”
  • Under “Output Formats” specify that you want to retrieve a “Tab-separated file.” Include the entire period of record. Note that these are water years, running from October 1- September 30.
  • This should produce a page of text containing several rows of information about the data record, and then columns of data.
  • Bring the data into Excel
  • To copy the gage data from your browser window, Right click in the browser window  Select all  Ctrl-C. Alternatively, you can press Ctrl-A (Ctrl and A at the same time) to select everything in the window.
  • Then, open a workbook in Excel, and click on the top left cell of the first worksheet. On the Home tab, click Paste  Paste Special  text. This will past the gage data into the Excel sheet.
  • Make sure to Save your file, someplace where you will be able to find it. To do this, click File  Save As, and then specify a filename and location. You only need to name the file once. After that, as you work just periodically click on the floppy disk icon in the upper left of the Excel window to update your saved file.
  • Next, you can clean up the data by deleting the rows of extraneous information that appear at the top of the spreadsheet. Also – make sure to use only the data for the continuous record. For the Colorado River at Lees Ferry, the first data point should be for 1921, the first year of the continuous record.
  • Note: if your data (i.e., date and discharge values) do not appear in separate columns in the spreadsheet, select the data cells (all of column A) and then go to Data Text to Columns. Follow the prompts to choose the “Fixed width” option. Select break locations so that you end up with 6 columns: (1) USGS 9380000, (2) Dates of peak discharge occurrences, (3) Discharge values, cfs, (4) extra data codes, (5) State, feet, (6) more extra data codes. After you do this, scroll through the entire data series to make sure it looks OK. It may need some editing!

It should look something like this:

  • You can delete columns 4 and 6, and add titles. This will yield a spreadsheet like this:
  • Visualize the impact of Glen Canyon Dam on peak discharge: Graph the data.
  • Produce a graph of the peak discharge values. Customize your chart to provide a title, and axis labels (with units). Make the graph look good!
  • To create the graph, select the cells that you want to graph (press and hold the Ctrl key to select non-adjacent column cells), and click on the “Insert” tab  choose the chart type that you want. Try the Scatter Chart, with lines connecting the points. For this graph, you are selecting the Date column (x-axis values, or independent variable) and the Discharge column (y-axis values, or dependent variable). Remember, select the data first and then insert the graph!
  • Add a title and axis labels. Do this by selecting the chart, and then choosing Design  Add Chart Element (in the green Chart Tools area on the top ribbon). Make sure to include units for both axes.
  • Label the date of dam completion, and/or the pre-dam and post-dam periods. INSERT an arrow and text box to do this.
  • Copy the graph and paste it (as a graphic image, not a linked item) into the answer sheet.
  • Compare the pre-dam and post-dam peak discharge values: Calculate the mean, maximum, and minimum peak discharge for pre-dam (1921-1965) and post-dam (1966-present) periods.
  • Perform these calculations in Excel, not by hand. To calculate the mean in Excel, click on the cell where you want the answer to go, and then type “=average(“. Next, select the cells to average, and then type “)” and press return. The “=” tells Excel to put a formula into the selected cell. Inside the parentheses, select the cells on the spreadsheet that you want to average. For example, to get the mean of the data in Column J Cells 2 through 1778, you would type/see this formula “=AVERAGE(J2:J1778)” in the cell where you want the answer to go. Note: do not type the quotation signs, those are just to show you the text to type.
  • The method for finding the maximum and minimum values is similar to what is described above, except that you type “=Max(cellrange)” or “Min(cellrange)” to find these quantities. Note: do not type the quotation (“) signs, and do not type the word “cellrange” – that is where you select the cells that will go into the formula.
  • You will fill in this information into the table on the answer sheet. Make sure to include units.
  • Compare the pre-dam and post-dam peak discharge values: Calculate the percent change in peak discharge.

What was the percent change in the mean annual peak discharge between the pre-dam and post-dam period? Calculate this as:

Provide your answer on the answer sheet. Make sure to show your work.

  • Evaluate your results. Use your results (graph and calculations above) to evaluate the overall impact of Glen Canyon Dam on peak discharge of the Colorado River. Qualitatively, describe the impact of Glen Canyon dam on peak discharge (i.e., describe what you see in the graph). Quantitatively, describe the impact (i.e., describe the results of your calculations). Provide a thorough response on the separate answer sheet.
  • Implications of your results. Based on what you know about the importance of the natural streamflow regime to river processes, what are some of the likely impacts of Glen Canyon dam on downstream physical and/or ecological conditions (e.g., sediment transport and in-channel habitats, water quality, floodplain functioning, river ecology, etc.)? Discuss at least three potential impacts on the separate answer sheet.
  • Ethical Use of Hydrologic Data

Credible scientific information is the foundation of sound water management. In particular, discussions about dams and river management rely on accurate, credible data and a sound scientific understanding of hydrologic conditions and processes (Langlois 2017). Without scientific data and research, we would lack the information and understanding to make effective decisions about water allocation, conservation, dam operations, etc. In the United States, the federal government and many states (and even some local municipalities) invest in hydrologic data collection and scientific research. This includes streamflow data collection by USGS, and reservoir monitoring by the US Bureau of Reclamation USBOR, among others. In addition to conducting their own research, these agencies provide free, reliable, trustworthy data that can be used by both scientists and the public. However, in order to yield credible information and results, data analysis and scientific research must be unbiased.

  • Data Quality

Imagine that you work for an anti-dam environmental advocacy group, and your job is to assess the impacts of Glen Canyon Dam on Colorado River peak flows. You conduct the analysis, and present your graph and calculations from Part III of this assignment to your boss. Your boss is very complimentary of your work. However, they notice the high peak flow in 1983. That peak discharge value is unusually high for the post-dam period (the 15th largest flood in the whole data series, at 97,3000 cfs). Your boss asks you to investigate that data point. You do some research, and discover that the 1983 flood was caused by an extraordinarily long winter and high late-season snowfall, coupled with springtime rains and fast snowmelt which were not predicted by water managers. As described by Fradkin HYPERLINK “” (1995), Lake Powell reservoir levels had not been drawn down sufficiently to accommodate the extra snowmelt and rain, when warm spring and summer temperatures arrived. Dam managers were forced to allow as much water as possible to pass through the dam turbines and spillways, and the dam came very close to being over-topped. Your boss thanks you for your research, and then asks you to remove the 1983 data point from the discharge record (since it was the result of such unusual circumstances) and re-do your calculations of the dam impacts.

  • Re-calculate the post-dam peak discharge values and percent change: Without the 1983 data point

As you did above, calculate the mean, maximum, and minimum peak discharge for the post-dam (1966-present) period. However, this time delete the 1983 peak discharge value. Note: the easiest way to do this is to duplicate your calculations worksheet, and just delete the 1983 value. You will fill in this information into the table on the answer sheet. Make sure to include units.

Re-calculate the percent change in the mean annual peak discharge between the pre-dam and post-dam period, but this time use the post-dam mean calculated with the 1983 discharge value deleted. The pre-dam value is the same as in Part III above. Provide your answer on the answer sheet. Make sure to show your work.

  • Dam Impacts Report (Anti-Dam)

Your boss likes the new results better than the original ones, since they are more consistent with the anti-dam campaign you are working on. The boss wants the results of the new calculations (with the 1983 data point removed) included in the final report, rather than the original calculations. Do you agree with this choice? Is this an ethical use of data? Why or why not? Provide your answer (a short paragraph) on the answer sheet.

  • Choosing Evidence

Imagine that you work for a pro-dam hydroelectric advocacy group, and your job is to assess the impacts of Glen Canyon Dam on Colorado River peak flows. You conduct the analysis, and present your graph and calculations from Part III of this assignment to your boss. Your boss is very complimentary of your work. However, they are disappointed to see such a large impact of the dam on peak flows, since it is well known that the river flow regime profoundly impacts the functioning of river ecosystems. However, looking at your graph your boss notices the high peak flow in 1983. They are pleased to see that in that year a very large (and likely ecologically-important) flood occurred on the Colorado River downstream of Glen Canyon Dam.

  • Dam Impacts Report (Pro-Dam)

Your boss decides that it is not necessary to present the full data analysis from Part III in your report. Instead, they want you to focus solely on the 1983 flood event. They ask you to use this flood as evidence to support the argument that Glen Canyon Dam does not have a significant impact on Colorado River Peak flows. Do you agree with this choice? Is this an ethical use of data? Why or why not? Provide your answer (a short paragraph) on the answer sheet.

  • Hydrologic Data and River Management
  • Lake Powell Water Balance

The amount of water in Lake Powell at any given time reflects the relative balance (or imbalance) of water inputs and water outputs. Water comes into the lake from three main sources: The Colorado River flows into the lake from upstream, groundwater and soil water may seep into the lake, and a small amount of precipitation falls directly onto the lake. Water exits the lake by three possible routes: outflow downstream into the Colorado River via Glen Canyon Dam or its bypasses, evaporation, and seepage into the underlying bedrock. If water inputs exceed outputs, the amount of water stored in the lake increases. If water outputs exceed water inputs, the lake volume shrinks. Because of the importance of Lake Powell to water supplies in the Colorado River Basin, the US Bureau of Reclamation (USBOR) collects data on lake water levels, inflows, outflows, and evaporation. This data can be used to track water storage over time, so that water managers can plan for dam releases and water allocations.

  • Download a graph of the Lake Powell water storage data from the USBOR website.

Go to the USBOR Water Operations: Historic Data website:

Choose “Lake Powell” under the drop-down menu under Reservoir. Click on the buttons for “Monthly Data”, and “Archived Date (>120 Days)”. For the Output, select “Storage (af)” and for Output Format, select “Graph.” This will open a new window with a graph of monthly dates (x-axis values) and the corresponding lake storage volume in acre-feet (y-axis values). An acre-foot is the volume water that would cover one acre of surface area to a depth of one foot. 1 af = 1,233.5 m3.

Copy and paste the graph into the answer sheet.

  • Evaluate the graph. Describe the temporal pattern of water storage in Lake Powell. Distinguish and describe (including approximate dates) at least 3-6 different general time periods in the data record (e.g., a period of initial lake filling, period of high lake levels, etc.). Provide your answer on the answer sheet.
  • Causes of the decline in water storage. Since the 2000s, water levels in Lake Powell have been considerably lower than in prior decades such as the late 1970s and 1980s, and the late 1990s. Considering the inputs and outputs of water to the lake, what is one possible reason for the decline in lake volume? Describe a specific aspect of the water balance, and how this would result in lower lake levels. Provide your answer on the answer sheet.
  • Colorado River Restoration

Over the years, there have been many calls to remove Glen Canyon Dam in order to restore the Colorado River. Famously, David Brower (the first Executive Director of the Sierra Club) was a strong advocate for the dam’s removal (Langlois 2017). More recently, several non-profit groups have called for the decommissioning of the dam and the draining of Lake Powell to achieve environmental and water supply benefits. For example, the Glen Canyon Institute (GCI) has put forth a proposal called Fill Mead First, which would restore a free-flowing Colorado River through Glen Canyon and the Grand Canyon by moving water from Lake Powell into Lake Mead. According to GCI, this proposal has become increasingly feasible over the past two decades as water levels have declined in both reservoirs. They argue that because Lake Powell loses so much water to evaporation and seepage, it is detrimental to water supplies, in addition to its other negative environmental impacts.

  • Read the High Country News article by Langlois (2017), posted on Canvas and linked here.
  • Based on the recent decline in lake volume, some groups have called for the draining of Lake Powell. It is argued that the benefits of the lake are outweighed by the costs of water lost to evaporation and seepage, as well as the downstream negative impacts to the river ecosystem. Do you agree with this argument? Why or why not? Do the USGS data (peak discharge) and USBOR data (lake volume) that you examined for this assignment support the argument that Lake Powell should be drained and/or Glen Canyon Dam decommissioned? Explain. Provide your answer on the answer sheet.
  • Reference List

Fradkin, P. (1995, October 29). The year the dam (almost) broke. The Los Angeles Times.

Langlois, K. (2017, September 4). Down with the Glen Canyon Dam? High Country News.

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