Lab8_Rivers_LLL

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Feb 20, 2024

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Lab 8—River Behavior Variation 11 Questions Possible bonus points Goals and motivation for the lab Rivers are so important to humans and to the earth as a system, and so richly diverse in their character and behavior, and we know so much about them, that it was difficult to choose what to focus on in this lab, given all the possibilities. One important aspect is that rivers are like the blood of the terrestrial world, an arterial network that transports water, sediment, salts and other compounds dissolved in the water, and sometimes organisms. It is this flow that we are going to focus on for this particular lab, and specifically we are going to focus on characterizing the variation in flow of water and sediment. USGS Image of the Mississippi River drainage network. This is the land area from which water flows into the Mississippi River and eventually to the Gulf of Mexico, transporting the sediment that feeds the Mississippi delta. It also transports human contaminants introduced into the water along the river all the way to the Gulf, influencing the water quality and biologic activity there. Many of the contaminants serve as nutrients for the growth of marine microbes. The Gulf of Mexico and the Mississippi River are linked. Source: http://toxics.usgs.gov/hypoxia/mississippi/index.html Why focus on the variation in flow? For one thing, this is where natural hazard risk lies. Consider the following. The first piece of information often provided about something is its ‘average’ value. Humans, of course, have accommodated their behavior to fit ‘typical’ and/or average river behavior. Yet, in understanding natural systems it is the range of behavior , especially the extremes, that is of fundamental importance. After all, the poor soul who has his head in a lit oven and his feet in a bucket of ice is on average at a comfortable temperature. An average is not only an incomplete description of a variable phenomenon; sometimes, when presented in isolation, the average can be misleading. It is the extremes , often known as outliers, the very high flows, and the very low flows that are of concern in natural hazard risk assessment. For rivers the focus is on floods and droughts. How often do these types of events occur, and what specifically happens during them? What is the biggest or longest event possible? Such considerations are especially important to efforts at living sustainably. Sustainability by its fundamental nature takes a longer-term perspective, and a long perspective forces consideration of the outlier events, the less common but larger floods that one is likely to see in a longer time period. History, and the inappropriate human development of some river flood plains and of coastlines, tells us that this is a particularly hard perspective and lesson for people to take and learn. 1

From an ESS perspective there are related questions about the different size events, such as – what moves most of the sediment: the larger flood events, or the typical and more frequent, day- to-day type flows? It turns out that outlier events, the larger and rarer catastrophic events, can determine a lot of how a system operates in the long run. This theme will be reinforced and explored even further in the lab on coastal systems where we consider hurricanes. Satellite (LandSat) image of 2011 flood on the Missouri River (water in blue on this false color image). This flood was striking because it lasted so long and was driven by the convergence of a large snow pack in the Rocky Mountains and intense spring rains in the mountainous headwaters. The flooding was unusual in terms of the length of time it lasted (months instead of weeks or less). Image courtesy of NASA. For this lab you will choose an appropriate river in the central U.S. (Great Plains region) and learn about and report on the variation of water and sediment flow and possible reasons for that variation. The United States Geologic Survey (USGS) , a federal agency, maintains a country wide stream gauging network that feeds data from the gauging stations on the streams to an associated, real-time, on-line database that keeps track of rivers, and from which we will learn to find and extract information/data. Therefore, you will need to select a river that has gauging stations on it. Luckily, the network is extensive so you will have abundant possibilities from which to choose. We chose to focus on the Great Plains because UNO is located there, the rivers are very important to the local economy, and the rivers in this region show some interesting behavior. If your instructor permits, there is no reason you cannot choose a river elsewhere in the U.S. This lab has an open inquiry component to it in that you will choose the river you will focus on, and you will determine the exact time period of data you will look at. Results will differ among students. Experience suggests to us that some students may be uncomfortable with such open instructions at first, a response that may be amplified in an on-line setting. Yet, think about it – most real-life situations deal with open inquiry. Such open inquiry is arguably a truer form of exploration. Remember, it is OK to feel a bit lost at first, and your instructor can help when/if needed. Even after decades of practice under their belts, scientists often still feel lost when they first tackle a new project. Indeed, one goal of this lab in particular and the course in general is to make you more comfortable with open-ended explorations, which is an essential part of science. 2

Conceptual framework for thinking about river and sediment flow (3 pts) It may be helpful to review the tools that were addressed in your second lab that can be used to understand complex systems, such as rivers, and construct associated system diagrams for such systems. With such a perspective you can ask: what are the reservoirs, transfer processes, and variables that determine the flow of water at a point on a river? The amount of water flow is called the discharge , which has units of volume per unit time , and can be thought of as the volume of water that moves past a given point on a river over a specified amount of time. You can also describe the amount of sediment that the river is carrying in the same way—as sediment discharge . The units of sediment discharge are often mass per unit time (e.g. tons per day) instead of volume per unit time. Discharge naturally changes along the length of a river, usually (but not always) increasing downstream. This is because of new inflows or outflows that occur along the length of the river, the most obvious (but not only) of which is another river joining the river under consideration (e.g. the Platte River joining the Missouri River changes the water and sediment discharge downstream of their confluence). Basically, in this section of the lab we are looking in greater detail at part of the water cycle, and specifically the part dealing with surface water. However, it turns out that surface water is strongly linked to ground water, especially for rivers that flow over their own floodplains. What are the various reservoirs the water comes from? One can be snow pack or glacier ice up in the mountains that source the river and that melt in spring and summer. Another can be atmospheric moisture, otherwise known as rain. Yet another is groundwater (think of springs). For a lot of rivers in the U.S., another important reservoir is a dammed lake, where humans control the outflow for downstream river discharge. What are the various reservoirs the water can move to (i.e. what are some outflows)? The ocean is one obvious possibility. Inland lakes are another (think of the Great Salt Lake in Utah). Rivers can also leak water into the surrounding sediment and or bedrock, and so groundwater is another. Rivers can also lose water by evaporation, and by transpiration (where plants take water from the soil and then evaporate it) to the atmosphere. Significant amounts of river water are also diverted by humans for irrigation purposes. Some transfer processes include precipitation (snow and rain as two different possibilities), evaporation, melting, human control of dam releases, gravity induced flow, seepage into ground, spring flow, and irrigation withdrawals. Associated variables that influence the rate of transfer include: temperature, humidity, porosity (the percentage of space in sediment and rocks that water can move into) and permeability (the ability of water to move through sediment or rocks), ground surface slope, plant type. At this point you should open the companion PowerPoint file for this lab. This is a similar setup to the second lab, where you will use the components to assemble a system diagram, in this case, for the more focused reservoir—the water in a river upstream of a given point (e.g. upstream of a USGS gauging station). The resulting diagram will provide you with a conceptual model with which to think about the variation in water flow of a river. 3

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Question 1: Insert a copy of your PowerPoint water flow system diagram below. Bonus exercise (optional, 3 pts) : Create a similar system diagram for the sediment discharge for a river, considering the movement of sediment through the system instead of water. Be as thorough as with the water flow diagram. Finding your way around the USGS National Water Information System (NWIS) For this exercise, you must first choose the river section you will explore. Picking a river that you have some connection with (you live near it or paddled on it) can make this lab more relevant to you. To choose your river section, go to the web site: http://waterdata.usgs.gov/nwis/rt . A map of the United States will appear with multiple “dots” of different colors. An example of such a map for the state of Nebraska is shown in the screen shot below. Screen shot of USGS map of gauging stations for Nebraska. The dots represent some of the monitoring stations that measure streamflow conditions in the United States and Puerto Rico. From here you can click on a state that you would like to retrieve water data from. A new page will open with a map of the state chosen to the left. Data is available for any “dot” (gauging station) that you can click on, but there is at least 30 years of data for the colored dots, and so you should choose one of these . Mousing over a dot brings up a small window that provides some information for that station. Clicking on one of the dots brings up a new page where you can select options for the type of data you want and the form in which it is presented. Up near the top of the page is a tab “Data Inventory” with a drop-down menu. The two data sets you will need to have access to in order to complete the exercise below are “Monthly Statistical Data” and “Peak Streamflow.” If your station doesn’t have data for both of these data sets, choose a different site. Once you have selected one of these options, a new window opens in which you check 4

which data you want from a list of what is available (discharge data in your case). If there are specific years that you are interested in you can limit the search; if left blank the entire time period available is returned. You also specify the form you want the data in. Most of the time, the “Table” option is the best, but if that doesn’t work when you Copy and Paste into your Excel sheet, you can try the “Tab-separated” option. When you are ready, click on “Submit,” after which a new window with the data you need should open up. You can Copy and Paste out of this. Screen shot of USGS window for one of the Nebraska river gauging stations showing the drop-down menu you can use to obtain the data you will need for this exercise. 5

Screen shot showing the window that opens when the monthly statistics option is chosen in the previous window. As you explore this website, you will notice that there are many options in this very extensive site, and you can acquire the same data through other paths. If you are having trouble getting to your data please contact the instructor . A tutorial that will teach you about the NWIS site can also be found at https://help.waterdata.usgs.gov/tutorials . Remember, that the web is a very dynamic place and updates and changes are constant, and different browsers also work differently, so it may be that the website doesn’t quite work as described above. However, the website contains instructions so that you can also learn to navigate it that way. If we need to update the above instructions please let us know. Before you actually extract and analyze data from the USGS site we first want you to learn more about the section of river you have chosen using Google Earth . This will help you understand the plots you create using the data. Learning about your river via Google Earth (3 pts) Open up Google Earth and navigate along your chosen section of river, looking for 4 elements/features along the river that could influence the water discharge or sediment discharge (a dam would be one obvious possibility). Copy an image (Edit  copy image) of each element/feature and insert it in below. Remember to turn on the scale indicator in Google Earth so that it will be included in your image. For each element/feature, describe in a couple of sentences how it influences sediment or water discharge (using the possibilities described above, or others you think of on your own). You should also feel 6

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free to go back and modify your system diagram if viewing your study river in Google Earth triggers some thoughts of additional possibilities. Question 2: Element/feature 1 (insert image and description here) Question 3: Element/feature 2 (insert image and description here) Question 4: Element/feature 3 (insert image and description here) Describing the variation in flow with USGS NWIS site (5 pts) For this section open up the companion Excel sheet, migrate to the sheet entitled “10yrMonthlyHistogram” and familiarize yourself with the contents. Read the instructions carefully. You will extract three different 10-year periods of average monthly river discharge data for the river of your choice from the USGS website. Perhaps make one of those time spans 1930 to 1939, the decade of the infamous Depression Era. If a dam has been built on the river, you might want to compare a time period before and after the dam was built if the data are available, as you should see an interesting before versus after difference. Navigate to the first 10 years’ worth of data for your river section and copy the data from the USGS site and paste it into the companion Excel sheet (10 years times 12 months equals 120 numbers you will be cutting and pasting). Each number represents the average discharge for that particular month. The spreadsheet will then automatically find the maximum and minimum values in the dataset and plot a histogram with 10 bins (10 intervals for which it counts the frequency of data points in that interval) – see screenshot below. Ultimately, you will copy and paste the results from the Excel sheet into the questions listed below. Read on for more instructions. 7

Screen shot of Excel sheet you will use with the default data you will replace with your own data in the green cells, and the histogram chart output in the lower right corner. The blue cells are where the calculations are made by the sheet that allow the histogram to be plotted. You will note in the Excel sheet that two types of histograms can be constructed with the two different sheets labeled “10yearmonthlyhistogram” and “10yearlogmonthlyhistogram.” In the first version a histogram is constructed from the discharge values , and in the second the histogram is constructed from the log of the discharge values . The log value can be thought of as simply counting differently. Instead of adding a unit value to get the next value, you multiply. The logs we will use here are base 10, so that at each step you multiply by 10. In this way, the log of 10 is 1, the log of 100 is 2 and the log of 1000 is 3 and so on. It is often helpful to look at the log of values in analysis as they are easier to interpret, and you are free to use either histogram, but be consistent—use the same one for all questions in this section. Histograms are bar charts that show the relative frequency of different size classes in a population (see examples below). They answer the question how often small-versus-medium- versus-large events occur. Analyzing them is an extremely common and often first step in data analysis. You have probably encountered them before, but if not (which is quite OK) or if you want a refresher, the following Khan Academy site can help : https://www.khanacademy.org/math/arithmetic/applying-math-reasoning-topic/reading_data/v/histograms . 8

This plot is for the same data as above, but first the log of the discharges was taken, and then a histogram made of those log values. The results show more pattern or “structure” than the typical plot. This is because in the above plot so many of the values are crammed into the one histogram bar to very left, whereas here they are spread out more. It’s the same data, just sorted into different categories. For the questions below, note the name and number of the gauging station you extracted the data from. Then for each 10-year period you choose to analyze, provide the following: a copy of the relevant histogram(s) you generated from Excel, and report the range of average monthly discharges (minimum to maximum values). Name and number of gauging station: ____________________ Question 5: Histogram for decade _________ (e.g. 1930) to ___________ (e.g. 1939) (copy and insert histogram): Range of average monthly discharge values, Minimum: ________ Maximum: _______ Question 6: Histogram for decade _________ to ___________ (copy and insert histogram): Range of average monthly discharge values, Minimum: ________ Maximum: _______ Question 7: Histogram for decade _________ to ___________ (copy and insert histogram): 9

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Range of average monthly discharge values, Minimum: ________ Maximum: _______ Question 8: In a one-paragraph discussion, compare the results for your three different decades in terms of the ranges and the appearance of the histograms. Speculate on what might account for the variation among the different decades and be as specific as possible. Calculation of the 100-year flood (4 pts) If you want to know how much sediment a river can move over the long term, the larger discharges are quite important, and you might want to know how big those rarer floods are. If you were an engineer building a bridge, you may also want to know the size of the largest flood that could possibly occur during the expected life of the bridge, so the bridge design would allow the floodwaters to pass safely underneath. If you were an insurance agent selling flood insurance you may also want to have similar information in order to set higher premiums for those living on the 100-year floodplain. Plenty of people want this type of information, and you may have heard the term ‘100-year flood’ before. The USGS water data is used for all these purposes, and is a critical data resource with very significant economic value. Without it, and the associated zoning and planning, loss of life and property during floods would have been much greater than it has been. In this particular instance we can think of the 100 years as the recurrence interval of interest. A certain size flood (discharge event) is associated with a certain recurrence interval. One can also calculate recurrence intervals for earthquakes of a given size, or hurricanes of a given size, or many other phenomena. The skills you are learning here can be applied in many other situations. One might be tempted to think that this is cyclic behavior, such that, if a flood of a certain size just happened it would be much less likely to recur in 3 or 4 years than after 90 or 100 years have passed, but that is not the case. Technically, the probability of a flood of that size occurring in any given year would be the same no matter how long it has been since the last one. Another way of thinking about it is that the chance of having a 100-year flood in any particular year is 1%, and the chance of having a 50-year flood in any particular year is 2%, and the chance of having a 1000-year flood in any particular year is 0.1%. Arguably, the terms “100- year flood” and “recurrence interval” are quite unfortunate choices, as so many people naturally think of them in terms of more cyclic behavior, which, as stated, is incorrect due to the probabilistic nature of the events. This is one of many cases where words and phrases take on a specific and special meaning in science that needs to be learned. A more apt term might be “event size annual probability” but that is a mouthful, and the most important aspect is to understand the concept. The underlying question that permits the estimation of a recurrence interval is whether there is some consistent relationship between the size of a given flood and how often it occurs? Common sense and experience tells us there should be. Smaller events often occur more often 10

that larger events—smaller floods occur more commonly than do larger floods. But, can we be more precise, can we quantify the relationship to some degree? The answer in many cases is, yes. Below is one example, a plot generated in a copy of the Excel sheet that you will use to make a similar plot for your river. For this example, data from the Republican River in Kansas was inserted. As you can see there is an historic relationship between the recurrence interval (the log of which is plotted on the x axis) and the discharge (on the y axis) associated with that recurrence interval. Although this includes only 30 years of historic data, if the river continues to behave in the same way over a longer time as it did for those 30 years, then it could be reasonable to extrapolate the relationship between discharge and recurrence interval obtained from the historic data to estimate how large the 100-year flood would be. This underlying assumption that past behavior will be useful in predicting future behavior, is always worth scrutinizing. This approach, of finding a pattern in the behavior of past events that can be described mathematically, and using it to predict and extrapolate to the future is a very powerful approach. Plot of a 30-year long record at a gauging station on the Republican River in eastern Kansas. Data taken from USGS site. Since the x-axis is the log of the recurrence interval, a 1 on the x-axis is equivalent to a recurrence interval of 10 years and a 2 would be equivalent to 100 years. The equation shown in the box was calculated with an Excel sheet function and is the best-fit line, where, on average, the y values are as close to the line as possible. Note that the actual data (the blue diamonds) in the plot do not fall exactly on the mathematical line used to quantify the relationship. The R 2 value of 0.960 describes how good the fit is. In this particular case it indicates that 96% of the variation in river discharge can be explained by observed recurrence interval. However, the lack of a perfect fit does mean there is an error associated with the estimate of the 100-year flood. The amount of error can be calculated, but doing so will be considered beyond the scope of this lab (although, it is standard practice in this type of analysis). One of the options when selecting data from the USGS NWIS site is “Peak Streamflow” (in the same drop-down window you chose “Monthly Statistics” from—see above screen shot). Find 11

and select this option and acquire the data for the gauging station along your river. Under Output Formats , select Table so you can copy and paste into Excel the Streamflow (cfs) values. “Peak Streamflow” provides data on the largest flow for a given year, and is conventionally used in this type of recurrence interval analysis. If you don’t have 30 years of data for your chosen river gauging station (you should if you choose a colored dot on the original USGS state map) find another station that does, preferably one along the river you are focusing on. Navigate to the sheet “100YrFloodEstimate” in your companion Excel file (see screenshot below for example of what it should look like) and read the instructions carefully. The result should be a plot similar to the one above. Screenshot of part of the Excel sheet you should use (instructions in a yellow text box also exist, but are not included in this view). The cells in green are where you insert your data for your river, replacing the default values. The graph will automatically update. Note from the website which river and gauging station you are using, insert your Excel plot, and indicate the size of your 100-year flood in the questions below. Name and number of gauging station: ____________________ 12

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Question 9: Excel plot (copy and insert here): Question 10: Estimate of size of the 100-year flood (see instructions on excel sheet for calculating, and please show your work): As mentioned, this type of analysis is done on a routine basis, albeit typically with more data and a bit more sophisticated analysis routine. However, it is always useful to think carefully about basic assumptions. Here a basic assumption, as mentioned, is that the river system will continue to operate in the same way in the future as it did in the historic period from which data was used to establish the mathematical description of its behavior. As a wrap-up, write two or three sentences on whether the river system might behave differently in the future than it has in the past, and for what specific reason(s). If you think it might behave differently, speculate on whether the difference will either increase or decrease the 100-year flood size and the variability of discharge. Be sure to address any assumptions you are making as part of your answer. Question 11: Thoughts on continuity of behavior of your river section (address assumptions): Clearly there is also some limit to how far you can extrapolate the mathematical relationship, as otherwise you could compute the million-year flood that might cover the earth, which would be a physical impossibility (not enough water to do so). However, 500-year floods are calculated on a routine basis for critical structures such as nuclear power plants that often are situated along a river. Just to reiterate, this basic approach you have learned here, of plotting event size versus frequency of the event size to determine if there is a mathematical relationship, and then using that relationship to predict recurrence intervals, is used for many natural phenomena including earthquakes, hurricanes, and landslides. With some thought you can probably think of other phenomena it might apply to. It is a powerful analytic tool, but must be used with careful thought. Bonus project (optional) : The intent of this project is to explore the variability in estimates of the 100-year flood for a given river as a function of the length of the specific time period used to calculate the flood size. 1) Find a river on the USGS website with at least 90 years of Peak Streamflow. Name and number of gauging station: _______________________ 2) Calculate the 100-year discharge by extrapolating the relationship determined from the data for the first three decades, as you did before. Then do the same using the data for decades 2-4, then for decades 3-5, then for decades 4-6, and continue until you get to decades 7-9. 13

Decades 1-3: Decades 2-4: Decades 3-5: Decades 4-6: Decades 5-7: Decades 6-8: Decades 7-9: 3) Plot the variation in estimates for the 100-year flood calculated from the different three- decade blocks from Q2 against the particular recording intervals, and then report (in several sentences) on how your estimates changed. Plot: Report: 4) Next calculate the 100-year discharge using the record for the first 30 years, then 40 years, then 50 years, then 60 years, and so on until you have used the entire 90-year record. 30 years: 40 years: 50 years: 60 years: 70 years: 80 years: 90 years: 5) Plot the variation in estimates from Q4 and report on the results of how your estimates changed as the length of the recording period increased. Also report on the differences between the first and this plot. Plot: Report: 14

Possible points for this bonus project equal a full lab, since significant effort is involved. 15

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