Lesson 8: Streams and Flooding
This week's lesson on streams and flooding is our third to focus on a specific geologic hazard. Streams—like the Roaring River, at right—are crucial to human agriculture and civilization. The can also be deadly, however, when rain or snowmelt bring more water to their channels than they can carry and produce swiftly moving floods that sweep out across the surrounding landscape. Like the volcanoes we studied last week, streams and flooding are also relevant to people's lives and livelihoods here in Siskiyou County. As some of you may remember, heavy rains in January 2006 triggered flooding on the Shasta River that swept away riparian fencing and vegetation and required tens of thousands of dollars for repairs that had to be undertaken before that year's irrigation and grazing season could begin. Agriculture in the Shasta Valley depends on the river, but is at its mercy as well.
In this week's lesson Keller begins with an introduction to the basics of streams—calculating their discharges, classifying their sediment loads, and recognizing their channel types—and then brings these ideas together in a discussion of stream dynamics that explains how streams respond to changes in these parameters. He then explores how we analyze flooding through the calculation of recurrence intervals, how we can mitigate its effects, and how human actions may change a stream's behavior. Finally, this week's exercise will help you learn how geologists measure the discharges of actual streams and give you and opportunity to measure the discharge of a virtual stream.
As you read through this chapter on streams and floods and work through the River Discharge exercise, it will be helpful to take detailed notes so that you can keep track of major points and have ready access to them when we refer back to them later in the semester. Be sure that you have completed this week's exercise on determining river discharge and are prepared to meet the learning objectives outlined below before you move on to the quiz at the bottom of the page.
Weekly Learning Objectives
Upon successful completion of this week's lesson, a student is expected to be able to:
- Calculate the discharge of a stream from data on its width, depth, and velocity.
- Distinguish between the different types of sediment load a stream can carry, describe what material typically makes up each type, and .
- Predict how a stream is likely to respond to changes in the amounts of sediment or water it receives in order to maintain dynamic equilibrium.
- Determine whether a stream you see has a braided or a meandering channel, and indicate where erosion and deposition are likely to be occurring in and adjacent to such a stream.
- Estimate the recurrence interval of a flood of a given magnitude (discharge) using a discharge-frequency graph like Fig. 9E, and calculate the chance that such a flood will occur during a given year.
- Use data from a hydrograph like Figure 9.22, particularly lag time and peak discharge, to infer whether a region has undergone significant urbanization or not and explain your reasoning.
- Evaluate three alternative methods for controlling future flood damage along a stream that flows through a lightly urbanized area: channelization, stream restoration, and floodplain regulation, and briefly describe the strengths and weaknesses of each.
Reading and Browsing Assignment
- Read Chapter 9, focusing on the topics outlined in the learning objectives above.
- Take a few minutes to browse through the USGS Water Resources site. Just looking at the map on the first page, what can you tell about streamflows in California compared to the Northeast in the wake of Tropical Storm Lee? Using the boxes near the upper left corner of the page can you find out the daily mean discharge of the Shasta River near Montague on October 2, 2011? (Hint: Choose "Daily streamflow" from the '---Predefined Displays---' drop down list and type "Shasta River" in the right-hand box, choose "station name" from the adjacent drop-down menu and then click the 'go' button. Choose the link to the site on the Shasta near Montague and then near the top of the next page choose "Time Series: Real-time Data". The top graph on the page that comes up will show discharge for the past 7 days, and I estimate the average discharge on October 2, 2011 was about 140-145 cubic feet per second. (See how much it jumped up when irrigation season ended on 1-Oct?) Note that the vertical scale on this graph is a log scale; each of the divisions between 10 and 100, is ten larger than the one below it [60, 70, 80, etc.]).
- For similar information of California's streams, check out the California Data Exchange Center. This portal will take you to real-time or near real-time data on river discharges, snowpack amounts, statewide water conditions, and more.
Exercise 8: River Discharge (Due by 9:00 AM on 10-Oct-2011)
In this week's reading from the textbook you learned that a stream's discharge is the product of its channel width times its channel depth times its velocity. Unless a channel has a simple rectangular cross-section, however, you could never determine a stream's discharge this way. Instead, this week's exercise will give you an opportunity to learn how geologists actually measure stream discharges.
- Point your browser to the Virtual Courseware exercise on River Discharge. This program uses Java applets, so be patient as you load the intractive graphics if you are using a dial-up connection. You don't want to crash the program by trying to use the applets before they are fully loaded. Also, allow enough time to do this entire exercise in one session (about 1 hour) because, unfortunately, this program will not allow you to save your work and come back later.
- Work through the exercise by carefully reading each part of the text and then answering the accompanying questions. Don't worry if you get an answer wrong at first; the program will tell you it's wrong and give you a chance to fix it. You should take notes on key points as you work through this exercise so that you will be able to remember what you did later. Focus on understanding the basics of the discharge measurement process.
- When you get to the end of the exercise, fill in your first and last names, school (College of the Siskiyous) and address and press enter to see your "Virtual Hydrologist" certificate. Right-click on the blue part of the certificate, choose Copy Image, and paste it into whichever program you use to view graphics files (Windows Picture Viewer, Paintshop Pro, Photoshop, etc.). From there, save it as a graphics file (.jpg is best but .gif and .bmp work too). Be sure to remember where you saved the image and what you called it! You should view it to be sure that it saved correctly before you close or navigate away from the browser window that displays the certificate.
- Log on to the Etudes site, click on the "Assignments, Tasks and Tests" tool, and click the link for Exercise 8. Attach your certificate by using the "Browse..." button on the page and then selecting the name of the file you saved your certificate as. Finally, press the "Upload" button. This exercise is worth 10 points, and scores will be posted in the gradebook shortly after 10-Oct.
Quiz 8: Rivers and Flooding (Due by 9:00 AM on 10-Oct-2011.)
After you feel you have met the learning outcomes outlined above, please complete Quiz 8 in the Etudes "Assignments, Tests, and Surveys" area. There are ten questions, each worth one point. If you can answer all of them correctly it means that you know your way around the basics of rivers and flooding pretty well and are ready to move on to a more detailed look at landslides and subsidence next week. Please contact me if you have any questions or there are concepts you're not sure about.