TEACHER INSTRUCTIONS

Part II. Drainage basins & divides

  • 1. Find the Ohio River. It flows into the Mississippi at the southern tip of Illinois. Trace it in blue. Notice that it flows westward from Pennsylvania. But there is something different about its source. The Ohio is formed by two other rivers, the Allegheny and the Monongahella. They join at Pittsburgh to become the Ohio. Trace each of these rivers in blue as well. Do you see how your tree continues to grow? The Mississippi is the trunk and the Missouri and Ohio are its two biggest branches. Now trace in blue all the remaining rivers you can find feeding into the Mississippi.

    Teacher notes: With the addition of the Ohio, our "tree" takes on a more balanced appearance. Tracing the Mississippi and all its tributaries will reveal a center of America threaded in blue. Review student work to assure rivers outside this drainage basin have not been traced in blue. Especially be sure the Great Lakes, eastern seaboard, western coastal, and Rio Grande rivers are not yet colored.

  • 2. When you have finished coloring your blue rivers, you will see that most of America's middle is drained by the Mississippi. This is the Mississippi River "drainage basin." To the east and west of this basin are two mountain ranges; what are they? In 1682 Robert La Salle explored the Mississippi and claimed its drainage basin for France. What part of America was he claiming?

    Teacher notes: You may ask students to lightly shade this basin in blue using their "blue" rivers as guides. (La Salle's claim will be discussed in our next lesson set, "Rivers & Exploration," this fall.) Students should clearly recognize the Mississippi drainage basin as America's midsection.

  • 3. Look in Illinois above the place where the Ohio joins the Mississippi and see if you can find the Illinois River. Locate its source near the large lake northeast of Illinois. Notice it does not quite connect with the lake. What is the name of the lake? It is one of five we call the "Great Lakes." Follow the chain of these lakes eastward and notice they become the source of the "St. Lawrence River" separating Canada and America. This river feeds into what ocean? Trace the St. Lawrence River in green and color the Great Lakes green as well.

    Teacher notes: Hopefully the Illinois River is on your maps. If not, have students construct it with your guidance. The source of the Illinois is actually a junction of two forks one of which, the Des Plaines River, begins near Lake Michigan and very near the Chicago River which flows into the lake. (There is a historic portage site in a southwest suburb of Chicago.)

    The Great Lakes are interconnected and ultimately become the source of the St. Lawrence River. This also demonstrates how rivers may emerge from lakes and become boundaries. This significance will be expanded in our fall lesson set. This also introduces the use of "green" to identify a new drainage basin, the St. Lawrence.

  • 4. The point where the Illinois River is closest to Lake Michigan is on a divide. This is an imaginary line that separates drainage basins. Water from north and east of this line goes into the Atlantic Ocean. Water south and west of this line enters another part of the Atlantic. What is it?

    Teacher notes: The geographic concept of "divides" is interesting. They are represented by the highest points between drainage systems. The Atlantic divide has an interesting shape. It will be seen as students continue tracing their green rivers. This divide differentiates the Atlantic Ocean drainage system from the Gulf of Mexico. It proceeds around the Great Lakes and down the Appalachian Mountain chain to central Florida.

  • 5. Sometimes rivers form so close to a divide that boats and goods can be carried from one to the other. Before there were roads, this was a good way for travelers to go from one river or lake to another. We call it a portage. There are many portages between the Mississippi drainage basin and the Great Lakes. Some of these portages were made into canals. A canal is a man-made river which usually uses dams and locks to control flow of water each way. There is a very important canal on the Chicago River between the Illinois and Lake Michigan. It allows boats to pass all the way from the St. Lawrence to the Gulf of Mexico!

    Teacher notes: The divide between the Des Plaines River (Illinois River) and the Chicago River is a classic, but by no means rare example. Portages were important elsewhere, even in the high Sierra of the west. But they were most used where most negotiable, such as here in Illinois.

    Keys to a good portage site are: 1. A short separation between rivers, 2. No steep rise between them, and 3. Navigable rivers at each end. These are also key ingredients for a practical canal.

    Since rivers at each end of a canal almost certainly differ in elevation, dams or locks are necessary to control water flow between them. (Locks, of course, if boats are to navigate the canal.) The Illinois-Chicago River connection is especially unique because the direction of flow is controlled so the system may flow to or from Lake Michigan! A unique canal & lock activity is included with issue #20 of Our Land of Liberty.

  • 6. Now you can trace all the other rivers flowing into the Atlantic Ocean east of the Mississippi. Use your green pen for this. See how the Appalachian Mountain chain divides your green and blue rivers. Why is this?

    Teacher notes: It is okay for students to trace in green rivers entering the Gulf of Mexico east of the Mississippi. We are not fully defining the Atlantic divide. But they should recognize the part the Appalachians play in this.

  • 7. Now go west of the Mississippi and find the Rio Grande River. It forms part of the boundary between America and Mexico. Trace it in green as well as the other rivers entering the Gulf of Mexico west of the Mississippi. Where is the source of the Rio Grande?

    Teacher notes: The Rio Grande bears some similarity with the St. Lawrence. It forms part of our nation's boundary. Since it does not enter the Mississippi, but does enter the Atlantic (Gulf of Mexico), we color it green. Its source is in Colorado. Thus we have a green and blue eastern 3/4 of our nation.

  • 8. Now you have colored most of America's rivers, but one important part remains blank. Which is it? Use a red pen and trace the rivers there. Notice the two biggest ones, the Columbia and the Colorado. One goes into the Pacific near Portland, Oregon. The other goes into the Gulf of California in the far southwest. What separates these rivers from the green and blue ones you've already traced?

    Teacher notes: It should be obvious that only those rivers west of the Rocky Mountains remain uncolored. These rivers enter the Pacific Ocean either directly, or through the Gulf of California. (Remember this gulf is also part of the Pacific.) The dramatic difference between drainage systems will be obvious given the color choices used.

  • 9. This marks another divide. It is a very special one. We call it the "Continental Divide" because it separates rivers feeding the Atlantic from those feeding the Pacific. It divides our North American continent. What do you notice about the parts of America you have now colored? (As you study America's past, you will see how these rivers and divides changed our nation.)

    Teacher notes: Between every set of divergent rivers there is a divide. But the "Continental Divide" stands out as a major American geographic feature like the mountains that define its course. Defining this divide will be important to our next lesson set, "Rivers & Exploration." (Coming in November.)

    • Copyright 2000 by Robert C. Law. Permission to copy granted provided this credit line is included:
    Our Land Publications - 4861 Chino Ave. - Chino, CA 91710 - 1-800-777-5292
    email: ourland@gte.net

    Links: Student text for this part - Part 1 - Part 3 - Part 4 - Return to lesson introduction - Home