1. WTBW What direction does water move and why?
How big can a watershed (drainage basin) be?
What is topography?
What is the largest watershed (drainage basin) in the United States?

2. Effect of Rainfall on Streams
Watersheds
Effect of Rainfall on Streams

3. A watershed = a drainage basin is an area in which precipitation runs off into or towards a single exit point.
PMC, Figure 10.1

4. River systems develop in watersheds wherever there is enough precipitation to keep water flowing perennially.
PMC, Fig. 10.4

5. A detailed look at part of the Mississippi River system near St
A detailed look at part of the Mississippi River system near St. Louis, MO. The colors indicate elevation, with reds and oranges being the highest and the greens the lowest. The area in the image on the right is enclosed inside the red box on the left-hand image.
H&C, Fig. 12.2A
H&C, Fig. 12.2B

6. An even more detailed look at tributaries of the Missouri River near St. Louis, MO. The area in the image on the right is enclosed inside the red box on the left-hand image. Notice how the branching networks look similar across a range of scales: this is called fractal scaling : it’s typical of many stream systems.
H&C, Fig. 12.2B
H&C, Fig. 12.2C

7. Remember that in a drainage basin water flows from areas of high elevation to low and thus from the branches of the ‘tree’ toward its main trunk.

8. The Fella River basin (northern Italy)
covers an area of 273 square miles,
slightly smaller than the Schuylkill River in Montgomery County. Dark lines on this map are streams.
“Although the river and hillside waste do not resemble each other at first sight, they are only the extreme members of a continuous series and when this generalization is appreciated one may fairly extend the ‘river’ all over its basin and up to its very divide. Ordinarily treated the river is like the veins of a leaf; broadly viewed it is the entire leaf.”
W.N. Davis, 1899 (quoted in Fractal River Basins, I. Rodriguez-Iturbe & A. Rinaldo, 1997)

9. Fella River basin
A watershed is defined by a point along the flow path (e.g. a stream) through which the water exits.
The line which surrounds the watershed is called the watershed’s divide.
Notice that the divide is drawn so that it separates streams which flow through the blue point from those which do not flow through the blue point.

10. Fella River basin
A watershed could be defined by any point along the stream; the red point is farther along the same channel: it includes the watershed defined by the blue point.

11. Fella River basin
The pink point is on the main channel; although this point is not far away from the red point its watershed is much larger. It includes the ‘red’ watershed.

12. Fella River basin
The green point defines the entire Fella River Basin as it is shown here.

13. WTBW Name the watershed(s)/drainage basin(s) that we live in.
What challenges do humans cause in watersheds?
Why is it important to manage watersheds properly?

14. All supplies of water for human use come from watersheds, either from surface water (streams, lakes, reservoirs), or from groundwater (wells).
People affect how watersheds work in all kinds of intentional and unintentional ways. Examples:
Household:
Oil, pesticides, pharmaceuticals dumped onto ground or into sewer.
A pool or washer drains into a sewer or directly to a stream
Urban/suburban:
Runoff washes oil, antifreeze, gasoline, salt, and other chemicals and metals from streets into a storm water inlet (leads to stream).
Municipal:
Treated sewage is returned to a stream
Construction:
Soil washing off a construction site enters a stream.
Industrial:
Industrial waste enters a stream through a pipe.
Agricultural:
Runoff from farm fields adds fertilizer to a stream.
Good watershed management is important for clean, plentiful water supply.
Who is responsible for watersheds?

15. Stream systems ordinarily include small streams in narrow valleys around the edges of the “leaf”; which feed into larger streams in wider valleys.
PMC,
Fig. 10.2

16. The valley that a stream flows through and the surrounding land is not coincidental to a stream.
A stream both erodes (B) and deposits (C) sediment to create its valley and adjacent land, such as a flood plain.
Over time, sediment in a stream comes into steady state. When humans alter the river or its watershed, the system moves out of steady state.
PMC,
Fig. 10.2, B & C

17. Watersheds in and around Chester County.

18. In PA, decisions about development and land use are made primarily at the township level. As you can see, townships do not correspond to watersheds.
The problem of watershed management keeps a lot of geologists and other professionals employed!

19. Pause/Reflect/Research
I a) What stops river from over flowing it’s banks? Why are flood plains fertile?
II Diagram of River channel with major components?
III List and describe several causes of flooding.
IV  How can floods be prevented? Why should floodplains not be used?