Geology 200

Study Guide

Unit 7:
Running Water

Overview

The presence of significant amounts of water on Earth sets it apart from other planetary bodies within the solar system. A significant proportion of this water occurs as running surface water that is related to other bodies of water on Earth through the hydrological cycle. Running water is a dominant geomorphic agent on the surface of the Earth. Through erosion, sediment transport, and deposition, moving water contributes significantly to landforms and landscapes at the surface of the Earth.

In this Unit, we begin by exploring the relationships between running water and other water bodies on Earth by looking at the hydrological cycle. The unit then goes on to examine how streams erode, and how they transport and deposit sediment, by looking at drainage basins, drainage patterns, and factors that affect stream erosion and deposition. The causes and effects of flooding and methods used to control flooding are discussed next. The unit concludes with a look at stream valley development.

Objectives

After completing this unit, you should be able to

1. describe the hydrological cycle, and outline the role played by surface water in the cycle.
2. define the terms drainage basin and divide.
3. describe dendritic, radial, and rectangular drainage patterns.
4. outline factors that affect stream erosion and deposition, including velocity, gradient, channel shape, channel roughness, and discharge.
5. explain the roles played by hydraulic action, solution, and abrasion in stream erosion.
6. define bed load, suspended load, and dissolved load as they pertain to sediment transport.
7. describe the process of saltation.
8. describe landforms produced by stream deposition including point bars, braided streams, meandering streams, flood plains, deltas, and alluvial fans.
9. explain the range of processes that can cause flooding.
10. outline measures that can be taken to control flooding.
11. describe processes important in stream valley development.

Section 1: The Hydrologic Cycle and Surface Water

The hydrologic cycle describes the continuous movement of water on and below the Earth’s surface as well as in air. Energy from the sun drives this perpetual cycle. Section 1 opens with a brief overview of the hydrologic cycle, followed by a look at running water in streams. The section concludes with some information about drainage basins and drainage patterns.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• Introduction to Chapter 10: Streams and Floods (pp. 230-231).
• “Earth Systems—The Hydrologic Cycle” (pp. 231-232).
• “Running Water” (pp. 232-233).
• “Drainage Basins” (p. 233).
• “Drainage Patterns” (pp. 233-235).

Study Questions

1. What is the hydrologic cycle?
2. What proportion of rainfall typically ends up as runoff in rivers? In what situations do you sometimes get 100% of a rainfall forming runoff?
3. How does the cross section of the channel of a river vary between its source and its mouth?
4. What causes sheet erosion?
5. What is a drainage basin?
6. Describe a radial drainage pattern. What topography promotes its development?
7. What type of rock promotes the development of a dendritic drainage pattern? —a trellis drainage pattern?

 Answer Key

Section 2: Factors Affecting Stream Erosion and Deposition

A major component of a river’s interaction with humans is associated with the erosional and depositional activities of the river. In this section we take a close look at the factors that influence the erosion and deposition associated with rivers. You will learn that a stream’s velocity and discharge play a major role in determining erosion and deposition, and you will be introduced to the factors that control stream velocity.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• “Factors Affecting Stream Erosion and Deposition” (pp. 235-237).

Study Questions

8. What two factors control stream erosion and deposition? Which of these two factors is more important, and what factors control it?
9. Why are silt and clay more difficult to erode than sand despite being lighter than sand?
10. What is the gradient of a stream? By what units is a gradient measured?
11. How does the geology through which a stream flows influence the channel width, and how does this in turn influence the stream velocity?
12. Give an example of how human interference can influence stream velocity.
13. Why do some streams in regions with a humid climate display an increase in velocity downstream despite a decrease in gradient?

 Answer Key

Section 3: Sediment Erosion and Transport

The ability of streams to carve the surface of the Earth cannot be exaggerated. Many of the landforms we see are products of water erosion. Streams are also efficient transport agents for materials produced by weathering and erosion. In this section we begin by looking at processes involved in stream erosion. After this, we will examine the mechanisms through which a stream transports its sediment load.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• “Stream Erosion” (pp. 237-238).
• “Stream Transportation of Sediment” (pp. 238-239).

Study Questions

14. In what three ways does a stream erode rock?
15. Distinguish between hydraulic action and abrasion as they relate to stream erosion.
16. What are the three categories of the sediment load transported by a stream?
17. Describe the processes of saltation and traction.
18. Which ions are commonly carried in solution by streams?

 Answer Key

Section 4: Stream Deposition

In the preceding section we learned that streams are effective transport agents of earth materials. At some points along the stream, sediment being transported will be deposited; this usually takes places when the stream velocity drops. Sediment deposition along a stream channel is only temporary, however, and transport occurs in intermittent episodes of transport and deposition. Deposited sediments play a major role in shaping the morphology of a stream bed and its depositional features.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• “Stream Deposition” (pp. 239-250).

Study Questions

19. Why are sediments deposited by streams in bars, and what kind of sediments are usually found in bars?
20. How do placer deposits form? Give examples of minerals that can form in this manner.
21. Explain how mid-channel bars can cause widening of the stream channel. What conditions favour the development of braided channels?
22. What types of sediment characterize meandering rivers? In which part of a river’s profile is meandering common?
23. How do point bars form?
24. What types of sediments form flood plains?
25. Where do deltas form?
26. What factors determine the shape of a marine delta in map view?
27. What factors determine whether a delta remains dry land or becomes submerged by water?
28. What causes sediments to be deposited in an alluvial fan? What differences in sediment grading can be noted between large and small alluvial fans?

 Answer Key

Section 5: Stream Valley Development

In this section we examine how streams create their valleys by eroding rock and sediment from their channels. In addition to learning about how the stream deepens its channel by downcutting, you will become acquainted with the concepts of lateral and headward erosion. The section concludes with a look at how stream terraces and incised meanders form.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• “Stream Valley Development” (pp. 250-254).

Study Questions

29. What are slot canyons, and what conditions favour their development? Why are they relatively rare?
30. What is downcutting? What conditions result in elevated rates of downcutting?
31. What is the difference between a graded and an ungraded stream?
32. Explain how a river can change from one of deposition to erosion during terrace development.

 Answer Key

Section 6: Flooding

Flooding occurs when streams overflow their banks. This may be the result of heavy rainfall or snow melt, such that more water accumulates than the channel can carry. The ability of the ground to absorb the precipitation received is important in determining how much water runs off directly into the stream, which is a major factor in influencing how rapidly the water level rises within a stream. Accordingly, factors such as topography and geology of the landscape are important in determining whether water reaches the stream rapidly or infiltrates into the ground first to be released gradually as groundwater. This section covers both the causes of flooding and methods used to estimate the size and frequency of floods.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• “Flooding” (pp. 254-260).

Study Questions

33. How can floods be of benefit to agricultural activities?
34. How does urbanization contribute to flooding?
35. What is a flash flood?
36. Describe how the erection of floodwalls can result in an increase in erosive power further downstream.

 Answer Key

Unit 7 Self Test

You have now finished Unit 7, so please complete and submit the associated laboratory exercise and assignment. Instructions can be found in the Assignment Drop Boxes section of the course homepage.