Fluvial Processes and Landforms

Chapter 17

Fluvial Processes and Landforms

OVERVIEW

This chapter focuses on running water as a land-forming agent. It examines the processes by which running water moves sediments and shapes landforms at the Earth’s surface. Landforms produced by running water dominate most of the Earth’s terrestrial environments.

·  Water is one of the four active agents of denudation (the others being wind, waves, and glacial ice) that erode, transport, and deposit sediments at the Earth’s surface to produce erosional and depositional landforms.

·  The term fluvial is applied to the processes and landforms associated with the action of running water.

·  Fluvial processes can erode and transport soil particles from slopes and uplands causing soil erosion.

·  Rates of soil erosion and soil formation are in equilibrium on the slopes of most natural landscapes. This is known as the geologic norm. Disturbance of this equilibrium by human activity or natural catastrophes can lead to accelerated erosion.

·  Some eroded soil particles are deposited immediately at the base of slopes to form colluvium, while others enter streams and are carried downstream before being deposited as alluvium along valley floors.

·  A stream can erode material from its bed and banks. Soft materials can be effectively eroded by hydraulic action, while hard bedrock materials can only be eroded by abrasion.

·  The stream load or sediment carried by a stream is transported in three ways: as dissolved load, suspended load, and bedload. Suspended load is usually the largest of these components.

·  Stream capacity to carry solid sediment is dependent on stream flow velocity, which, in turn, is dependent on channel gradient.

·  Streams tend to a graded condition over time such that the channel gradient and stream capacity are adjusted to move the average amount of water and sediment supplied by slopes.

·  Indicators of a graded condition are the development of a floodplain and a smooth stream profile.

·  Grade is maintained as landscapes are eroded toward base level and, in tectonically stable areas, erosion can eventually lead to the formation of a peneplain.

·  Floodplain development involves lateral channel shifting by bank erosion on the outside of channel bends and deposition of alluvium in point bars on the inside of channel bends. It results in valley widening and the development of alluvial meanders.

·  Large rivers with low gradients and wide floodplains are called alluvial rivers. Meandering or lateral shifting of alluvial rivers produces cutoff meanders, oxbow lakes, and other distinctive landforms.

·  Tectonic and environmental changes can cause aggradation and degradation in alluvial rivers and lead to the formation of alluvial terraces and entrenched meanders.

·  Fluvial processes are very effective in shaping desert landforms because of the sparse vegetation cover.

·  Some of the more distinctive fluvial landforms of arid regions are alluvial fans, pediments, and playas.

KEY TERMS

STUDY QUESTIONS

1.   What are the four principal agents of denudation? Which is most important in shaping the landforms of terrestrial environments?

2.   Distinguish between erosional and depositional fluvial landforms, and give an example of each.

3.   What is the geologic norm?

4.   Describe the processes of splash erosion and sheet erosion. How and why does removal of the vegetation cover affect these processes?

5.   What landscape evidence might indicate the development of accelerated erosion?

6.   What is the difference between colluvium and alluvium? What is the source of the material for these deposits?

7.   Hydraulic action and abrasion are both forms of stream erosion. How do they differ, and under what conditions might each dominate?

8.   What is stream load, and what are its three components? Which component is dominant in most large rivers?

9.   How are stream capacity, flow velocity, and channel gradient related?

10.   What is a graded stream? How would a graded stream be characterized in system terms? Your answer should consider the system inputs and outputs and the relationship between them.

11.   Describe the typical sequence of evolution of a fluvial landscape.

12.   Outline the processes involved in floodplain formation.

13.   What are some of the characteristic landforms found on the floodplains of alluvial rivers, and how do they develop?

14.   Why is fluvial action an important land-forming process in regions which receive relatively little precipitation?

15.   What is a pediment, and how does it develop?

CHAPTER QUIZ

Multiple Choice Questions

1.  Which of the following is an erosional fluvial landform?

a)  fan

b)  floodplain

c)  spur

d)  playa

2.  Stream laid sediment is called:

a)  colluvium

b)  alluvium

c)  regolith

d)  sandstone

3.  Which of the following would suggest that stream aggradation may be in progress:

a)  natural levees

b)  oxbow lakes

c)  a braided channel

d)  entrenched meanders

4.  A gently rolling land surface of low elevation resulting from a long period of denudation is called a(n):

a)  alluvial terrace

b)  floodplain

c)  peneplain

d)  col

5.  A gently sloping rock floor veneered with alluvium is known as a(n):

a)  pediment

b)  playa

c)  alluvial fan

d)  bluff

True/False Questions

1.   Because they receive only small amounts of precipitation, arid and semi-arid regions are largely unaffected by fluvial processes. (T/F)

2.   Fluvial landforms and fluvial processes dominate continental land surfaces. (T/F)

3.   Niagara Falls is an example of a large waterfall formed by tectonic activity. (T/F)

4.   The largest component of transported matter in a stream is carried as bedload. (T/F)

5.   Stream channels receiving runoff from slopes that suffer accelerated soil erosion can become choked with alluvium. (T/F)

Short Answer Questions

1.   Sketch a diagram showing the way in which the suspended sediment load of a stream normally varies with discharge.

2.   Describe the two principal mechanisms of stream erosion, and identify the conditions under which each is expected to be dominant.

Short Essay Questions (1 - 2 paragraphs)

1.  Describe the process of floodplain formation, and discuss the impact of this process on the shape of the valley and its side slopes.

2.  Assume that a large landmass has been uplifted by tectonic forces. Describe the changes that it would undergo if fluvial denudation were to occur for a long period of time with no further tectonic disturbance.

3.  How would fluvial processes be affected if the vegetation cover was removed from a hilly landscape in a humid region?

Internet Resources

1.  The Floodplain Management Web site Learning Center: <http://floodplain.org/p-learn.htm#THE%20LEARNING%20CENTER

2.  Fluvial landforms from space: <http://daac.gsfc.nasa.gov/DAAC_DOCS/geomorphology/GEO_4/GEO_CHAPTER_4_TABLE.HTML

3.  Photographs of fluvial activity: <http://collections.ic.gc.ca/geoscience/dynamicEarth_naturalhazard_4_e.html>; <http://collections.ic.gc.ca/geoscience/dynamicEarth_naturalhazard_5_e.html>; <http://collections.ic.gc.ca/geoscience/dynamicEarth_naturalhazard_6_e.html>

4.  River channels, sediment, and runoff: <http://waterknowledge.colostate.edu/geomorph.htm>

5.  Simulation of bedload transport: <http://www.gly.fsu.edu/faculty/schmeeckle/bedanim.html

6.  Virtual Field Trip - Tomorrow River: <http://www.uwsp.edu/geo/courses/geog391/toriv/G391MAIN.HTM

7.  River landforms: <http://library.thinkquest.org/20035/newpage3.htm

8.  Stream features of North Dakota: <http://www.ndsu.nodak.edu/nd_geology/nd_strm/index_st.htm

9.  An elementary presentation on river features with many good photos: <http://www.kented.org.uk/ngfl/rivers/River%20Articles/rivart.htm>