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

fluvial landforms  stream deposition  alluvial river     
fluvial processes  stream load        aggradation        
erosional          dissolved load     degradation        
landforms          suspended load     meander cutoff     
depositional       bedload            oxbow lake         
landforms          stream capacity    natural levee      
accelerated        graded stream      alluvial fan       
erosion            alluvial meander   pediment           
colluvium          peneplain          playa              
alluvium                                                 
stream erosion                                           
stream                                                   
transportation                                           

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?