Learning Objectives
After reading and studying this chapter, students should:
• Understand basic river processes.
• Understand the nature and extent of the flood hazard.
• Understand the effects of urbanization on flooding in small drainage basins.
Chapter Summary
This chapter focuses on river processes and flooding. The first portion of the chapter focuses on the
purpose rivers serve in the hydrologic and rock cycles, on the physical processes of drainage basins and
river channels, of sediment transport, and of erosion and deposition. A subsequent section addresses
Chapter Outline
I. Rivers: Historical use
A. Americans have lived and worked on floodplains for more than 200 years
B. Floodplain
1. the flat surface adjacent to the river channel that is periodically inundated
C. Two parallel trends in floodplain settlement
II. Streams and rivers
A. Hydrology: study of processes in the water cycle
C. Base level
1. theoretical lowest level to which a river may erode
3. base level may be temporary, as with a lake
III. Sediment in rivers
A. Sediment load
1. total load
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Rivers and Flooding
Rivers and Flooding
4. dissolved load
IV. River velocity, discharge, erosion, and sediment deposition
A. Rivers are a transportation system involving erosion and deposition of sediments
B. Discharge
1. volume of water moving past a particular location in a river (in a unit of time)
C. Faster flowing river transports has greater erosional capacity than does slower moving river
1. alluvial fans
3. distributary channels (see A Closer Look: History of a River)
D. Reasons for erosion or deposition correlated to physical properties of the river
2. composition of channel bed and banks
4. land use
E. Sediment transport measures
2. capacity
V. Effects of land-use changes
A. Dynamic equilibrium
B. Land use change from forest to agriculture
1. increased soil erosion
3. increasing channel slope
4. new dynamic equilibrium may be reached
C. Example: Piedmont region of SE United States
D. Dam construction
2. below dam, water released has minimal sediment
3. channel erosion will predominate below dam
VI. Channel patterns and floodplain formation
A. Configuration of channel as seen in aerial view
2. meandering
B. Meandering stream features
2. point bars
4. riffles
VII. River flooding
A. Definitions
1. flooding is the natural process of overbank flow
3. stage
4. flood stage
B. Flash floods and downstream floods
2. downstream floods: caused by storms of long duration over large area
a. examples: 1993 Mississippi River Flood and 1997 Red River, North Dakota Flood
3. Characteristics of downstream floods
VIII. Urbanization and flooding
A. Human use of urban environments
2. percentage of impervious cover and percentage of area served by storm sewers are a measure
B. Urbanization causes many changes in rainfall-runoff relations
1. increased runoff
3. influences limited to smaller floods of greater frequency; larger, less frequent floods are not
significantly affected
5. Flashy discharge
C. Effects of other development
1. Debris dams develop upstream of bridges, and release flood waves when they fail
IX. The nature and extent of flood hazards
A. Flooding is one of the most universally experienced natural hazards
B. Factors that cause flood damage
1. land use on floodplain
3. rate of rise and duration of flooding
5. sediment load deposited
C. Effects of flooding
1. primary: caused directly by the flood
2. secondary: caused by disruption and malfunction of services and systems
X. Adjustments to flood hazards
A. Historically, humans have attempted to prevent flooding
2. straightening stream
4. deepening stream
B. Recent years, alternative adjustments
2. controlling land use on floodplains
C. The structural approach
2. reservoirs
4. some engineered structures actually increase the flood hazard (see A Closer Look: Mississippi
River Flooding, 1973 and 1993)
D. Channelization
E. Channel restoration: alternative to channelization
2. allowing stream to flow freely
4. Kissimmee River, Florida restoration
F. Flood insurance
2. Special Flood Hazard Areas
G. Flood-proofing
1. raising building foundations
3. waterproofed doors, walls, and windows
H. Floodplain regulation
1. Hydrologic definition of floodplain
3. need to recognize that the floodplain belongs to the river system
I. Flood hazard mapping
1. Rocky Mount, NC, after Hurricane Floyd
3. Emotional issues
K. Personal adjustment: What to do and what not to do
1. see Table 9.2
XI. Perception of flooding
A. People are tremendously variable in their knowledge of flooding, anticipation of future flooding,
Answers to Review Questions and Critical Thinking Questions
Review Questions
2. Total load includes bed load, suspended load, and dissolved load.
4. The principal lessons learned from the Ventura River flood are that it is important to assess the
historical behavior of a river and to adjust engineering models accordingly.
6. Braided channels are multiple, relatively straight channels separated by bars and islands, while
meandering channels are single, winding channels with diverse habitats.
8. Pools are deep, slow-moving parts of the channels, while the riffles that join them are shallow and
relatively fast moving.
9. Upstream floods affect the upper portions of drainage basins and typically result from intense storms
11. Urbanization affects the flood hazard by increasing impermeable surfaces and serving greater areas
by storm sewers; these factors increase the severity of low and intermediate frequency floods.
12. The major factors that control flood damage are land use on the floodplain, the magnitude and
13. Primary effects of flooding include injury, death, and damage to farms, buildings, railroads, etc.
14. Floodplain regulation strives to obtain the most beneficial uses of floodplains while minimizing
16. Channel restoration includes a series of techniques to reestablish natural processes in stream
Critical Thinking Questions
1. The plan of action should include assessment of the permeability of existing surfaces in the basin, an
2. The plan should include an assessment of the physical condition and processes of the existing stream,
3. An answer to this question will be highly dependent upon the student’s location. It will require the