Page 524 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

488 part III The Earth–Atmosphere Interface
water velocity and forms a point bar deposit. When a me- ander neck is cut off as two undercut banks merge, the me- ander becomes isolated and forms an oxbow lake.
The drop in elevation along a river from headwaters to mouth is usually represented in a side view called a longitudinal profile. A graded stream condition occurs when the slope is adjusted so that a channel has just enough energy to transport its sediment load; this repre- sents a balance between slope, discharge, channel charac- teristics, and the load supplied from the drainage basin. Tectonic uplift may cause a stream to develop entrenched meanders as it carves the landscape during uplift. An in- terruption in a stream’s longitudinal profile is called a nickpoint. This abrupt change in slope can occur as the stream flows across hard, resistant rock or after tectonic uplift episodes.
braided stream (p. 465) meandering stream (p. 468) undercut bank (p. 468) point bar (p. 468)
oxbow lake (p. 468) graded stream (p. 472) nickpoint (p. 472)
14. Describe the flow characteristics of a meandering stream. What is the pattern of flow in the channel? What are the erosional and depositional features and the typical landforms created?
15. Explain these statements: (a) All streams have a gradient, but not all streams are graded. (b) Graded streams may have ungraded segments.
16. Why is Niagara Falls an example of a nickpoint? Without human intervention, what do you think will eventually take place at Niagara Falls?
■ Describe the depositional landforms associated with floodplains and alluvial fan environments.
Alluvium is the general term for the clay, silt, sand, gravel, or other unconsolidated rock and mineral fragments de- posited by running water. The flat, low-lying area adjacent to a stream channel that is subjected to recurrent flooding is a floodplain. On either bank of some streams, natural levees develop as by-products of flooding. On a flood- plain, riparian marshes are common, and yazoo streams may develop, which flow parallel to the river channel but are separated from it by natural levees. Entrenchment of a river into its own floodplain forms alluvial terraces.
Along mountain fronts in arid climates, alluvial fans develop where ephemeral stream channels exit from can- yons into the valley below. A bajada may form where in- dividual alluvial fans coalesce along a mountain block. Runoff may flow all the way to the valley floor, where it forms a playa, a low, intermittently wet area in a region of internal drainage.
alluvium (p. 473) floodplain (p. 473)
natural levee (p. 473) yazoo stream (p. 473) alluvial terrace (p. 473) alluvial fan (p. 474) bajada (p. 474)
playa (p. 474)
17. Describe the formation of a floodplain. How are natural levees, oxbow lakes, riparian marshes (backswamps), and yazoo tributaries produced?
18. Describe any floodplains near where you live or where you go to college. Have you seen any of the floodplain features discussed in this chapter? If so, which ones?
19. What processes are involved in the formation of an alluvial fan? What is the arrangement, or sorting, of alluvial material on the fan?
■ List and describe several types of river deltas, and explain flood probability estimates.
A depositional plain formed at the mouth of a river is called a delta. Deltas may be arcuate or bird’s foot in shape, or estuarine in nature. Some rivers have no deltas. When the mouth of a river enters the sea and is inundated by seawater in a mix with freshwater, it is called an estu- ary. Despite historical devastation by floods, floodplains and deltas are important sites of human activity and set- tlement. Efforts to reduce flooding include the construc- tion of artificial levees, bypasses, straightened channels, diversions, dams, and reservoirs.
A flood occurs when high water overflows the natural bank along any portion of a stream. Human- constructed artificial levees are common features along many rivers of Canada and of the United States, where flood protection is needed for developed floodplains. Both floods and the floodplains they occupy are rated statistically for the expected time interval between floods of given discharges. For example, a 10-year flood has the statistical probability of happening once every 10 years. Flood probabilities are useful for floodplain zoning.
delta (p. 474)
estuary (p. 475)
flood (p. 478)
artificial levee (p. 482)
20. What is a river delta? What are the various deltaic forms? Give some examples.
21. Describe the Ganges River delta. What factors upstream explain its form and pattern? Assess the consequences of settlement on this delta.
22. What is meant by the statement, “The Nile River delta is disappearing”?
23. Specifically, what is a flood? How are such flows measured and tracked, and how are they used in floodplain management?
24. What is channel avulsion, and how does it occur?