Page 490 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

454 part III The Earth–Atmosphere Interface
Earth’s rivers and waterways form vast arterial net- characteristic processes and produce recognizable land-
forms, yet also can behave with randomness and seem- ing disorder.
In this chapter: We begin by examining the organiza- tion of river systems into drainage basins and the types of drainage patterns. With this foundation, we examine gradi- ent, base level, and stream discharge. We then discuss fac- tors that affect flow characteristics and the work performed by flowing water, including erosion and transport. We also examine the effects of urbanization on stream hydrology and the impacts of dams on sediment regimes. Deposi- tional features are illustrated with a detailed look at flood- plains and the Mississippi River delta. Finally, we examine floods, floodplain management, and river restoration.
Drainage Basins
and Drainage Patterns
Streams, which come together to form river systems, lie within drainage basins, the portions of landscape from which they receive their water. Every stream has its own drainage basin, or watershed, ranging in size from tiny to vast. A major drainage basin system is made up of many smaller drainage basins, each of which gath- ers and delivers its runoff and sediment to a larger basin, eventually concentrating the volume into the main stream. Figure 15.1 illustrates the drainage basin of the Amazon River, from headwaters to the river’s mouth (where the river meets the ocean). The Amazon has an average flow greater than 175 000 m3 ∙ s−1 and moves millions of tonnes of sediment through the
works that drain the continents, and at any mo-
3
ment, approximately 1250 km of water is flowing
through them. Even though this volume is only 0.003% of all freshwater, the work performed by this energetic flow makes it a dominant natural agent of landmass de- nudation. Rivers shape the landscape by removing the products of weathering, mass movement, and erosion and transporting them downstream. Rivers also serve society in many ways. They not only provide us with essential water supplies, but also receive, dilute, and transport wastes, provide critical cooling water for industry, and form critical transportation networks.
Of the world’s rivers, those with the greatest dis- charge (the streamflow volume past a point in a given unit of time, discussed in this chapter) are the Amazon of South America, the Congo of Africa, the Yangtze (Chàng Jiang) of Asia, and the Orinoco of South Amer- ica. In North America, the greatest discharges are from the Missouri–Ohio–Mississippi, St. Lawrence, and Mackenzie River systems (Table 15.1).
Hydrology is the science of water and its global circulation, distribution, and properties—focusing on water at and below Earth’s surface. Processes that are re- lated expressly to streams and rivers are termed fluvial (from the Latin fluvius, meaning “river”). There is some overlap in usage between the terms river and stream. Specifically, the term river is applied to the trunk, or main stream of the network of tributaries forming a river system. Stream is a more general term for water flowing in a channel and is not necessarily related to size. Fluvial systems, like all natural systems, have
TABLe 15.1 Largest Rivers on Earth Ranked by Discharge
Rank by Discharge
Average Discharge at Mouth
(× 1000 m3 ∙ s−1)
River (with Major Tributaries)
Outflow/Location
Length (km)
Rank by Length
1
180
Amazon (Ucayali, Tambo, Ene, Apurimac)
Atlantic Ocean/Amapá-Pará, Brazil
6800
1*
2
41.0
Congo (Lualaba)
Atlantic Ocean/Angola, Congo
4630
10
3
34.0
yangtze or Chàng Jiang
East China Sea/Kiangsu, China
6300
3
4
30.0
Orinoco
Atlantic Ocean/Venezuela
2737
27
5
21.8
La Plata estuary (Paraná)
Atlantic Ocean/Argentina
3945
16
6
19.6
ganges (Brahmaputra)
Bay of Bengal/India
2510
23
7
19.4
yenisey (Angara, Selenga or Selenge, Ider)
gulf of Kara Sea/Siberia
5870
5
8
18.2
Mississippi (Missouri, Ohio, Tennessee, Jefferson, Beaverhead, Red Rock)
gulf of Mexico/Louisiana
6020
4
9
16.0
Lena
Laptev Sea/Siberia
4400
11
17
9.7
St. Lawrence
gulf of St. Lawrence/Canada and United States
3060
21
36
2.8
nile (Kagera, Ruvuvu, Luvironza)
Mediterranean Sea/Egypt
6690
2*
*Measurement in 2007 places the Amazon first in length; see Figure 15.1.