Urbanization and the Disruption of Matter and Energy Flows within Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 424

additional surplus inputs during this period, the organic surplus is reduced and then fol-
lowed by a decline in the numbers of organic-consuming organisms. The system returns
to equilibrium. Positive feedback occurs when the outcome of a process within a system
speeds up or magnifies the system's activity or work output. Anthropogenic measures
designed to modify a stream channel often initiate positive feedback. When a dam for
sediment control is placed in a stream, the downstream segment of the stream becomes
hungry for sediment. Since a stream's equilibrium depends largely on the amount of water
and sediment it transports, the stream hunts for more sediment. Downstream of the dam
the stream initiates the erosion of its own banks. This outcome sometimes prompts engi-
neers to cover the natural streambanks with concrete. Besides the ecological insult to the
bankside habitat, this measure reduces the stream's width, and lower volumes of runoff
become sufficient to trigger a flood. It can get nastier. Positive feedback may form a loop,
with each successive iteration producing a more unstable condition. For these reasons,
identification of the existing and potentially destructive positive feedbacks imparted by
humans on watersheds is a necessary step for achieving sustainable watersheds. In urban
watersheds, this is especially challenging because of their complexity and the large quan-
tities of energy and matter humans contribute.

12.4 Urbanization and Watersheds: Disruption
of Matter and Energy Flows
Urbanization encompasses the processes contributing to urban growth, especially an
increased number of people coming to the cities to live and the subsequent city expan-
sion through the annexation of surrounding land and adjacent communities (Rugg 1972).
Globally, the trend toward urbanization is increasing. By 2030, nearly two-thirds of the
world's population will live in urban areas (Figure 12.6). In North America, the degree of

Percent urban
1950

87

84

2007 2030

79

78

76

72

64

60

54

51

51

49

42

41

37

29

17

15

Latin Am./
Caribbean

North
America

Africa Asia Europe

World

FIGURE 12.6
World urbanization trends 1950-2030. (From United Nations, World Urbanization Prospects: The 2005 Revision ,
New York, 2006; Haub, C., World Population Data Sheet , Population Reference Bureau, Washington, DC, 2007.)