Urbanization and the Disruption of Matter and Energy Flows within Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 423

physical systems operating within and between these spheres are present in watersheds.
For example, when soil erosion occurs after a rainstorm, the hydrologic cycle operating
within the hydrosphere interacts with the geomorphic system of the lithosphere. A geo-
morphic system is characterized by the removal, transport, and deposition of material by
wind or water.
The activity of individual systems and their interactions with other systems produces
work—the application of force over distance. In watersheds, most of the work is accom-
plished by the movement of materials through the motion of fluids (wind and flowing
water). On sloped terrain, gravitational force also performs some of the work. The work
produced at the watershed scale can also be thought of as their function. Erosion, there-
fore, is a function of watersheds, as is providing pathways and sinks for nutrients.
Performing work also requires the presence of energy necessary to create a force. Over
most of the nearly 4.6 billion years of Earth's history, nature has crafted its energy alloca-
tions in a sustainable way. Humans, however, have introduced extra energy into natural
systems. This is not surprising, for, unlike animals, humans must produce their means of
subsistence. The development of large-scale agriculture, manufacturing, and residences
represents a tremendous amount of energy. It is this extra energy input by humans into
the watershed system that causes malfunctions (Table 12.1).
Some of the key functions of watersheds are identified at the general level of systems
interaction shown in Table 12.1. Human inputs and the resulting malfunctions are shown
in the third column. The list of functions performed by watersheds expands significantly
if subsystems of the major systems listed in the table are considered. For example, when
the subsystems of precipitation and runoff interact within the hydrologic system, one of
the outcomes is infiltration, which has the function of providing soil moisture recharge.
Table 12.1 also demonstrates another general property of systems—feedback. Feedback
is an outcome of a process within a system that affects the overall function of the system.
Negative feedback results when an output from a process in a system slows down or
dampens the overall operation of the system. For example, if a surplus of organic matter is
input into a stream by a storm, more organisms with the purpose of decomposing organic
matter will come on the scene and work until the original balance is achieved. Barring

TABLE 12.1
Watershed Functions and Associated Malfunctions from Human Inputs

System Interaction: Outcome/Human Input Function/Malfunction

Climatic Ecologic: formation of biomes Provision of species habitat

Global warming—excess energy in the atmosphere Species extinction

Ecologic Hydrologic: Inputs of nutrients into streams Nutrient transport

Excess nutrients input into streams by wastewater
discharges

Cultural eutrophication

Hydrologic Tectonic/geomorphic: Formation of drainage

networks

Movement of water from
headwaters to mouth

Modification of stream channels (straightening,
deepening, and widening)

Flooding, unnatural channel
adjustments leading to more
artificial channel modifications

Tectonic/
Geomorphic

Climatic: uplift from magma plumes creates more
precipitation due to a higher elevation of the land
mass (this occurred with the East African rift)

Erosion, transport, and
deposition of surface materials
(rock and debris)

Building on unstable soil Accelerated erosion rates