Urbanization and the Disruption of Matter and Energy Flows within Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 417

12
UrbanizationandtheDisruptionofMatter
and Energ yFlowswithinWatersheds
12.1 Introduction
The preceding chapters have emphasized that any effort to attain sustainable urban
watersheds must include a thorough investigation and understanding of the interactions
between geology and contamination. Since nature does not isolate geology and contami-
nation from other physical processes such as the hydrologic cycle, it is important to investi-
gate the linkages between geology and contamination to other processes occurring within
urban watersheds. This will help ensure when planning measures are implemented they
are informed by the broadest foundation of the applicable science.
Watersheds are physical systems, consisting of interconnected components performing
work over a defined geographic scale. Much of the work watershed systems accomplish
involves the transport of soil and nutrients by wind and water. The energy required for
these tasks comes from the sun and goes through numerous conversions. Before human
activities impacted watersheds, their balances of matter and energy were in dynamic equi-
librium. Occasionally, fluctuations occurred during floods and other disturbances, but,
over time, the streams within the watersheds returned to a condition where they flowed
along a gradient reflecting the balance between their erosion, transport, and deposition.
Over many years, an overhead view of the streams in this balanced state would reveal
local changes in response to natural fluctuations in sediment load and runoff—but overall
the channel would maintain its average morphology. A balance between the recharges
from precipitation and discharges to surface water also kept the aquifer systems in a long-
term balance.
With the advent of large-scale agriculture and urbanization, the flows of matter and
energy into and within watersheds changed dramatically. Sediment loads into urban
streams increased significantly as soil was dislodged from construction sites (Wolman and
Schick 1967), and floods occurred more frequently as urban development created more
impervious surface and greater volumes of runoff (Klein 1979). Water quality also declined
from the discharges of factories, urban and agricultural runoff, automobile exhaust, con-
tributions from contaminated groundwater, and atmospheric deposition. The severity of
pollution in North American streams was symbolized by the Cuyahoga River in Ohio
catching fire in 1969.
In the United States, the response to these problems of excess flows of matter and energy
and to the threats caused by pollutants has been significant. Erosion and sedimentation
controls have been implemented at the state and local levels. Developing property within
sensitive floodplain zones has become more difficult, and the Clean Water Act passed
in 1972 created an effective mechanism for controlling pollution discharges from point
sources. There is, however, a long way to go before our streams and aquifers achieve any-
thing close to the type of balance found in predevelopment landscapes.