Regions 15These flows emphasize that the city is also both a source and a sink. The
city’s economic activity may depend on the flows of commuters, and its semi-
natural parks and greenspaces depend on continued native species dispersing in
from outside. Put another way, a city is swamped by commuters, bathed by out-
side air pollutants, protected from flooding by suburban wetlands, nourished by
market-gardening food products, and enriched by outside recreational opportu-
nities. And of course the outer cropland/woodland zone is also a source and sink.
These numerous flows and movements among city, suburbs, and surroundings
represent a regional system with many feedbacks. An urban region is eternally
working.
An outward expanding city pushes these flows outward. However, the areas
and the spatial arrangement of city, suburbia, and cropland/woodland also
change. A larger city means bigger inward and outward flows. If the suburban
landscape noticeably widens, its inward and outward flows also increase, but the
linkage between city and outer cropland/woodland ring becomes more tenuous.
People in the city are further divorced from natural and agricultural landscapes.
When an outer cropland/woodland area shrinks significantly, the flows do too.
In effect, outward urbanization and its spatial arrangement become critical in
determining how the urban region of the future will work.
The change in width of suburbia and the spatial pattern of urbanization
point to another important little-analyzed role of the suburban landscape. It
serves as afilter,selectively reducing flows between city and cropland/woodland
outside. Wind may blow agricultural dust inward and city air pollutants out-
ward, sometimes unaffected by suburbia. But streams and rivers that flow from
outer areas into the city pass through suburbia. Suburbs may have a high or low
impermeable-surface cover (Arnold and Gibbons1996,Formanet al.2003). With
considerable hard surface, much rainwater is added to the streams, increasing
flood hazard in the city ( Jared2004). Alternatively, ample wetlands and other
natural vegetation in suburbs absorb rainwater, helping to protect the city from
flooding.
Moving in the opposite direction, city residents crowd highways on week-
ends to recreate in outer woodland areas. Narrow commercial suburban high-
ways squeeze the traffic flows. However, providing a richness of small recre-
ational locations across suburbia that attract and are used by city residents
would reduce the congested inward and outward weekend flows. As for any fil-
ter, thedegree of filtering depends on the amount of input and the prevention
of clogging. The suburban landscape varies in width and the greatest filter-
ing may occur in the widest portions. In short, adjustments in the suburbs
and in channeling urbanization spread can make an urban region work much
better.