About gravel in rural versus urban streams
Urbanization of streams occurs when natural biota,
soils, sediment and topography are altered through human activities
such as deforestation and devegetation, and construction of buildings,
roads, parking lots, etc. Urbanization can change the hydrologic character
of streams in many ways. Because of increased amounts of impervious
surface area (such as parking lots), water that falls in the drainage
basin is often transported more rapidly to the stream, and peak discharge
of the stream during rainfall events is often much larger than normal.
Increasing the peak discharge can have profound effects on sediment
erosion, transport and deposition, changing the habitats and ecology
of the stream. Understanding the role of urbanization is important
if we want to mitigate negative effects of urbanization and restore
urban streams to a more "natural" condition.
Pizzuto et al (2000) have studied 8 pairs of rural
and urban streams from southeastern Pennsylvania in an attempt to
quantify differences between these two types of streams. The pairs
of streams, not adjacent geographically, were chosen because of their
similar drainage basin area (similar size catchment). Their study
indicated that "urban stream channels ... are wider, straighter
and smoother than their rural counterparts." They also demonstrated
that the urban streams were not extensively scoured or cleared of
their coarse bed load, that urban streams still had plenty of bed
gravels.
Pizzuto et al also presented an analysis of the
size distribution of gravel particles (not including sand) from the
beds of 1 rural and 1 urban stream (see the table and figure). Note
that except for the smallest bin (2-8 mm), all the bin widths are
"multiples of 2" (this is the standard way of representing
particle size data in geology). The particles analyzed ranged from
pebble size (2-64 mm) to cobble (64-256 mm) to boulder (>256 mm),
and the ranges for the two streams were very similar. The median of
the particle sizes was essentially the same for both streams (about
30 mm). Both streams show a similar shape to the distribution of gravel
sizes.
The only consistent difference between these two streams was the
presence of a secondary mode of medium sized pebbles in rural streams
(see graph). They found the same thing with almost all of the other
streams analyzed. Pizzuto et al concluded that particles of this size
are depleted from urban streams. Why are these particles missing?
[I can think of two radically different answers to this question].
How might the absence of these particles impact the ecology of the
stream bed? Can the absence of this secondary mode be used as a indicator
of the degree of urbanization?
Reference: Pizzuto JE et al (2000), Comparing gravel-bed rivers in
paired urban and rural catchments of southeastern Pennsylvania; Geology,
v. 28, # 1, pp. 79-82.
See also Hollis GE (1975), The effects of urbanization on floods
of different recurrence interval; Water Resources Research, v. 11,
pp. 431-435.
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