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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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 Comparing rural and urban rivers in Pennsylvania. Particle size for river bed gravels given in millimeters. Frequency of particles normalized to total. Pizzuto et al (2000) rural urban size (mm) rel freq rel freq 2 to 8 0.11 0.01 8 to 16 0.19 0.11 16 to 32 0.12 0.18 32 to 64 0.17 0.25 64 to 128 0.24 0.33 128 to 256 0.14 0.11 256 to 512 0.02 0.01 >512 0.02 0.00

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