MORPHOLOGY IN URBANIZED STREAMS OF THE PUGET SOUND LOWLAND REGION
Boyle, Pamela Roxana
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Increased runoff from urbanization may result in erosion to the stream channel and banks, leading to channel incision, bed changes, loss of instream debris and habitat, and an overall reduction of heterogeneity and channel complexity. These impacts are especially evident in low gradient, gravel-bed, meandering streams - the major type of stream in the Puget Sound Lowland region. The failure of many stream restoration projects is due to a lack of understanding of how morphological features of a stream respond to hydrological changes. Single cross-section methods (instead of reach-level) are generally used and may not adequately portray the complexity, or variation, of the stream channel and bed. Three main hypotheses in this thesis are: 1) a single cross-section taken within a reach does not adequately describe a stream compared to a mean value calculated from several measurements; 2) urban streams with more urbanized drainage areas have higher shear stresses, and thus move larger bed particles and have higher reach mobility; and 3) urban channels have less channel complexity than non-urban channels. Results showed that a single cross-section may not adequately describe the morphological variables of a stream reach; however, this method may be appropriate for calculating reach shear stress. In addition, shear stress and mobility were not found to increase with increasing urbanization. Furthermore, complexity was not found to decrease with increasing urbanization. These two latter results indicate that urbanization (or percent imperviousness) alone cannot be used as a variable to investigate changes in stream morphology and hydraulics. In fact, a measure of sediment supply could be considered an additional independent variable by which to study urbanization impacts to streams. Substrate distributions from this thesis also support this finding.