Spatial Effects of Urbanization on Physical Conditions in Puget Sound Lowland Streams

Abstract

Urban development threatens local and global ecosystems. In the Puget Sound region, urbanization has dramatically altered stream ecosystems by changing their flow regimes and their physical attributes. This study of urbanization effects on streams had three objectives: 1) to assess physical stream conditions within and among four watersheds spanning a range of urbanization, 2) to use a geographic information system (GIS) to comprehensively characterize the urban landscape within these watersheds, and 3) to relate in-stream physical conditions to the landscape conditions of each watershed. To address the first objective, I used a rapid stream assessment technique to document the condition of several physical attributes along the mainstem channels of the four watersheds. The assessed streams had considerable heterogeneity in physical condition. In order to rate the overall physical condition, six attributes were used as components of a multi-metric index, termed the physical stream conditions index (PSCI). The PSCI helped quantify relationships between stream and landscape conditions. Landscape conditions of each watershed were measured using several GIS-derived landscape metrics (the second objective). The final objective was to consider how the location and distribution of urban land might affect the degree of impact to a stream’s physical attributes. Physical conditions (as measured by the PSCI) were best explained by three of the landscape metrics: the quantity of urban land in that part of the watershed draining to the sampled site, the quantity of urban land within 500 m upslope of the sampled site, and the proximity of the sampled site to the closest upstream road crossing. A stream’s physical condition improved downstream from degraded reaches when the stream flowed through portions of intact forested riparian buffers devoid of road crossings. In sum, the results of this study suggest that if urban development can be built such that riparian areas are untouched, functioning stream reaches may be better preserved. Further, similar studies using GIS-based landscape analysis may quickly target rehabilitation efforts to stream reaches that have realistic opportunities for improvement.