Up an Urban Creek: The Role of Development Patterns in Stream Health

Abstract

Population growth in urban and sub-urban areas imposes increasing pressure on stream ecosystems. The "Urban Stream Syndrome" explains the observed negative trends in stream conditions associated with increasingly urbanized watersheds. Urban planners are challenged with addressing increased growth and development while also protecting critical areas such as streams and rivers that provide many services to urban dwellers as well as crucial habitat to fish and wildlife. The purpose of this study was to identify which patterns and characteristics of urban development play a role in stream health and might be managed by local planners. Available data for an index of macroinvertebrate communities, the Benthic Index of Biotic Integrity (BIBI), were used to assess stream health in twenty-two moderately-urbanized drainage basins (13 to 15 percent impervious area) in the Puget Sound lowlands. Land cover patterns in the basin and the riparian area (100 m) were calculated using Fragstats v4 and correlated with stream BIBI score and the metrics that comprise the score. I assess the correlations between infrastructure intensity (road density, number of road crossings, and number of stormwater outfalls) and these response variables. The selected landscape metrics did not explain the variability in the BIBI score. The values in this study may have been within the natural range of variation of the BIBI, and possibly the index only provides a coarse level of information about stream condition. Urban patch density was positively correlated to intolerant taxa richness. Number of road crossings and number of stormwater outfalls were negatively related to intolerant richness. No metrics were related to riparian land cover composition or configuration. These findings show that more dispersed urban land are associated with less disturbance in macroinvertebrate assemblages, as well as fewer road crossings and fewer stormwater outfalls. Planners interested in maintaining stream biological integrity should focus on maintaining patchier development patterns, improving road/stream crossings, and shifting from traditional stormwater infrastructure to more natural drainage systems.