Presentation Title

Urban impacts on the biological health of Portland streams

Abstract

The ecological health of Portland stream communities varies widely. Fish and bugs score poorly against regional benchmarks, yet there are clear signs of ecological values that persist in the face of urbanization. The ten most commonly encountered fish are native, two of the five most common are salmonids, and bug communities in some streams within Forest Park meet or approach values expected of regional reference streams. A set of upland and instream indicators of urbanization were used to investigate the key drivers of stream health in Portland. Macroinvertebrate health was most strongly related to canopy cover, particularly within the riparian zone, and stormwater indicators (impervious and zinc) accounted for additional variability in models with riparian canopy. Culverts were the strongest predictor of fish communities: there were no fish present in 45% of the reaches above impassable culverts. In contrast, fish were always found in reaches above fully- or partially-passable culverts. Evaluating additional factors beyond culverts was hampered by 1) the strong effect of culverts, 2) confounding effects of land use (the best habitat in Portland is often located above impassable culverts), and 3) a strong watershed effect – the Columbia Slough, Johnson Creek and West Hills streams have very different fish communities, and likely always did. Next steps to advance the findings beyond these analytical roadblocks include accounting for spatial autocorrelation, improving impervious indicators, applying causal inference, and using inverse distance weighting to better account for proximity of land use impacts. Initial results suggest accounting for spatial autocorrelation is particularly important.

Subjects

Land/watershed management, Water quality, Habitat assessment

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Urban impacts on the biological health of Portland streams

The ecological health of Portland stream communities varies widely. Fish and bugs score poorly against regional benchmarks, yet there are clear signs of ecological values that persist in the face of urbanization. The ten most commonly encountered fish are native, two of the five most common are salmonids, and bug communities in some streams within Forest Park meet or approach values expected of regional reference streams. A set of upland and instream indicators of urbanization were used to investigate the key drivers of stream health in Portland. Macroinvertebrate health was most strongly related to canopy cover, particularly within the riparian zone, and stormwater indicators (impervious and zinc) accounted for additional variability in models with riparian canopy. Culverts were the strongest predictor of fish communities: there were no fish present in 45% of the reaches above impassable culverts. In contrast, fish were always found in reaches above fully- or partially-passable culverts. Evaluating additional factors beyond culverts was hampered by 1) the strong effect of culverts, 2) confounding effects of land use (the best habitat in Portland is often located above impassable culverts), and 3) a strong watershed effect – the Columbia Slough, Johnson Creek and West Hills streams have very different fish communities, and likely always did. Next steps to advance the findings beyond these analytical roadblocks include accounting for spatial autocorrelation, improving impervious indicators, applying causal inference, and using inverse distance weighting to better account for proximity of land use impacts. Initial results suggest accounting for spatial autocorrelation is particularly important.