Use of a Biological Index to Identify Fine Sediment Related Macroinvertebrate Impairments in Oregon

The state of Oregon and the U.S. Environmental Protection Agency have identified 27 waterbodies in the mid-coast subbasins of Oregon as biologically impaired due to a detrimental change in the resident macroinvertebrate community. For these biological impairments the pollutants or condition causing this impairment was unknown. Our goal was to identify which impairments have fine sediments as a contributing pollutant. We utilized a biological sediment tolerence index derived by paired macroinvertebrate and fine sediment data at 446 sites across Oregon. We developed sediment stressor benchmarks from this index using classification and regression tree (CART) modeling to factor out natural gradients in the index values at 278 reference sites. The CART model produced five reference groups based on four predictors (stream power, precipitation, erodible lithology, elevation). The 90th percentile of reference residuals was used to identify test sites with index values beyond what was commonly observed at environmentally similar reference sites. We assessed 33 sites in the Oregon’s mid-coast watersheds and identified 18 sites (55%) having fine sediments as one of the likely causes for impairment.