Biological indices indicate stream conditions in King County, WA have improved over the last twenty years, despite increased development and warming stream temperatures. To help explain these surprising yet encouraging trends, we used a new thermal stressor index and improved estimates of thermal preferences that Shannon Hubler and others developed for freshwater macroinvertebrates in the Pacific Northwest. We found macroinvertebrate taxa that are increasing in density across lowland streams are more tolerant of warmer water than those that are not increasing or spreading. In addition to providing insights on how macroinvertebrate communities may be responding to warming streams, the thermal stressor index and improved taxon-specific thermal preferences can be used to help identify stressors and prioritize streams for targeted restoration actions. The thermal stressor index can also be used to biologically validate modeled stream temperatures, including providing biological evidence that temperatures may be too warm for certain macroinvertebrate taxa and salmonids.