The Benthic Index of Biotic Integrity (B-IBI) describes the biological condition of stream sites and their surrounding habitat based on the diversity and relative abundance of the benthic (bottom dwelling) macroinvertebrates found at the site. Ten measures of biological condition are scored and summarized as the B-IBI, which ranges from a score of 0, indicating very poor stream condition, to 100, indicating excellent condition.

• Stream conditions are mixed across Puget Sound. The biological condition category at 718 sites ranged from excellent or good (41 percent) to fair (22 percent) to poor or very poor (37 percent).
• Over time, most B-IBI scores have either improved or remained stable. Analysis of trends at 188 sites with at least 10 years of monitoring data show that B-IBI scores improved at 22 percent of sites and declined at only 2 percent of sites. Scores at three-quarters of sites (76 percent) have not changed significantly over time.
• Overall, these trends are encouraging. While development has increased in the region, stream health appears to have improved in more than one of five streams and declined in only one of 50 streams. This may be due in part to stream and riparian restoration as well as historic and ongoing actions that aim to control and treat stormwater (see the Stormwater Strategic Initiative for more information).