Fish habitat restoration and interannual variability in intensively monitored watersheds

The Western Washington Intensively Monitored Watersheds (IMW) study was designed to assess the cumulative effects of extensive stream habitat restoration over long durations at ‘watershed’ geographic extents. We conducted annual surveys from 2004 through 2024 at hundreds of randomly selected reaches (“sites”) throughout 10 independent watersheds (“streams”) clustered into 3 regional complexes. Our objectives were to describe cumulative change in habitat measures that are common aims of fish habitat restoration and to test for differences between watersheds with different restoration efforts. In most cases, we did not observe the expected directional changes in habitat measures summarized to whole-watershed extents nor did we observe the expected differences relative to the level of restoration effort. However, we did observe high interannual variation in fish habitat measures. The magnitude of these year-to-year fluctuations was frequently much greater than observed long-term trends. Despite physiographic differences among study watersheds, all watersheds exhibited substantial spatial variation among sites in the degree of interannual variation, indicating a widely shared pattern of within-watershed heterogeneity in habitat stability. Our results underscore the importance of anticipating and accounting for year-to-year changes in stream habitat when assessing habitat conditions and planning and implementing fish habitat restoration.