Historical Changes in Pool Habitats in the Columbia River Basin

An historical stream survey (1934-1945) was compared with current surveys (1987-1997) to assess changes in pool frequencies in the Columbia River Basin. We surveyed 2267 km of 122 streams across the basin, representing a wide range of lithologies, stream sizes, land use histories, ownerships, and ecoregions. Based on pool classes inherited from the historical surveys, the frequencies of large (≥20 m2 and ≥0.9 m depth) and deep (≥20 m2 and ≥1.8 m depth) pools have decreased significantly (P < 0.01) since the 1930s. We classified streams as natural or commodity based on their watershed management and land use histories. Natural streams were in watersheds minimally affected by human activities (e.g., wilderness or roadless designation, limited entry), with only 12% having roads in riparian areas. Commodity streams were defined as having watersheds managed predominantly for extraction of resources via timber harvest, livestock grazing, and other human activities. Ninety percent of these streams had roads in the riparian areas. In natural streams, large- pool frequencies increased or remained the same in 96% of the streams (88% for deep pools). In commodity streams, large- and deep-pool frequencies decreased in 52% and 54% of the streams, respectively. Despite differences in stream size and the level of human activities, the magnitude and direction of these changes were consistent. Land ownership did not influence trends; pools decreased significantly on both private and public lands. Only where entire watersheds or headwaters were designated as wilderness or roadless areas did pools consistently remain unchanged or increase. Pool frequencies decreased in all ecoregions except the North Cascades ecoregion. We developed regional histories of human activities for the Columbia River Basin. Human activity histories were typically of low spatial resolution and available for broad geographic areas only; we rarely were able to obtain information at the scale of individual watersheds. Consequently, we were unable to test the relationship between temporal and spatial patterns in human activities and their influence on site-specific trends in pools. Despite our inability to isolate causal mechanisms, management emphasis and human activities clearly influenced trends in pools. We conclude that the persistent effects of human activities have simplified stream channels and reduced large- and deep-pool frequencies in watersheds outside of designated wilderness and roadless a