Historical changes in stream habitats in the Columbia River basin Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/4m90f090q

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  • Knowledge of how stream habitats change over time in natural and human-influenced ecosystems at large, regional scales is currently limited. A historical stream survey (1934-1945) was compared to current surveys to assess changes in poo1 habitats in the Columbia River basin. Streams from across the basin, representing a wide range of geologies, stream sizes and land-use histories, were used to evaluate habitat change. We classified streams as managed or unmanaged, based on their land-use histories. Managed basins were watersheds managed predominantly for multiple-use (e.g., timber harvest, livestock grazing) and unmanaged basins were minimally affected by human disturbance (e.g., wilderness/roadless areas). The quantity and quality of pool habitats increased or remained the same in unmanaged streams, and decreased in managed streams since the 1930s. Despite differences in stream size and land-use history, the magnitude and direction of theses changes were consistent. In addition, the decrease in pool habitats did not differ between public and private lands. Ecoregions were used to assess regional patterns to these changes. Our analysis showed that pool habitats decreased in all Ecoregions except the North Cascades Ecoregion. Regional land-use histories were documented for the study streams. The overgrazing of most rangelands had been documented by 1900. Grazing practices began to change after 1930, but current information suggests that while uplands have improved, riparian areas have not. By World War II, stream habitats had been affected by the loss of riparian vegetation, large woody debris, and aquatic habitats due to splash dams, log drives, and riparian timber harvest. Timber harvest expanded to the uplands after World War II, as the demand for timber expanded. Rapidly developing road networks increased runoff and sedimentation, which continued the impact of timber harvest on already damaged stream ecosystems. Almost 90% of managed streams had roads along the channel or within the floodplain. "Stream improvements," such as channelization and stream cleaning, also affected stream ecosystems. We concluded that the chronic and persistent effects of land-use practices had simplified stream channels and reduced habitat complexity in most managed watersheds in the Columbia River basin.
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