ESTIMATING SEDIMENT PRODUCTION IN A LARGE WATERSHED / Évaluation de la production en sédiments d'un grand bassin versant |
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| Authors: | W E BULLARD |
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| Institution: | Watershed Management Water Supply and Pollution Control Program, PHS , Portland, Oregon |
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| Abstract: | Abstract Estimates of average annual sediment production in the 4,560 square mile Umpqua River basin in southwest Oregon were developed to assess impacts of land condition, use, and management in the watershed on quality of the water produced. Specific attention was given to channel bankcutting, scour, and deposition; to signs of erosion on the land, occurrence of landslides, and effects of cultivation, timber harvest, grazing, and road construction. Sediment production was estimated and ratings given each tributary drainage in five classes: slight, low, moderate, high, and very high; corresponding to average sediment loads of less than 50 ppm, 50 to 150 ppm, 150 to 500 ppm, 500 to 1500 ppm, and over 1500 ppm carried by the average annual flow. Areas and rating were checked against measured sediment loads, allowing for the fact that the sampling did not include the infrequent high flows which carry a disproportionate share of total sediment load. Information on geology was available for half the basin; on soil characteristics for a fifth of the area, including some of the forested mountain land. Aerial photo coverage was complete. Suspended load sampling at ten stations, done at two-week intervals for the preceding five years, furnished a fair indicator of sediment movement. Soils and geology information included erodibility and slump and slide hazard indices, and was used to guide field ratings and to extend them to areas not visited. Aerial photographs were used for extrapolation to areas for which soils and geology had not been mapped. |
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