Graduate Thesis Or Dissertation
 

Turbidity in Prineville Reservoir as related to soils and hydrology

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/bk128f042

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  • Persistent levels of high turbidity in the outflow from the Prineville Reservoir led to development of relationships between watershed soils, land use, and resource management and water quality in the Upper Crooked River, Central Oregon. Seven stations, strategically located for measurement of runoff volume and water sample collection, were established late in 1972. Six stations were on tributaries to the Crooked River; the seventh was on the main stream above the Prineville Reservoir. During runoff events, suspended sediment samples were obtained on each tributary above the confluence with the Crooked River. Greater stream sediment loads were associated with tributary watersheds under shrub-grass-juniper cover that have steeply rolling, dissected terrain with common rock outcrops. Those watersheds also have dominantly medium-textured, moderately deep soils. Steeper gradients are associated with these watersheds. Streams contributing high sediment loads were not necessarily those found to cause the long-term turbidity. This phenomenon was usually associated with those watersheds having a preponderance of soft, tuffaceous sedimentary rock. The results indicate major runoff events carry the greatest sediment loads to the reservoir and cause considerable turbidity. However, some turbidity-causing material is transported during smaller events as well. Freshets on watersheds with erodible soils cause turbidity during the convective storm season. Turbidity values in streams decreased between storm and runoff events, especially on forested watersheds. Sensitive watersheds (Camp Creek, TomVawn and Eagle Creek), with easily eroded soils,and specific reaches of the main channel, supply disproportionate amounts of material, causing long-term turbidity. X-ray diffraction analyses of suspended sediments, and the clay fraction of soil samples showed a predominance of smectites. Significant amounts of amorphous material were associated with the smectites in samples capable of creating long-term turbidity in the Prineville Reservoir. Field reconnissance of watershed land use and management indicated situations where domestic animal grazing, timber harvest and associated road building, and fire prevention practices which may unduly contribute to erosion and turbidity problems. Primary consideration of soil-hydrology relationships in land use planning and management is needed to reduce the severe erosion observed on certain tributaries. Particular attention should be directed toward minimizing erosion on soil with characteristics associated with long-term turbidity. Reservoir management and control should include recreational boating regulations to reduce shoreline turbulence. Limitations on boat draft, engine size, and total use will aid in reducing summertime shoreline turbidity. Recommended conservation and stabilization practices include: 1. Riparian fencing with controlled access for protection of stream bank soils, channel, and vegetation from external forces; 2. Retention of felled junipers, and continued felling, to extend soil moisture for germination and growth of grass; 3. Selective stream channel clearing to reduce debris-caused dams; 4. More awareness of soil-hydrology interrelationships by management agencies and private owners to better protect against turbidity-causing practices; and 5. Range management designed to maintain and improve grass cover and range condition overall.
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