Graduate Thesis Or Dissertation
 

The suspended sediment regimes of two small streams in Oregon's Coast Range

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  • The protection of water quality and the maintenance of productive anadromous fisheries is a primary concern in the Pacific Northwest. Excessive suspended sediment loads is a principle water quality problem on small wildland watersheds in this region (Anderson, 1971; Brown, 1972). Man's activities have been shown to increase sedimentation rates in some cases (Burns, 1970; Megahan, 1972). However, more research is needed to define the basic sedimentation processes and factors before adequate assessments of man's impacts can be determined on a broad basis. This paper presents the results of a study of the suspended sediment regimes for two small mountain watersheds located in Oregon's Coast Range. Suspended sediment concentrations in these kinds of watersheds are typically variable over short time spans. In- channel sources of fine sediment, particularly sediment stored in the bed gravels of armored stream segments, may be a major factor influencing the sediment regimes of these watersheds. A The primary objective of the study was to characterize the temporal variability in suspended sediment concentration on the two watersheds. In addition, nephelometric and gravimetric sampling procedures and the potential contributions of in-channel sources of suspended sediment were evaluated on the Oak Creek watershed. The temporal variability in suspended sediment concentration during storm events and on a seasonal basis was determined using intensive automatic and manual sampling procedures. Sieve analysis of bed material composition and channel profile measurements were utilized to define the potential availability of suspendable material within the channel systems. It was found that : (1) Stream bed gravels are a significant potential source area of suspendable material. (2) A decline in the suspended sediment concentration in the stream channel at a given flow occurs during the falling stage of individual runoff events and with successive events over the winter runoff season. This phenomenon can best be described as a flushing process, where the depletion of suspendable sediments may be associated with the successive release and capture of fine material by the bed armor layer. (3) Sampling of sediment concentration did not appear to be significantly influenced by horizontal concentration gradients. However, vertical concentration gradients, particularly in the transition zone between suspended load and bed load, did prove to be significant. (4) Basic soils and geomorphic parameters provided useful indexes for comparing the sediment regimes of these watersheds.
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