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<br />The risk that these sources may contribute sediments to Willow Creek varies based on many <br />variables including vegetative cover, slope, the degree of disturbance, and the distance to the <br />Willow Creek channel. In the GIS, these watershed characteristics were combined spatially <br />using a mathematical manipulation to create a map indicating the potential for areas to contribute <br />sediments to the Willow Creek stream system. Sediment sources that may be critical to protect <br />are indicated on the map. <br /> <br />Sediment Transport and Debris <br />The channel bed in the majority of the study reach is "armored". This armoring will tend to <br />resist hydraulic stresses under typical flow conditions, but will be broken down and transported <br />as "bedload" during high flood flows. In a steep mountain stream such as Willow Creek, <br />bedload transport (rather than suspended sediment transport) is the channel-defining process. <br /> <br />Shear stress data from the HEC-RAS model and approximate field data of sediment size were <br />used to calculate a potential bedload transport rate by sediment size fraction for every cross- <br />section in the HEC-RAS model for the various flood flow levels. Potential bedload rates are <br />very high in Willow Creek and East Willow Creek and rather extreme in the steepening section <br />of West Willow Creek to the Commodore Mine. <br /> <br />The bedload transport rate in tons per hour was converted to a total weight or volume of <br />sediment transported during a flood event using very approximate assumptions about the <br />duration of flood events. Willow Creek has the potential to produce enormous volumes of <br />bedload during high flow events. Man has probably increased sedimentation rates in the area, <br />and this bedload potential must be considered to protect hydraulic structures as well as control <br />channel stability and flooding. <br /> <br />Many areas of the Willow Creek channel were noted to have significant amounts of timber <br />debris. This timber debris will be easily mobilized during small flood events and could easily <br />plug culverts or other structures. Several locations where timber debris has collected were noted <br />in the text. Much of this debris could be removed from the channel relatively easily. The area <br />below the Commodore Mine has an enormous amount of deteriorating mine cribbings and other <br />timber debris. Timber removal in this area could be more difficult, but could be a very important <br />measure to lower the risk of flood damage. <br /> <br />IV <br />