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2.4.2 - Morphological Description Willow Creek can be described using the geomorphic characterizations developed by Dave Rosgen. Upper Willow Creek between the Army Corp flume and cross-section 15 may have once been a B3 stream, although the entrenchment caused by the push-up levees may now cause the stream to function more as a G3 stream. Similarly, the Bachelor Loop road restricts the stream reach upstream of cross-section 15 to the confluence to a G3 stream functionality. West Willow Creek transitions to an A3 stream downstream of the Commodore Mine tailings pile, switches to an A2+ above the Commodore until cross-section 42, but then returns to an A3 stream. Although its slope levels begin most similar to a B3 or G3 stream, East Willow Creek is primarily an A3 stream. These characterizations are made over large reaches, but a number of different classifications could probably be identified on more of a bend-by-bend scale. The channel bed in the majority of the study reach is armored. Fine particles such as silts, sands, and even small gravels have been removed from the top channel bed layer, leaving an armoring of cobbles and boulders. Typical of other high-energy armored streams, many cobbles have become tightly packed and almost "locked together". This was particularly noted in cross- sections 30 to 35 in West Willow Creek. This armoring will tend to resist hydraulic stresses under typical flow conditions, but will be broken down and transported as bedload during high flood flows. Flood flows may even erode into the finer more heterogeneous alluvial material under the armored layer. 2.4.3 Sediment Transport Riverine sediment transport is divided into two general processes. Stream energy leading to buoyancy effects can transport fine sediments such as silts and fine sands in suspension. This is known as suspended sediment transport. Alternatively, high shear stress working against the stream channel can mover larger particles. Once a threshold (critical shear stress) is reached, larger materials such as gravels, cobbles, and even boulders can begin to roll and bounce along the channel bottom. This process is known as bedload transport and the amount of material being transported is simply known as the bedload. In a steep mountain stream such as Willow Creek, bedload is the channel-defining process. When undisturbed, fine particles in the channel bed are washed away and a layer, or armoring, of larger cobbles will protect finer particles deeper in the bed. Fine particles from disturbed areas or from further upstream may be transported in the stream, but will generally wash through the system without depositing. These suspended fine sediments are definitely a concern for their effect on water quality. Therefore, any potential sources of fine sediments should be controlled. However, significant channel instabilities, deposition, and damage to hydraulic structures will be controlled by bedload transport of larger materials. Therefore, the primary transport mechanism examined in the current study is the bedload transport of large materials. Bedload transport is a very complex and difficult process to quantify. Numerous equations have been developed since Duboys developed a bedload equation in 1879, and results from these equations can vary widely. The equations are empirical in nature and were often developed to match conditions in the particular set of rivers being considered and may not be accurate when applied to other rivers. Unfortunately no single equation has proven to be the "best", and results from all equations are considered to be good estimates rather than highly accurate quantifications. 2-45