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Fluvlel Proce._ . e A further distinction of note is the type of sediment being ttansported. 1\\'0 classifications of material type in ttansport can be made. They are the bed material load and the wash load. Wash load consists of the finer particles (usually silts and clays) which are washed through the channel under virtually all ftow conditions. Wash load has as a source streambank and upland areas within a basin. and is not fOlmd in appreciable quantities in the slream bed. Bed material load is the remainder of the sediment being ttansported. It consists of the particles typically found in the slream bed, which is the principal source of this sediment although the banks and upland areas contribute to the supply as well. Sediment may be ttansported as bed load or as suspended load. Bed load is defined as that portion of the load moving on or near the bed by rolling, saltation, or sliding. Suspended load, by definition. moves in suspension. Bed material load is ttansported both in suspension and as bed load. Wash load, on the other hand, travels only in suspension. Both the supply and transport capacity of a stream must be analyzed to quantify sedimentation. Quantifying supply involves conducting a sediment yield study, for which there are many methods. Only those based on direct field measurement are considered a rigorous approach; mathematical methods are trend indicators at best USACE EM 1110-2-4000 (1989) presents a discussion of the various methods. Transport capacity is typically computed mathematically using one or more sediment discharge fonnuJas. Selection of an appropriate fonnula for the sediment being investigated is critical to any reasonable quantification of the sediment load. Several of the references cited in this manual provide good guidance on the selection of these fonnulas. Remember that it is the bed material, not the wash load, which is critical to the channel morphology processes. This is the fraction of the load which must be assessed. 2.3.4 Aggradation and Degradation. The tenns aggradation and degradation generally refer to lrends in the location and behavior of the slream bed profile. An aggrading slream is one in which a progressive buildup or raising of the channel bed, ftood plain, or other surface is occuring due to sediment deposits. Degradation is a progressive lowering of the channel bed due to scour. True aggradation or degradation is usually associated with observed changes over time. not nonnal ftuctuations within a dynamic system. The fact that a reach of a river scours during a ftood event does not make it a degrading reach. Aggradation and degradation are a function of sediment yield, water yield, bed sediment size, and channel controls. Land use changes, climatic cycles, and impacts to sediment or water discharge (such as occurs with a reservoir) all affect bed level within a river system. These processes must be understood to evaluate the impacts of aggradation and degradation upon channel stability. e Quantification and prediction of aggradation and degradation are often accomplished through the use of sediment transport models. Other methods include periodic surveys of bed elevation to monitor change, and analysis of stage-discharge lrends. Care should Colorado Erosion Control Manual 9