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I I I I I I I I I I I I I I I I I I . deposit in the pools where velocities are low. No deposition of these materials will occur on the riffles because the flow is more rapid over the crest of the riffles and the transport capacity is higher than in the pool. As discharge is increased, the flow regime changes to a condition where the velocities in the pools are large enough to re-mobilize deposited sands. These transported sands will not deposit on the riffles because the transport capacity in these locations has also been increased. During hydrograph recession, and ensuing low flows, fme material in motion will redeposit in the pools. From this, it is clear that there are two cases of flow and transport of sediment in a pool-riffle steep mountain stream. The first case is for low flows where fine sediments are trapped in the pools between riffles. This finer material (usually sand) typically overlays a coarser material substrate found in the pools. At higher flows these sands are removed and transported as suspended load over a coarse bed, stable channel. The pools will infill with fine material as the discharge decreases. For this type of steep mountain channel, it is proper to classify it as a stable, supply limited channel, but not an "equilibrium" channel. During low flows, pool reaches may be in equilibrium or even have an excess of sediment supply, but the riffle reaches will have an excess transport capacity. At higher flows excess transport capacity can be observed in both the pools and riffles. 5.13 Terminolo~. Understanding the various types of sediment transport and their definitions is also important in the analysis of sediment transport. Sediment particles are transported by flowing water in one or more of the following ways: 1) surface creep or contact load (rolling or sliding along the bed), 2) siltation (movement in a cyclical manner involving periods of rest and movement, and 3) suspension. Sediments transported by surface creep and siltation are referred to as bedload, and those transported by suspension are called suspended load. Total sediment load is the sum of the bedload and suspended load. The bedload is typically 5-25 percent of the suspended load. The total sediment load may also be defmed as the sum of bed-material load and wash load. The bed-material load is the sum of the bed load and suspended bed-material load, where bed-material load is defined as that portion of the total sediment load composed of grain sizes found in the channel bed. In contrast the wash load is defmed as that portion of the total sediment load composed of particles finer than those found in appreciable quantities in the bed. Note that the wash load is typically supply limited due to the easy transport of fine material (e.g., silts and clays and in some cases sands and fme gravels - see below), while the bed-material load is typically limited by the transport capacity of the channel. Figure 5.1 illustrates the use of this terminology in the defmition of total sediment load. Depending on the objectives of a given study, different size fractions or modes of transport may be the focus of an analysis. For analysis of channel morphology, the material that forms the bed and banks (bed-material load) will typically be most important. If the focus is on turbidity, which is an optical property, then the wash load may have more bearing than the bed..materialload. For the purposes of this study the term sand load will also be used. The sand load is defined as the portion of the total load consisting of sand and fme gravels which is transported either in suspension or by rolling, sliding or saltation over the bed. The total load using this definition will then be the sum of the sand load and any transported silt/clay material. It should be noted that the wash load in gravel bed streams cannot be defined simply as the silt/clay sized particles. Rather, the wash load consists of those particles which move in suspension and are not found in appreciable quantities in the bed. Simons and Senturk (1977) proposed defining the limiting grain size between wash load and suspended load as the d10 of the bed material. With this defmition of wash load and using the gradation curves for the bed material presented in Chapter 3.0, the division between wash load and suspended bed material load would be between 2-10 mm for the Yampa River in Yampa Canyon. This is much coarser than 0.64 mm which is usually considered the division between wash load and suspended bed material load for sand bed streams. 5-3