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I Ie I I I I I I I . I I I I I I I I' I I - II discharges in the main channel are reduced, sediments from the tributary that previously were eroded will no longer be carried away and serious aggradation with accompanying flood problems may arise. An insight into the direction of change, the magnitude of change, and the time involved to reach a new equilibrium can be gained by studying the river in a natural condition; having knowledge of the sediment and water discharge; being able to predict the effects and magnitude of man's future activities; and applying to these a knowledge of geology, soils, hydrology, and hydraulics of alluvial rivers. The current interest in ecology and the environment have made people aware of the many problems that mankind can cause. Previous to the present interest. in environmental impact, very few people interested in rivers ever considered the long-term changes that were possible. It is imperative that anyone working with rivers, either with localized areas or entire systems, have an understanding of the many factors involved, and of the potential for change existing in the river system. Two methods of predictinq response are employed. They are the physical and the mathematical models. Engineers have long used small scale hydraulic models to assist them in anticipating the effect of altering conditions in a reach of a river. With proper awareness of the large scale effects that can exist, the results of hydraulic model testing can be extremely useful for this purpose. A more recent and alternative method of predicting short-term and long-term changes in rivers involves the use of mathematical models. To study a transient phenomenon in natural alluvial channels, the equations of motion and continuity for sediment laden water and the continuity equation for sediment can be used as discussed in Chapters III and IV.