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I Ie I I I I I I I .. I I I I I I I ,. I I - 7 has resulted from lateral tilting. Major shifts in position of the Brahmaputra River toward the west arc attributed by Coleman (1969) to tectonic movements. Hence, neotectonics should not be ignored as a possible cause of local river instability. tconq-term _ climatic _ fluctuations have caused maior cha~_~rives !11orpt}Q!Qgy, Floodplains have been destroyed and reconstructed many times over. The history of semi-arid and arid valleys of the western United States is one of alternating periods of channel incision and arroyo formation followed by deposition and valley stability which have been attributed to climatic fluctuations. It is clear that rivers can display a remarkable propensity for change of position and morphology in time periods of a century. Hence rivers from the geomorphic point of view are unquestionably dynamic, but does this apply to modern rivers? It is probable that 9Urinq--,~iod of several years, neither neotectonics nor a proqressive climate chanqe will have a detectable influence on river character and gehavior. What then causes a river to appear relatively unstable from the point of yiew of the hiqhway enqineer or the environmentalist? It is the slow but implacable ~hift of a river channel throuqh erosion and deposition at bends, the shift of a ghannel to form chutes and islands, and the cutoff of a bend to form oxbow lakes. Lateral migration rates are highly variable; that is, a river may maintain a stable position for long periods and then experience rapid movement. Much therefore depends on flood events, bank stability, permanence of vegetation on banks and the floodplain and watershed land use. A compilation of data by Wolman and Leopold shows that rates of lateral migration for the Kosi River of India range up to approximately 2500 .feet per year. Rates of lateral migration for two major rivers in the United States are as follows: Colorado River near Needles, California, lD to 150 feet per year; Mississippi River near Rosedale, Mississippi, 158 to 630 feet per year. Archaeologists have also provided clear evidence of channel changes that are completely natural and to be expected. For example, the number of archaeologic sites of the floodplains decreases significantly with age because the earliest sites are destroyed as floodplains are modified by river migration. Lathrop (1968), working on the Rio Ycayali in the Amazon headwaters of Peru, estimates that on the average a meander loop on this rivJlr begins to form and cuts off in 5000 years. These loops have an amplitude of 2 to 6 miles and an average rate of meander growth of approximately 40 feet per year.