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PARSONS The geologic history of the Murrumbidgee River, in a relatively and part of New South Wales, Australia, was deduced by Schumm (1968), who found that traces of old aggraded and abandoned river channels or paleochannels were present on the floodplain of the modern river and on the surface of the bordering plain. At least two generations of the ancestral river preceded the modern river. The modern Murrumbidgee River has a meandering planform, is relatively narrow and deep, and transports relatively small quantities of sediment. In contrast, the earliest paleoriver of which there is evidence on the Murrumbidgee plain was relatively straight, wide, and shallow. Later, the paleoriver came to more nearly resemble the modern river in planform, but was much larger than the modern river. Schumm (1968) ascribes these dramatic changes in fluvial morphology to the effects of climate changes on the hydrologic regime of the drainage basin. The earliest channels (wide, straight, and braided) appear to have been developed during a relatively drier period, when a reduction in vegetation cover would have allowed additional sediment to be moved from the floodplain, and less- frequent flood events, but possibly of greater magnitude than at present, would have transported relatively large quantities of sediment for short periods of time. A later, more humid climatic regime, corresponding to a threshold excursion, would have promoted vegetation growth, reducing the quantities of sediment available for transport, while simultaneously increasing the mean annual discharge. A climate of this type would have resulted in the development of a narrower, deeper channel having a more meandering planform. In contrast to the effects of past climatic change, anthropogenic regulation of the river in modern times, through construction of water diversion and retention structures, has caused relatively little change in the planform and dimensions of the channel, and the changes that have occurred have been neither progressive nor systematic. Schumm (1968) concluded that the modern channel of the Murrumbidgee River, though somewhat influenced by diversion and control structures, has responded primarily to climatic threshold excursions and erosional conditions in the drainage basin. The planform of the Platte River in Nebraska, and the processes active in shaping the river, were examined by Smith (1970 and 1971). Unlike many braided streams, particularly those associated with glacial features or located in glacial outwash plains, the Platte River in Nebraska carries a dominantly sand bedload. In streams carrying coarser - grained bedload material, especially those with poorly -sorted gravel beds, braiding is caused by the construction during period of high discharges of low, linear, mid - channel mounds. Formation of the mounds requires only that a stream at some point becomes unable to transport part of its coarsest load. The coarse sediment is deposited and traps additional sediment causing the mound to be built upward and in a downstream direction. The resulting deposit is elongate in the direction of the stream current, convex upward or slightly inclined on top, and it usually displays a pronounced fining trend in sediment gradation in the downstream direction. These mounds divide the channel into smaller branches as high flows recede and the mounds become exposed. Such linear mounds (called longitudinal bars by Smith [1970]) dominate the upper reaches of the South Platte River in Colorado, and in many other coarse -bed streams. In braided streams that carry well- sorted, sandy sediments, however, bars are more typically transverse, with wide, flat- topped tabular bodies and sinuous to lobate depositional fronts. Bars of this type characterize the braided reaches of the lower Platte in Nebraska (Smith, 1970 and 1971). Braiding in reaches of the Platte River in which sand -sized material comprises most of the bedload is an intermediate to low- discharge -12- S:\ES \WP\PROJECTS\3- States\A 1 Final Tech Memo.doc