Abstract

Anastomosing channels form the characteristic stream pattern of the Riverine Plain of southeastern Australia. Investigations of the Ovens and King Rivers in northeastern Victoria show that the individual channels and reaches of channels vary markedly in channel morphology and age. Young channels have large bankfull flows, low sinuosity, large meander wavelength, steep gradients, and are unstable. They are incised channels that are rapidly eroding their bed and banks. Old channels, however, have small bankfull flows, high sinuosity, small meander wavelength, gentler gradients, and are relatively stable. The multiple channels develop by avulsion. As individual channels become older and more sinuous, they also become more hydraulically inefficient. Thus, increasing proportions of flood discharge are displaced overbank. This overbank flow concentrates in relatively straight flood-plain depressions, and it erodes a new channel, which develops by both up- and down-valley incision. Down-valley incision is initiated at outflow points and up-valley incision is initiated where the overbank flow rejoins the main channel. With time, both channel segments join to form a new anabranch of the anastomosing-channel system.

Large anastomosing channel networks have been identified in ancient rocks by stratigraphers and sedimentologists. On the basis of studies of the anastomosing Ovens and King Rivers, it is very likely that these channel networks are composed of channels of different ages; hence, even at one stratigraphic level, not all of the anabranches functioned simultaneously. Nevertheless, as a result of anabranch incision, which intersects older channels at depth, a three-dimensional network of paleochannels can form that permits fluid flow throughout the network.

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