Patterns of anabranching channels: The ultimate end-member adjustment of mega rivers

Large fluvial systems adjust to a combination of controls to form distinctive channels, which represent a dominant factor in the evolution of floodplain geomorphology and sedimentology. Fluvial geomorphology has commonly classified river channels into meandering, straight and braiding patterns, which are seen to represent a continuum of channel geometry. Anabranching patterns, rivers with multiple channels, however, are characteristic of many rivers. The identification of a combination of variables that discriminates specific channel patterns has been a significant focus of research in fluvial geomorphology. The development of this body of knowledge, however, has been established from medium and small rivers, and laboratory flume studies. Very few of these research ideas developed from analysis of large fluvial systems.This paper assesses the pattern of channel adjustment of large fluvial systems by employing hydraulic geometry, discharge, w/d, slope, grain size, stream power, specific stream power, and Froude number (Qmean > 1000 m3/s). The study demonstrates that methods currently used to discriminate channel patterns are not useful when applied to very large rivers. Further, with the exception of the Lower Mississippi, alluvial rivers with mean annual discharges greater than ~ 17,000 m3/s, here classified as mega rivers, do not generate single thread meandering or typical braided patterns. These mega rivers develop anabranching patterns.