2019 AAPG Annual Convention and Exhibition

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Is Braided vs. Meandering a Valid Distinction?

Abstract

The idea that some rivers are meandering and some rivers are braided is one of the more enduring concepts in geology. The implication is that these are distinctive river types. Recognition of transitional forms has pushed most workers into an amended view that these patterns are stations in a continuum. What if, however, a clearly braided river was to meander? The pre-channelization Missouri River is a fully meandering river that is also a pervasively braided river. One 84 km reach is unchannelized and still retains this braided meandering character. The Missouri River generates highly sinuous meander loops by attaching mid-channel bars to the inside point of loops to generate assemblages of compound bars. This differs from single-channel meandering rivers, like the Mississippi River, that attach unit bars to the inner accretionary bank to generate compound point bars. Both accretion of mid-channel and unit bars to the inner point of a loop expands meanders. Both single channel and braided rivers can thus meander. While the braided meandering pattern of the Missouri River is unusual, it is not unique. This means that meandering and braided patterns are not mutually exclusive patterns. Both single-channel and braided rivers thus can be straight or sinuous. Meandering in both cases is a means for lowering channel slope relative to valley slope to balance stream power with river load. Braided streams require this compensation less commonly than single-channel rivers as they can also adjust friction by widening and adding anabranches. Braided meandering streams are thus less common, and most braided rivers are straight to low sinuosity. That they do exist though means that the distinction between braided and meandering is not a valid distinction, though it has endured as a practical distinction.