Reverse meanders, pseudo point bars, and the enigma of meandering in braided rivers

Abstract

The mechanisms by which braided rivers meander are enigmatic in systems that lack defined point bars. Examination of the Missouri River floodplain reveals that meanders form by accretion of mid-channel bars onto the side of the river as an alternative to recognized accretion of lateral bars on the river bend. Mid-channel bars migrate asymmetrically within the channel then collide with the river bank. Successive accretion of mid channel bars onto one bend creates a mosaic of amalgamated fusiform bar elements with a bounding river loop that resembles point bars but lacks well developed scroll bars. This accretion process may be observed using maps and aerial photos from late 1800 until present. Preferential flow on the bank side of the bar can backfill loops causing a bar to amalgamate to the channel bank and forcing the river to meander backward, allowing the river to migrate multiple directions. This distinction of bar growth is significant because it describes a mechanism for meandering previously not reported.