AN ALTERNATIVE MEANDER MECHANISM FOR BRAIDED RIVERS

Geologists commonly understand that braided rivers meander, but they do not completely understand how this process occurs, which often differs from typical point bars of single-channel meanders. An examination of the Missouri River made evident that meanders created by traditional accretion of lateral point bars on the interior and meanders created by accretion onto the cut-bank side of the river occur just as frequently. Mid-channel bars migrate asymmetrically within the channel then collide with the outer loop allowing the addition of successive braid bars to the cutback side of the river thus creating backfilled or reverse meanders. The cutback braid channel then becomes abandoned, molding the bar to the floodplain and shifting the main channel inward a bar width. One could easily mistake a reverse meander deposit as a forward filled loop. Accretion of mid channel bars onto the cutback/outside of the loop creates a mosaic of amalgamated fusiform bar elements that contrast with the more arcuate scroll bars that typify a more familiar laterally accreting point bar. This distinction of bar growth is significant because it describes a mechanism for meandering previously not understood and that differs from the floodplain architecture of a “normal” meander loop fill and scour. Using maps and aerial photos from late 1800 until present, one can see a temporal occurrence of the process described above. Modeling shows that preferential flow on the non-accreting side of the bar causes a bar to amalgamate to the channel bank where sediment accumulates, thus leaving a reverse meander. The outward expansion or inward shrinkage of loops reflects consistent preferential accretion of bars through multiple accretionary events and effectively allows a braided river to meander across the floodplain.