REVERSE MEANDERS; AN ALTERNATIVE METHOD TO THE CREATION OF MEANDER LOOPS BY A BRAIDED RIVER

Mapping of the Missouri River floodplain near Nebraska City, Nebraska by TCU student mappers through the USGS EDMAP program reveal new insights into preservation patterns for meander loops occurring within braided rivers. Braided rivers are understood to meander commonly but the processes by which this occurs may differ from more typical point bars of single-channel meanders. While analyzing several stretches of river in the Langdon, Nebraska City, and Hamburg quadrangles it became evident that while some of the meanders are created by the more traditional accretion of lateral point bars on the loop interior, an equal number of them have been created by accretion onto the cut-bank side of the loop. These loops are filled by adding successive braid bars against the cutback side of the river thus creating backfilled or reverse meanders. Mid-channel bars migrating asymmetrically within the channel collide with the outer loop. The cutback braid channel then becomes abandoned, molding the bar to the floodplain and shifting the main channel inward. Deposits of these reverse meanders can be easily mistaken for forward-filled loops. Since the reverse meanders fill by accretion of mid-channel bars onto the outside/cutback of the loop, they are characterized by bar deposits that are a mosaic of amalgamated fusiform bar elements, as opposed to the more arcuate scroll bars that typify the more familiar laterally accreting point bars. This distinction of bar growth is significant in that it defines differing river processes and builds contrasting floodplain architecture.