Changes in Braided River Morphology Resulting from Aggradation and Degradation

Abstract

Forty-three experiments were completed in a large flume (1.8 X 18.3 m) to examine the effects of sediment load, sediment size, and channel gradient on braided-river morphology. The laboratory results were supported by field research and analogs from the published literature. Subaerially exposed braid bars were formed by the dissection and further accretion of stalled linguoid dunes, which are actively migrating bedforms. Bar-forming processes and channel dynamics were the same in both sand-and gravel-bed channels. However, linguoid dunes stalled and were transformed into braid bars more frequently in gravel-bed channels, and therefore, gravel-bed channels were more intensely braided. Aggradation resulted in the development of more bars rather than in an increase in the size of preexisting braid bars. The number of actively migrating linguoid dunes also increased with sediment load in the highly mobile sand-bed channels. Conversely, in degrading channels, the number of braid bars decreased and average bar size increased due to the coalescence of smaller braid bars. The degrading gravel-bed channels remained braided as the channels degraded, whereas rapid incision in the upstream portion of sand-bed channels resulted in the development of a single channel flanked by terraces. The downstream reach remained braided as a result of continued delivery of sediment from the incising reach. Rapid incision upstream and continued deposition downstream resulted in the development of a greater number of inset erosional terraces upstream, which correlate temporally with higher and younger terraces downstream.