Sediment Transport and Morphological Change in a River Meander without Vegetation

It has long been assumed that vegetation plays a key to keep outer bank erosion synced with inner bank accretion, so as to facilitate meandering of streams. By extension, it is presumed that prior to the evolution of vascular land plants, meandering streams should have been uncommon on Earth because no plants stabilized bank sediments. Nonetheless, evidence of well-developed fluvial meanders on Mars indicate that the process can occur without the help of stabilizing plant cover. This observation suggests that we do not fully understand the fundamental cause of meander development in rivers. By virtue of cohesive forces, mud can provide strength, but the mechanism of its initial accretion and the relative abundance of mud versus sand in bank deposits is poorly known. The Quinn River in Nevada (slope = 0.00015, bankfull width = 19.8m), an actively meandering river with sparse vegetation on the floodplain, provides an opportunity to explore these issues in depth. The river cuts through silt and clay deposits of paleo-lake Lahontan, which generates a muddy load that combines with fine sand from upstream sources. During moderate discharge we mapped bed topography and measured velocity fields and suspended sediment concentration profiles in three sections of a sharp river bend. Flocculated clays and silt accounted for 90% of the suspended load. A brief rain storm caused turbidity to double and led to inner bank deposition of mud flocs that settled and traveled as ripples. Upon deposition and accretion this sediment gained significant shear strength. Stratigraphic sections across the inner bank show that lateral accretion deposits consist of alternating sand-rich and mud-dominated interbeds. On close inspection, however, the latter contain abundant coarse silt to fine sand size mudstone fragments, suggesting that these beds may potentially reflect bedload transport. To explore flow and deposition processes at the observed discharge and at flows that rise to overbank conditions, we are testing the utility of Delft3D in explaining our observations. Delft3D treats mud and sand transport calculations separately, yet we find all mud deposits are sandy, and all sand deposits are muddy. The intermingling of fine grained aggregates, flocculated muds, and sand grains appears to be the underlying reason for cohesion of deposits.