MUD MEANDER MORPHODYNAMICS ON EARTH AND MARS (Invited Presentation)

Recent studies in arid environments have demonstrated that active meandering can occur in rivers lacking vegetated banks, if formative flows contain a sufficient silt and clay (mud) component. Such rivers may constitute an analog to sinuous paleochannels on Mars. The Quinn River in northwestern Nevada is one such mud-dominated system exhibiting long-term meandering involving extensive lateral floodplain reworking and frequent cutoffs. The river flows across the lakebed of the Pleistocene Lake Lahontan. Reworking of the lacustrine deposits supplies the mud component of the sediment supply to the Quinn River. Only fine sand is delivered to the river from the upstream watershed. The steep inner bank and point bar deposits are dominated by centimeters-thick layers ranging from nearly pure mud alternating with layers containing more than 50% sand. The interplay between the locally-derived mud and the upstream-sourced sand appears to underpin the meandering behavior and to modulate floodplain deposition.

In the Spring of 2017, we measured the velocity field and suspended sediment at one cross-section and the velocity field through a sequence of cross-sections through a tight bend. A rainstorm during the field work generated local runoff, raising the concentration of the flow, but not significantly the discharge. Migrating ripples of flocculated mud developed along the inner bank of a bend. This revealed that bands of mud-rich layers may record temporal change in mud input from local sources. To explore the conditions leading to inner bank mud accretion, we modified (in Delft3D) the use of standard cohesive entrainment and deposition models by assigning flocculated settling velocities to the mud and by including the sand fraction with the cohesive entrainment (as mud and sand are always co-deposited). A non-dimensional ratio of shear stress to a deposition function delineated areas of predicted erosion and deposition by grain size. Sandy mud is predicted to be deposited in active currents where the boundary shear stress-driven entrainment is less than the local settling velocity- concentration product. Post deposition drainage and consolidation leads to strengthening sufficient to slow erosion once these deposits are exposed to channel shifting, thereby sustaining river meandering in the absence of vegetation.