"Single-thread rivers without land plans: A simple mechanistic model to interpret pre-Silurian and Martian fluvial deposits"
Speaker: Mathieu Lapotre, a John Harvard Distinguished Science Fellow
Abstract: To meander, alluvial rivers need cohesive banks to hold water within a single, relatively narrow and deep channel (low width-to-depth ratio) and thus develop the helical flow that is paramount to sustaining lateral accretion. Although many cohesive agents could in theory provide the required bank strength, reproducing single-thread rivers in flume experiments has proven difficult and, in the majority of few successful attempts, required the presence of overbank vegetation. Furthermore, few single-thread rivers exist on Earth today where there is no vegetation. Owing to these observations and the rough temporal coincidence between the colonization of land by plants and the appearance of lateral accretion sets in the Early Paleozoic record, it is generally thought that plants are essential for rivers to be single-threaded and meander. However, more and more pre-vegetation single-thread-river deposits have been identified in recent years, and observations from another world – Mars – are clearly at odds with the plant-bank-stabilization hypothesis. Thus, whereas plants may play a critical role in promoting the formation of single-thread rivers on Earth today, an alternative mechanism is required to explain the fluvial deposits of pre-Silurian Earth and early Mars. Although other cohesive agents may be possible, clays are perhaps the lowest common denominator between Earth and early Mars. We herein explore the stability of single-thread rivers within muddy floodplains in the absence of land plants. Specifically, we build a theoretical model to predict the equilibrium width-to-depth ratio of rivers as a function of bank materials based on an empirical flow stress-partitioning model, a model for bank cohesion as a function of grain size, and the assumption that flow stresses along riverbanks are near the threshold for bank erosion. The model indicates that single-thread rivers may form readily within clay-rich banks, and quantitative predictions are supported by experiments and field observations. This simple model has several important implications for quantifying riverbank stabilization by vegetation, understanding the hydraulic geometry of unvegetated fluvial systems, interpreting pre-Silurian fluvial deposits, and unraveling the hydrologic and climate history of Mars.
Lunch will be provided. As always, please plan to bring reusable plates and cutlery to reduce waste.