Model for the Formation of Single-Thread Rivers in Barren Landscapes and Implications for Pre-Silurian and Martian Fluvial Deposits

Flume experiments and field observations show that bank vegetation promotes the formation of narrow and deep single-thread channels by strengthening riverbanks. Consistent with this idea, the pre-Silurian fluvial record generally consists of wide monotonous sand bodies often interpreted as deposits of shallow braided rivers, whereas single-thread rivers with muddy floodplains become more recognizable in Silurian and younger rocks. This shift in the architecture of fluvial deposits has been interpreted as reflecting the rise of single-thread rivers enabled by plant life. The deposits of some single-thread rivers, however, have been recognized in pre-Silurian rocks, and recent field studies have identified meandering rivers in modern unvegetated environments. Furthermore, single-thread-river deposits have been identified on Mars, where macroscopic plants most likely never evolved. Here we seek to understand the formation of those rarely recognized and poorly characterized single-thread rivers in unvegetated landscapes. Specifically, we quantitatively explore the hypothesis that cohesive muddy banks alone may enable the formation of single-thread rivers in the absence of plants. We combine open-channel hydraulics and a physics-based erosion model applicable to a variety of bank sediments to predict the formation of unvegetated single-thread rivers. Consistent with recent flume experiments and field observations, results indicate that single-thread rivers may form readily within muddy banks. Our model has direct implications for the quantification of riverbank strengthening by vegetation, understanding the hydraulic geometry of modern and ancient unvegetated rivers, interpreting pre-Silurian fluvial deposits, and unraveling the hydrologic and climate history of Mars. Plain Language Summary Plants tend to strengthen riverbanks, favoring the formation of single-thread rivers (i.e., relatively deep flows within a single channel) over that of braided rivers (i.e., relatively shallow flows distributed among several interlaced channels). In parallel, geologists have observed a shift in the structure of river deposits coincident with the evolution of land plants, commonly interpreted as the signature of the rise of single-thread rivers, sparked by plant life. However, recent studies have identified single-thread-river deposits in both modern unvegetated environments and in rocks that predate the greening of the continents, and deposits of ancient single-thread rivers have also been identified on Mars, where large plants realistically never evolved. Thus, single-thread rivers can clearly form in vegetation-free environments; here we seek to understand how. Employing a conceptual model based on the mechanics of river flow and bank erosion, we show that sticky mud may strengthen riverbanks enough to resist erosion and prevent river braiding, suggesting that mud alone could have enabled the formation of single-thread rivers on ancient Earth and Mars. The model may help to quantify plant-driven riverbank strength, understand river geometry in barren landscapes, interpret ancient river deposits on Earth, and, possibly, decipher the climate history of Mars.