Mud cohesion governs unvegetated meander migration rates and deposit architecture

Abstract

Vegetation is thought to be a main source of riverbank cohesion, enabling meandering and a deposit architecture characterized by sandy channel belts isolated in mudstone. However, early Earth and Mars had meandering rivers without vegetation, implying that other sources of bank strength can allow meandering with potentially different deposit characteristics. Here we studied the Amargosa River in Death Valley, California, USA, as a modern analog of meandering rivers without vegetation. We monitored flow and erosion at two bends and used radiocarbon dating of strandlines to quantify flood frequency. We also sampled cutbank mud and constrained an erosion theory using flume experiments. Cutbank erosion occurred for floods with >2 yr recurrence intervals, and 18 cm occurred for an ∼6 yr reoccurrence, bankfull event. Mud set the rate of meander migration: salt crusts rapidly and completely dissolved during floods, vegetation was absent, and mud entrainment theory matched observed erosion rates. Flood-frequency analysis showed that most bank erosion occurs at flows below bankfull, challenging the threshold channel hypothesis. We used meander migration rates to constrain the time scale of channel-belt formation and compared it to the time scale of avulsion. These calculations, combined with floodplain facies mapping and core sedimentology, indicated a likely deposit architecture of sandy point bar accretion sets intermixed with muddy overbank facies. This deposit architecture is characteristic of vegetated meandering rivers, but due to muddy banks, occurred for the Amargosa River in the absence of plants.