Stuck at the Bar: Larger-Than-Average Grain Lag Deposits and the Spectrum of Particle Mobility

Abstract

Larger-than-average grain deposits in gravel bed rivers potentially exert a distinctive influence upon fluvial morphodynamics and flow resistance. They are products of historical contingency, sourced from rare events that supply atypically coarse material. Larger-than-average grain lag deposits are emblematic attributes of the Tongariro River, New Zealand. They are deposited on bar edges and heads. Derived from lahar valley floor deposits that subsequently became terraces, these materials are less likely to be reworked across a range of flows compared to other bar material. Conceptual models that consider channel configuration and incorporate distributions of particle mobility and flood flows are necessary to assess the role of larger-than-average grain deposits on river morphodynamics.