Evaluation of Morphodynamic Controls on the Preservation of Fluvial Meander‐Belt Deposits.

The way river morphodynamics influence the preservation of point‐bar deposits at multiple scales is not fully understood. Employing time‐lapse trajectories of natural rivers, a numerical model is used here to simulate planform evolutions of meander‐belt reaches that embody different transformation behaviors and cutoff processes. Proxies for temporal durations are obtained considering the surface area over which a river migrated and channel migration rates that relate to average channel radius of curvature through constant, monotonic, and non‐monotonic relationships. The preservation of meander‐belt deposits over different timescales is assessed at three architectural hierarchies: (a) pairs and (b) sets of accretion packages, and (c) meander‐belts. Results confirm that sediment preservation decreases in a predictable way with the accumulation time; however, accretion rates decay with time in a way that does not follow the expected power‐law. This is interpreted to reflect the effect of the onset of geomorphic thresholds of channel transformation and cutoff. Plain Language Summary: Larger sediment volumes tend to record slower rates of deposition, because the likelihood of incorporating significant gaps in sedimentation increases with time. Hence, over timescales from seconds to millions of years, accumulation rates decrease as a power of time. This research determines whether this relationship holds true for channel belts produced by meandering rivers, and in particular for deposits that may develop over timescales of days (beds in point bars) to millennia (meander belts consisting of amalgams of bars and abandoned channels). To do this, a numerical model is applied to reconstruct the planform evolution of natural rivers. This way, sedimentation, erosion, and sediment preservation can be quantified, and proxies for time can be established based on the area over which a river migrated. The results show that, over this time window, the dependency of accumulation rates with time is more complex than anticipated. This is likely due, in part, to the sudden onset of changes in the style of river evolution (e.g., from meanders swinging laterally to sweeping downstream) and of bend cutoffs, which drive significant erosional reworking. Key Points: The preservation of meander‐belt deposits is assessed at multiple scales through numerical simulations of river migration historiesRates of planform growth of fluvial meander belts decay with time in a way that does not regularly follow a simple power‐law relationshipGeomorphic thresholds of meander‐transformation change and bend cutoff likely account for nonlinearity in stratigraphic completeness [ABSTRACT FROM AUTHOR]