Dynamic preservation of Holocene meander cutoffs in laterally migrating rivers

Abstract

When rivers keep meandering long enough, their channel belts widen to a certain width due to lateral channel migration processes. A meandering river tends to shift and sharpen its bends as the outcome of channel migration, and at some point cut them off. This leaves a cutoff meander element (usually a point bar complex with an oxbow lake) with a certain area and in a certain lateral position. Over time, continued channel migration and new cutoffs will erode, rework, and reincorporate parts of the older abandoned meanders. While vertical preservation in a long stratigraphic section is well understood, specific understanding of the forcing mechanisms and robust quantitative metrics of migration and lateral (meander) preservation rates and temporal development still are underdeveloped.

Here, previously proposed conceptual models of fluvial preservation were evaluated through numerical modelling. A morphodynamic, two-dimensional model (Nays2D) was used to simulate the evolution of a single-thread meandering river that is dominated by chute cutoffs. This river was analogous to the lower reach of the river Rhine in Germany but downscaled to flume dimensions. Four model scenarios were run with different mean water discharge, which produced channel patterns ranging from barely mobile enough for meandering to highly mobile, or wandering, with multiple threads. Two model runs showed extensive meander formation by a continuous single-thread river (𝑃 ≈ 1.5).

It was observed that preservation was generally higher for extensive point bars (with scroll bars) and marginal deposits than for smaller bars and sediment deposited where the dominant channel migrates little over time (>90% versus 0–30%, respectively). In contrast, preservation was more homogeneous for channel patterns without extensive point bar development. Additionally, preservation and reworking frequency were found to be logarithmically related to each other. Furthermore, at the meander-belt scale, the Sadler effect was prevalent (𝑅2 > 0.95), which makes the preservation of fluvial sediment packages predominantly depend on their age of deposition. At smaller spatial scales, this effect appeared weaker and was particularly modulated for extensive individual bars. Preservation at the scale of accretion packages was dominated by variability or randomness.

Because these results were largely consistent with previous quantitative analyses for other (natural and modelled) meander belts, an updated conceptual model of preservation was proposed. In this model, drivers of preservation depend on spatiotemporal scale, with each hierarchical level being informed by processes from the levels above and by additional processes at the specified level. It is suggested that preservation of individual bars or meanders is affected by the channel pattern, meandering style, river avulsions, and lithology for the Holocene time period.