The along-channel variation of sinuosity (channel path length divided by down-valley distance) in lowland meandering rivers, especially near the river outlet, is a planform signal used to interpret the sedimentary record on Earth and ancient fluvial systems on Mars. Wu et al. (2023) measured 21 lowland rivers on Earth and 6 ancient Martian systems and found a bimodal pattern: instead of the previously suggested downstream decrease, sinuosity either increases or remains essentially constant toward the coast. They proposed, and supported with calibrated channel-migration simulations, that this pattern is set by a competition between the timescale over which a channel develops steady-state sinuosity by lateral meandering and the timescale on which avulsion (wholesale relocation of the channel to a new floodplain course) resets the channel. The open problem is what physically sets these near-outlet profiles: whether the pace-competition hypothesis can be developed from a calibrated model consistent with 27 systems into a predictive account, tested against independent data, of which lowland rivers show substantially increasing and which roughly constant near-outlet sinuosity — or whether that account can be empirically refuted.

Background

River sinuosity is the ratio of the length of a channel's centerline to the length of its down-valley axis; values above 1 indicate a meandering (sinuous, laterally migrating) planform. In lowland river corridors near the coast, several processes jointly shape the along-channel sinuosity pattern. First, meandering: bends grow by bank erosion and point-bar deposition, so sinuosity increases through time toward a steady-state value set by the balance of bend growth and cutoff events, in which a loop is abandoned when its two limbs intersect (a neck cutoff) or when a new channel shortcuts across a bar (a chute cutoff). Second, avulsion: the relatively sudden relocation of flow to a new course on the floodplain, which abandons the old sinuous channel and initiates a new, comparatively straight one. Third, the backwater zone: the seaward reach, typically of length hb/Sh_b/S (bankfull flow depth hbh_b divided by channel-bed slope SS), within which water surface elevations are controlled by the receiving basin rather than by the upstream river; backwater hydrodynamics concentrate avulsion nodes and modulate flow, sediment transport, and bank erodibility along the reach. Prior work established individual controls on sinuosity growth: sediment supply enhances point-bar growth and speeds sinuosity increase (Constantine et al. 2014; Ahmed et al. 2019, across the Amazon Basin), and avulsion locations on deltas are organized by backwater effects (Chatanantavet et al. 2012). Earlier studies had suggested that lowland rivers become less sinuous (straighter) downstream toward the outlet, and this supposed decline has been used when interpreting ancient fluvial-deltaic deposits. Wu et al. (2023, Nature Geoscience) tested this expectation by measuring sinuosity versus normalized downstream distance for 21 lowland rivers on Earth (e.g., Mississippi, Indus, Danube, Mackenzie, Rhine) and 6 ancient river systems on Mars (where former channels are preserved as fluvial sandstone ridges in inverted relief). They found the opposite of the expectation: a dichotomy in which sinuosity either substantially increases toward the coast or remains roughly constant. Numerical simulations of channel migration, with parameters from empirical hydraulic-geometry relations and erosional coefficients calibrated against historical migration rates, reproduced both pattern types as outcomes of a competition between the timescale to reach steady-state sinuosity and the avulsion timescale (distances normalized by the avulsion length LaL_a); the accompanying Research Briefing summarizes that the patterns reflect channel age and lateral migration rate, associated with sediment supply and discharge variability. Martian ridge systems show compatible patterns, suggesting similar dynamics operated on Mars. What the paper provides is a calibrated, data-consistent model on 27 systems; what does not yet exist is a predictive, independently validated theory of the downstream sinuosity profile.

Problem Statement

Establish what physically sets the downstream (along-channel) sinuosity profile — sinuosity being channel centerline length divided by down-valley distance, considered as a function of distance from the river outlet — of lowland alluvial meandering rivers, in the near-outlet reach affected by avulsion (the wholesale, relatively sudden relocation of flow to a new channel course on the floodplain) and by backwater hydrodynamics (the near-outlet reach within which water levels are controlled by the receiving basin rather than the upstream river), together with the comparable ancient Martian inverted fluvial-ridge systems for which the same profile measurement is made. In particular: can the pace-competition account of Wu et al. (2023), in which near-outlet sinuosity reflects the competition between the timescale for a channel to develop steady-state sinuosity by lateral migration (migration rate set by discharge variability, sediment supply, and bank erodibility) and the timescale on which avulsion resets the channel, be developed and tested into a predictive theory that (i) predicts, from independently measurable quantities, which lowland rivers belong to the variable-sinuosity class (sinuosity substantially increasing toward the outlet) and which to the constant-sinuosity class (sinuosity roughly constant toward the outlet), the two classes identified in Wu et al. (2023), where the reported observable is the binned average of Ω\Omega in bins of width 0.1 La0.1\,L_a plotted against distance from the outlet normalized by LaL_a (Fig. 2; Extended Data Figs. 1–3) and the authors' public code applies a Mann–Kendall trend test at the 5% significance level to the binned profiles; and (ii) reproduces the sinuosity profile of individual systems, expressed as sinuosity Ω\Omega versus distance from the outlet xx (river kilometer RK on Earth, projected distance PD for the Martian ridges) normalized by the avulsion length LaL_a — the avulsion length of the system, that is, the distance from the outlet to the avulsion location marked in the source's profiles, so that x/La=1x/L_a = 1 falls at the avulsion node; not the along-valley spacing between successive channel resets, and not to be identified with the backwater length hb/Sh_b/S — and is subjected to independent empirical tests wherever the required inputs (avulsion length and recurrence interval, historical migration rates, discharge variability, sediment supply, channel age) are independently obtainable? The 2023 demonstration is a calibrated, per-system consistency check (erosional coefficient fitted to measured migration rates, channel age specified and first-cutoff time simulated for each river), so what remains open is prediction beyond that calibration — or a demonstrated empirical refutation of the pace-competition account.

References

  1. Wu, C., Kim, W., Herring, R., Cardenas, B. T., Dong, T. Y., Ma, H., Moodie, A., Nittrouer, J. A., Tsai, F., Li, A. (2023). Lowland river sinuosity on Earth and Mars set by the pace of meandering and avulsion. Nature Geoscience 16(8), 747-753. DOI 10.1038/s41561-023-01231-1. https://doi.org/10.1038/s41561-023-01231-1
  2. The sinuosity patterns of lowland meandering rivers on Earth and Mars [Research Briefing, anonymous]. Nature Geoscience 16(8), 667-668 (2023). DOI 10.1038/s41561-023-01233-z. https://doi.org/10.1038/s41561-023-01233-z
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