199-5 Hovering over the Threshold: Reconciling Stochastic and Equilibrium Models of Long-Term Stream Channel Incision

Session: Advances in Fluvial Processes and Sediment Transport, Part II

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This work is motivated by a desire to improve numerical models for fluvial landscape evolution in “ordinarily complex” geological situations. We are specifically interested in river systems on length scales of hundreds of kilometers, where strong lithologic variations are common, and where the downstream attrition of coarse sediment can have a strong influence on fluvial behavior. In reviewing the pantheon of potential fluvial transport and erosion formulations, two edge cases stand out. On one end of the spectrum are stochastic-threshold models. These directly address time-varying water discharge by integrating erosion and sediment transport across a frequency distribution of flows. However, stochastic-threshold models usually rely on the approximation that channel width depends only on water discharge, irrespective of variations in channel gradient, sediment grain sizes, or local bank material strength (rock, vegetation, or sediment), all of which are known to influence channel geometry. At the other end of the spectrum are equilibrium-threshold models, which explicitly allow channel width adjustment in response to changes in slope, discharge, or grain size, based on gravel-channel adjustment theory. Yet equilibrium-threshold models, at least as implemented so far, are limited by an “effective discharge” approach in which some typical discharge, like bankfull flow,  is assumed to provide a reasonable surrogate for the full flow distribution. Our aim is to identify the conditions under which the limiting approximations underlying these two types of models—fixed-width and effective discharge, respectively—are appropriate. Review of existing data sets reveals plenty of examples of bedrock-incising rivers that show the kind of equilibrium scaling usually associated with alluvial gravel-bed rivers, suggesting that encounters with bedrock do not necessarily negate alluvial-like channel adjustment. We also find that the predicted width scaling depends on hydraulics, namely that width becomes more sensitive to gradient in steep, rough channels with near-critical flow. Analysis of bedload transport efficiency under a distribution of flood flows highlights the importance of valley confinement. All else equal, the effective discharge assumption introduces more error in narrow canyons than in broad, unconfined floodplains. To quantify this effect, we compute the time-integrated bedload transport rate in idealized valley cross-sections with varying degrees of confinement and compare them to predictions from equilibrium-threshold models that assume an effective discharge. We consider the implications of these findings for long-term landscape evolution.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9838

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