Geomorphic covariance structure of a confined mountain river reveals landform organization stage threshold

Abstract

Significant growth in mountain rivers research since 1990 has promoted the concept that canyon-confined mountain rivers have complex topographic features nested from base-to flood-stages due to canyon structure and abundant large bed elements.Nesting means literally structures inside of structures. Mathematically, nesting means that multiple individual features and repeating patterns exist at different frequency, amplitude, and phasing, and can be added together to obtain the complete structure.Until now, subreach-scale landform structure, including nesting, has not been quantified sufficiently to understand morphodynamic mechanisms that control and respond to such organization. Geomorphic covariance structure analysis offers a systematic framework for evaluating nested topographic patterns. In this study, a threshold stage in mountain river inundation was hypothesized to exist. Above this stage landform structure is organized to be freely self-maintaining via flow convergence routing morphodynamics. A 13.2 km segment of the canyon-confined Yuba River, California, was studied using 2944 cross-sections. Geomorphic covariance structure analysis was carried out on a meter-resolution topographic model to test the hypothesis.River width and bed elevation had significantly less variability than previously reported for lower slope, partially confined gravel/cobble river reaches. A critical stage threshold governing flow convergence routing morphodynamics was evident in several metrics. Below this threshold, narrow/high "nozzle" and wide/low "oversized" were the dominant landforms (excluding "normal channel"), while above it wide/high "wide bar" and narrow/low "constricted pool" were dominant. Threestage nesting of base-bankfull-flood landforms was dictated by canyon confinement, with nozzle-nozzle-nozzle nesting as the top permutation, excluding normal channel.

Identifying key channel variability functions controlling ecohydraulic conditions using synthetic channel archetypes

“…Importantly, this framework can now be readily applied to evaluate other sub‐reach‐scale variability functions based on distinct statistical properties of geomorphic attributes such as geomorphic covariance structures (Brown & Pasternack, 2014; Pasternack et al, 2021), coefficient of variance of width and depth (Guillon et al, 2020; Lane et al, 2017) or fractal dimension (Guillon et al, 2020) for different channel type. This would help us to estimate hydraulic and ecological responses of a watershed with limited topographic information by enabling one to synthesize a generic channel corridor without any field survey data.…”

“…Given a nonuniform channel with an undulating riverbed, it is more accurate to extract flow‐dependent channel features from 2D model outputs than through pure geometric analysis of a DEM. The latter could be done using the procedure explained in Pasternack et al (2021) when no 2D model is available or desired.…”

“Process‐based similarity” revealed by discharge‐dependent relative submergence dynamics of thousands of large bed elements

“…incised or canyon-confined channels) with LBEs relegated to the low-flow portions of the valley bottom, though the prospect of spatially uniform increases in depth is questionable in most rivers. In the context of rivers with hierarchically nested LBE non-uniformity (Pasternack et al, 2021;, conditions to achieve Style 3 generally require depths at previously wetted LBEs to increase at relatively uniform rates with magnitudes nearly equal to the shifts in central tendency (e.g. depth increases are normally distributed with low variance and means close to shift magnitudes), and depths at newly wetted LBEs to increase rapidly such that submergences are similar to the shifted distributions (Figure S1b).…”

“…The study segment is comprised of a low‐sinuosity, boulder‐bedded, fifth‐order mountain river confined within a steep‐walled bedrock and forested hillside canyon. The mean bed slope is 2%, but there are significant, high amplitude bed and width undulations at multiple frequencies (Pasternack et al, 2021), including shorter intervals of 10–100 m (10 0 –10 1 channel widths) having slopes > 10%. The river segment was delineated into six parsimonious reaches (slopes of 0.8–2.6%) based on major channel‐bed slope breaks (Figure S2).…”

Channel Constriction Predicts Pool‐Riffle Velocity Reversals Across Landscapes

“…Bankfull flow has been shown to be a good approximation for effective discharge (Lenzi et al., 2006), especially in temperate rivers, but research suggests that river channel morphology is a product of all flows rather than the effective discharge (Pittaluga et al., 2014). In addition to the magnitude of channel forming floods, the complexity of mountain rivers can produce nested structures that are likely to adjust river bedforms in different ways (Pasternack et al., 2021). Pool‐riffle couplets may exhibit velocity‐reversal conditions at different flow conditions (Strom et al., 2016), changes in flow and sediment supply may lead to alteration of pool‐riffle couplets and associated processes (Caamaño et al., 2009; Chartrand et al., 2018; Morgan & Nelson, 2021), or a stochastic forcing (e.g., large woody debris) may create a pool at a higher flow that persists at lower flows such as bankfull (Buffington et al., 2002).…”