Hierarchically nested river landform sequences. Part 2: Bankfull channel morphodynamics governed by valley nesting structure

Abstract

River corridors exhibit landforms nested within landforms repeatedly down spatial scales. In Pasternack et al. (), a new, scale‐independent, hierarchical river classification was developed that uses five landform types to map the domains of a single fluvial process – flow convergence routing – at each of three–five spatial scales. Given those methods, this study investigated the details of how flow convergence routing organizes nested landform sequences. The method involved analyzing landform abundance, sequencing, and hierarchical nesting along the 35 km gravel/cobble lower Yuba River in California. Independent testing of flow convergence routing found that hydraulic patterns at every flow matched the essential predictions from classification, substantiating the process–morphology link. River width and bed elevation sequences exhibit large, nonrandom, and linked oscillations structured to preferentially yield wide bars and constricted pools at base flow and bankfull flow. At a flow of 8.44 times bankfull, there is still an abundance of wide bar and constricted pool landforms, but larger topographic drivers also yield an abundance of nozzle and oversized landforms. The nested structure of flow convergence routing landforms reveals that base flow and bankfull landforms are nested together within specific floodprone valley landform types, and these landform types control channel morphodynamics during moderate to large floods. As a result, this study calls into question the prevailing theory that the bankfull channel of a gravel/cobble river is controlled by in‐channel, bankfull, and/or small flood flows. Such flows may initiate sediment transport, but they are too small to control landform organization in a gravel/cobble river with topographic complexity. Copyright © 2018 John Wiley & Sons, Ltd.

Hierarchically nested river landform sequences. Part 1: Theory

“…We have piloted this methodology using three river datasets, one from a gravel/cobble river (see Pasternack et al, 2018) and two from mountain bedrockboulder rivers (Gore and Pasternack, 2016). We have piloted this methodology using three river datasets, one from a gravel/cobble river (see Pasternack et al, 2018) and two from mountain bedrockboulder rivers (Gore and Pasternack, 2016).…”

“…We have piloted this methodology using three river datasets, one from a gravel/cobble river (see Pasternack et al, 2018) and two from mountain bedrockboulder rivers (Gore and Pasternack, 2016). For example, using the base flow gravel/cobble bed lower Yuba River data from Pasternack et al (2018) as an example,~60% of stations with -0.5 < Ws·Zs < 0.5 had individual Ws and Zs values also meeting that same criterion. Conceptually, the more that Ws·Zs threshold values deviate from zero, the more uniform a river will seem.…”

“…A similar but more exacting and specific system requires both variables to individually exceed a threshold (Figure 4(a)). The mechanistic validity of the approach selected for this study was tested in Pasternack et al (2018). The mechanistic validity of the approach selected for this study was tested in Pasternack et al (2018).…”

“…How wide bar versus oversized velocity might compare as well as how constricted pool versus nozzle velocity, is an open question investigated in Pasternack et al (2018), given the possibility of a narrower effective flow width in one or more landform types. These relative magnitudes should hold across all discharges.…”

“…The question is how strongly must geometry deviate from the average to be considered significant enough to denote a new landform type as a starting point before many datasets can be analyzed for possible threshold criteria? We have piloted this methodology using three river datasets, one from a gravel/cobble river (see Pasternack et al, 2018) and two from mountain bedrockboulder rivers (Gore and Pasternack, 2016). Using those datasets, sensitivity analysis was done to answer this, but these details are too lengthy to cover herein.…”