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1. Quantitative relationships between river and channel-belt planform patterns

   https://doi.org/10.1130/G49935.1  

   Dong, Tian Y. ; Goudge, Timothy A. ( June 2022 , Geology)

   Abstract Channel planform patterns arise from internal dynamics of sediment transport and fluid flow in rivers and are affected by external controls such as valley confinement. Understanding whether these channel patterns are preserved in the rock record has critical implications for our ability to constrain past environmental conditions. Rivers are preserved as channel belts, which are one of the most ubiquitous and accessible parts of the sedimentary record, yet the relationship between river and channel-belt planform patterns remains unquantified. We analyzed planform patterns of rivers and channel belts from 30 systems globally. Channel patterns were classified using a graph theory-based metric, the Entropic Braided Index (eBI), which quantifies the number of river channels by considering the partitioning of water and sediment discharge. We find that, after normalizing by river size, channel-belt width and wavelength, amplitude, and curvature of the belt edges decrease with increasing river channel number (eBI). Active flow in single-channel rivers occupies as little as 1% of the channel belt, while in multichannel rivers it can occupy >50% of the channel belt. Moreover, we find that channel patterns lie along a continuum of channel numbers. Our findings have implications for studies on river and floodplain interaction, storage timescalesmore »of floodplain sediment, and paleoenvironmental reconstruction.« less
2. Differential Bank Migration Limits the Lifespan and Width of Braided Channel Threads

   https://doi.org/10.1029/2021WR031236  

   Chadwick, A. J. ; Steel, E. ; Passalacqua, P. ; Paola, C. ( August 2022 , Water Resources Research)

   Successful management of flooding and erosion hazards on floodplains depends on our ability to predict a river channel's shape and the lifespan during which it will continue to flow. Recent progress has improved our understanding of what sets the lifespan and width of single‐thread channels; the next challenge is to extend this knowledge to braided channels and their interwoven sub‐channels (threads). In this study, we investigate the lifespan and width of braided channel threads in a large experimental data set, coupled with particle‐image velocimetry‐derived measurements of riverbank erosion and accretion. We find that, unlike single‐thread channels, braided channels in the experiment do not exhibit an equilibrium between bank erosion and accretion. Instead, bank erosion outpaces lateral accretion, causing individual threads to widen and infill until they are abandoned. Thread lifespan is limited to the time it takes for threads to triple their width: tripling of the width yields enough bank material to aggrade more than half the channel depth, at which point flow is rerouted to a narrower thread. In consequence the width of active threads is limited to three times their initial width. Threshold channel theory accurately predicts the median thread width, which is roughly double the initialmore »width and two‐thirds the limiting width. The results are consistent with existing field data and suggest that differential bank migration is sufficient to explain why braided channels show greater width variability and higher width‐to‐depth ratios than their single‐thread counterparts.

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3. Resistance Is Not Futile: Grain Resistance Controls on Observed Critical Shields Stress Variations

   https://doi.org/10.1029/2018JF004817  

   Yager, Elowyn M. ; Schmeeckle, Mark W. ; Badoux, Alexandre ( December 2018 , Journal of Geophysical Research: Earth Surface)

   Estimates of the onset of sediment motion are integral for flood protection and river management but are often highly inaccurate. The critical shear stress (τ^*_c) for grain entrainment is often assumed constant, but measured values can vary by almost an order of magnitude between rivers. Such variations are typically explained by differences in measurement methodology, grain size distributions, or flow hydraulics, whereas grain resistance to motion is largely assumed to be constant. We demonstrate that grain resistance varies strongly with the bed structure, which is encapsulated by the particle height above surrounding sediment (protrusion,p) and intergranular friction (ϕ_f). We incorporate these parameters into a novel theory that correctly predicts resisting forces estimated in the laboratory, field, and a numerical model. Our theory challenges existing models, which significantly overestimate bed mobility. In our theory, small changes inpandϕ_fcan induce large changes inτ^*_cwithout needing to invoke variations in measurement methods or grain size. A data compilation also reveals that scatter in empirical values ofτ^*_ccan be partly explained by differences inpbetween rivers. Therefore, spatial and temporal variations in bed structure can partly explain the deviation ofτ^*_cfrom an assumed constant value. Given that bed structure is known to vary with applied shear stresses andmore »upstream sediment supply, we conclude that a constantτ^*_cis unlikely. Values ofτ^*_care not interchangeable between streams, or even through time in a given stream, because they are encoded with the channel history.

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4. River channel width controls blocking by slow-moving landslides in California's Franciscan mélange

   https://doi.org/10.5194/esurf-7-879-2019  

   Finnegan, Noah J. ; Broudy, Kiara N. ; Nereson, Alexander L. ; Roering, Joshua J. ; Handwerger, Alexander L. ; Bennett, Georgina ( January 2019 , Earth Surface Dynamics)

   Abstract. To explore the sensitivity of rivers to blocking from landslidedebris, we exploit two similar geomorphic settings in California'sFranciscan mélange where slow-moving landslides, often referred to asearthflows, impinge on river channels with drainage areas that differ by afactor of 30. Analysis of valley widths and river long profiles over∼19 km of Alameda Creek (185 km2 drainage area) andArroyo Hondo (200 km2 drainage area) in central California shows avery consistent picture in which earthflows that intersect these channelsforce tens of meters of gravel aggradation for kilometers upstream, leadingto apparently long-lived sediment storage and channel burial at these sites.In contrast, over a ∼30 km section of the Eel River (5547 km2 drainage area), there are no knickpoints or aggradation upstreamof locations where earthflows impinge on its channel. Hydraulic andhydrologic data from United States Geological Survey (USGS) gages on Arroyo Hondo and the Eel River, combinedwith measured size distributions of boulders input by landslides for bothlocations, suggest that landslide derived boulders are not mobile at eithersite during the largest floods (>2-year recurrence) with field-measured flow depths. We therefore argue that boulder transport capacity isan unlikely explanation for the observed difference in sensitivity tolandslide inputs. At the same time, we find that earthflow fluxes per unitchannel width are nearlymore »identical for Oak Ridge earthflow on Arroyo Hondo,where evidence for blocking is clear, and for the Boulder Creek earthflow onthe Eel River, where evidence for blocking is absent. These observationssuggest that boulder supply is also an unlikely explanation for the observedmorphological differences along the two rivers. Instead, we argue that thedramatically different sensitivity of the two locations to landslideblocking is related to differences in channel width relative to typicalseasonal displacements of earthflows. A synthesis of seasonal earthflowdisplacements in the Franciscan mélange shows that the channel width ofthe Eel River is ∼5 times larger than the largest annualseasonal displacement. In contrast, during wet winters, earthflows arecapable of crossing the entire channel width of Arroyo Hondo and AlamedaCreek. In support of this interpretation, satellite imagery shows thatimmobile earthflow-derived boulders are generally confined to the edges ofthe channel on the Eel River. By contrast, immobile earthflow-derivedboulders jam the entire channel on Arroyo Hondo. Our results imply that lower drainage area reaches of earthflow-dominated catchments may be particularly prone to blocking. By inhibiting the upstreampropagation of base-level signals, valley-blocking earthflows may thereforepromote the formation of so-called “relict topography”.« less
5. Quantifying bankfull flow width using preserved bar clinoforms from fluvial strata

   https://doi.org/10.1130/G48729.1  

   Greenberg, Evan ; Ganti, Vamsi ; Hajek, Elizabeth ( May 2021 , Geology)

   Abstract Reconstruction of active channel geometry from fluvial strata is critical to constrain the water and sediment fluxes in ancient terrestrial landscapes. Robust methods—grounded in extensive field observations, numerical simulations, and physical experiments—exist for estimating the bankfull flow depth and channel-bed slope from preserved deposits; however, we lack similar tools to quantify bankfull channel widths. We combined high-resolution lidar data from 134 meander bends across 11 rivers that span over two orders of magnitude in size to develop a robust, empirical relation between the bankfull channel width and channel-bar clinoform width (relict stratigraphic surfaces of bank-attached channel bars). We parameterized the bar cross-sectional shape using a two-parameter sigmoid, defining bar width as the cross-stream distance between 95% of the asymptotes of the fit sigmoid. We combined this objective definition of the bar width with Bayesian linear regression analysis to show that the measured bankfull flow width is 2.34 ± 0.13 times the channel-bar width. We validated our model using field measurements of channel-bar and bankfull flow widths of meandering rivers that span all climate zones (R2 = 0.79) and concurrent measurements of channel-bar clinoform width and mud-plug width in fluvial strata (R2 = 0.80). We also show that the transversemore »bed slopes of bars are inversely correlated with bend curvature, consistent with theory. Results provide a simple, usable metric to derive paleochannel width from preserved bar clinoforms.« less