Sediment supply and channel morphology in mountain river systems: 2. Single thread to braided transitions

Abstract

Differences in sediment supply between single-thread and braided channel types provide a long-recognized, though difficult to quantify, pattern discrimination. Building on the results from our preceding paper, we present a multiscale assessment of the sediment supply, geomorphology, and sediment transport characteristics of braided, gravel-bedded reaches in the northern Rocky Mountains, USA. First, we present a quantitative, theoretically based discriminant function that stratifies single-thread and braided reaches on the basis of variations in bankfull sediment concentration and dimensionless discharge following the work of Millar (2005) and Eaton et al. (2010). This function correctly classifies 50 of the 53 channel types where bed load concentrations are known. Second, while channel pattern transitions are often linked to changes in slope, field studies along Sunlight Creek, Wyoming, show that downstream transitions between single-thread and braided reaches are instead caused by valley constrictions and changes in the grain size of sediment from tributaries. Finally, we demonstrate that the two-dimensional variability in flow properties in braided reaches produces locally high values of shear stress and bed load transport. Yet bed load measurements and sediment transport calculations also show that sediment transport rates between adjoining braided and single-thread reaches may be approximately equal where channels are near the pattern threshold and downstream variations in bank fortitude and channel constriction force pattern transitions. Taken together, these results indicate that high bed load concentrations are fundamental to the braided channel planform and that braided channels likely reflect a quasi-equilibrium state within watersheds with persistent high sediment supply.

The influence of a vegetated bar on channel-bend flow dynamics

“…1; see Supplement for more detail). The RMSE ranges obtained through calibration are consistent with values reported in other studies that have used FaSTMECH (e.g., Legleiter et al, 2011;Mueller and Pitlick, 2014;Segura and Pitlick, 2015), providing 10 confidence in model performance. Relaxation coefficients were set to 0.5, 0.3, and 0.1 for ERelax, URelax, and ARelax, respectively, through trial and error.…”

Rates of planimetric change in a proglacial gravel‐bed braided river: Field measurement and physical modelling

“…Particle size analyses for the bedload samples also allows an assessment of particle mobility at different transport rates and planimetric change rates. At the highest transport rate, D 90 is very close to that of the bulk size distribution of the bed material, so that this coarse fraction is also at or close to full mobility during the most active planimetric change periods (see Mueller and Pitlick, 2014;Peirce et al, 2018b). The exception was the final hydrograph at the lowest peak (D), during which the channel scoured deeply against the flume wall close to the flume outlet and produced substantial bedload locally at the outlet (see above).…”

“…All discharges high enough to cause planimetric change showed selective mobility (all sizes are mobile but not in the same proportion as the bed material grain size distribution) and events with extensive planimetric and morphological change had bedload close to equal mobility (bedload size distribution very similar to that of the bed material) including having D 90 of the bedload very similar to that of the bulk bed material (Mueller and Pitlick, 2014). All discharges high enough to cause planimetric change showed selective mobility (all sizes are mobile but not in the same proportion as the bed material grain size distribution) and events with extensive planimetric and morphological change had bedload close to equal mobility (bedload size distribution very similar to that of the bed material) including having D 90 of the bedload very similar to that of the bulk bed material (Mueller and Pitlick, 2014).…”

The morpho‐textural signature of large bedforms in ephemeral gravel‐bed channels of various planforms

“…Indeed, alternate bars in single‐thread channels remain in the same position (Crosato, Mosselman, Beidmariam Desta, & Uijttewaal, ) as do point bars in meandering rivers as long as sediment is sufficiently available to deposit and rebuild the bars. It is merely the variation in the availability of sediment and its flux that separates the planforms (single vs. multiple thread (Mueller & Pitlick, ). However, because process was not measured directly herein, similarity by processes, that is, the dynamic similarity, remains undetermined.…”

“…This was found to be true also for point bars in meandering rivers (Parker & Andrews, ) and in alternating bars in straight rivers (Lisle, Ikeda, & Iseya, ; Lisle & Madej, ; Venditti, Nelson, Minear, Wooster, & Dietrich, ). Bars in various river planforms differ by the cause of deposition, the flux of bedload (Mueller & Pitlick, ), and thus their location in the river. However, it is very likely that they do not differ by the in situ depositional process; hence, dynamic similarity most likely takes place after all.…”