A New Generation of Fluvial Facies Models Based on Discharge Variance, Not Planform

Historically, facies models for alluvial deposits have been based on the formative channel planform (meandering, straight, braided, anabranching). But planform-based models have limited predictive value because many aspects of alluvial deposit architecture are independent of planform. Fielding et al. (2018, SedGeo 365, 1–20) found that the coefficient of variance of inter-annual peak discharge (CVQp) for a suite of modern rivers correlated strongly with their preserved subsurface stratigraphy, independent of planform. They posited that discharge variability provides a more reliable basis from which to construct a new generation of fluvial facies models. Here, we expand the 2018 database, explore the controls on CVQp, and propose preliminary facies models for different CVQp river categories. Deposits of systems with low CVQp (<0.20–0.40) preserve macroform structure, show limited lateral lithological heterogeneity, are dominated internally by dune-scale cross-bedding, and show no influence from river bed in situ woody vegetation. Systems with moderate CVQp (0.40–0.70) show muted preservation of macroform structure, a greater diversity of mesoform structures, some lithological heterogeneity and some vegetational influence. Deposits of systems with high (0.70–1.0) and very high CVQp (>1.0) preserve few if any identifiable remnants of macroform structure, are dominated internally by upper flow regime structures, show significant internal heterogeneity, and may contain extensive large woody debris and in situ channel vegetation. There is no simple correlation between CVQp class and latitude, although there is some with climatic zone. The highest CVQp systems are concentrated in arid to semiarid climatic belts.