MINDING THE GAPS—FLUVIAL PALEODEPTH REVISITED

Paleo streamflow-depth (paleodepth) is a central parameter used to estimate paleoslope, paleodischarge, and measures of sediment transport and channel scaling in ancient fluvial systems. A common way to infer paleodepth from outcrop is to measure the height of fully-preserved large-scale inclined strata, or so-called ‘clinoforms’, interpreted to be lee slope deposits of ancient channel bars. The key assumption linking clinoform height to paleodepth is that crest and base of a clinoform represent the water surface and reference bed elevation respectively. The difference between these elevations is interpreted as paleodepth (e.g. reach-mean, mean-thalweg, or bankfull-flow depth). Despite the common application and importance of clinoform height as a scale for paleodepth, there is a lack of clarity regarding the scaling between clinoform height and reference flow depth. This is particularly true for low-frequency, near-bankfull flow events where only a handful of observations have been made while bars are actively migrating. Divergence between clinoform height and flow-depth during high-flow conditions would be the consequence of two gaps in deposition: (1) a gap between clinoform crest elevation and maximum water stage (crest gap), and/or (2) a gap between clinoform base elevation and the reference channel bed elevation (base gap). We examined the magnitude of these gaps by measuring the slipface heights of 46 large channel bars formed during the prolonged, steady flood of 2011 in a wide, sandy, braided reach of the Missouri National Recreational River along the South Dakota-Nebraska border. Clinoform heights were extracted from a LiDAR DEM taken shortly after flood recession. We compared clinoform heights to reach-averaged flow depth and reach-averaged thalweg depth. We show that the crest gap accounts for as much as 30 percent of the divergence between flow depth scaling and clinoform height, while the base gap can account for as much as 80 percent. We use estimates of surface velocity to examine hydraulic limits on bar crest elevation. We speculate that a combination of local Froude and sediment suspension conditions cause bed-configuration to transition from dunes to plane-bed as the crest approaches the water surface during a flood.