Are dynamic fluvial morphological unit assemblages statistically stationary through floods of less than ten times bankfull discharge?

Abstract

The morphological unit (MU) characterizes river landforms at the scale of ~0.5–5 channel widths. Geomorphic theory posits that ceteris paribus under a stationary flow and sediment regime, a river's channel pattern and its MU assemblage will also be stationary. This study tested that conjecture for the dynamic, gravel/cobble lower Yuba River. The MU assemblage consisting of eight in-channel bed types (i.e., chute, fast glide, pool/forced pool, riffle, riffle transition, run, slackwater, and slow glide) was mapped in 2006/2008 and 2014 for a ~34 km long by 100 m wide alluvial segment at ~1-m resolution using the Wyrick et al. (2014) methodology. Between these two surveys there were four brief floods of 6–9 times bankfull discharge and a total of 163 days above bankfull discharge, yielding an estimated 638,539 and 507,743 m3 of erosion and deposition, respectively. Bulk statistical change tests (e.g., MU area, count, spacing, diversity, lateral diversity, and adjacency), MU temporal transition analysis, and MU individual polygon tracking analysis were used to answer the scientific question. Even though the river has an abundant internal sediment supply and dynamic flow regime, the river's in-channel bed shifted toward a lower-relief morphology, with a widespread and significant fragmentation of individual MU polygons, especially the large ones. The MU at each location in the river predominantly changed, except for pool, which tended to stay the same because of local topographic forcing. Overall, the predominance of evidence leads to the conclusion that the MU assemblage was not stationary over the period evaluated. This is hardly the final word, given the relatively short duration compared to geological time and considering only one cobble/gravel river, but this study points the way toward future investigation into MU stationarity in light of modern spatially explicit fluvial geomorphic mapping methods.