Riffle-pool maintenance and flow convergence routing observed on a large gravel-bed river

Geomorphologists have studied and debated over the processes responsible for natural riffle-pool maintenance for decades. Most studies have focused on small wadable rivers, but they lack much description of overbank flood conditions or a spatially explicit characterization of morphodynamics. In this study, 1-m horizontal resolution digital elevation models were collected from a riffle-pool-run sequence before and after an overbank flood with a 7.7-year recurrence interval on the relatively large gravel-bed lower Yuba River, California. Digital elevation model differencing was used to quantify the magnitude and pattern of flood-induced morphodynamic change. Cross section based analysis and two-dimensional hydrodynamic modeling of flows ranging from 0.147 to 7.63 times bankful discharge were completed to evaluate the hydraulic mechanisms responsible for the observed topographic changes. One key finding was that riffle-pool relief increased by 0.42 m, confirming the occurrence of natural hydrogeomorphic maintenance. Spatially complex patterns of scour and deposition exceeding 0.15 m at the scale of subwidth morphological units were reasonably predicted by the two-dimensional mechanistic model that accounts for convective acceleration. The one-dimensional cross section based method underperformed the two-dimensional model significantly. Consequently, multiple scales of channel non-uniformity and a dynamic flow regime caused the observed maintenance of the pool-riffle morphology through the mechanism of “flow convergence routing” proposed by MacWilliams et al. [MacWilliams, M.L., Wheaton, J.M., Pasternack, G.B., Kitanidis, P.K., Street, R.L., 2006. The flow convergence-routing hypothesis for riffle-pool maintenance in alluvial rivers. Water Resources Research 42, W10427, doi:10.1029/2005WR004391].