The role of hydrologic processes and geomorphology on the morphology and evolution of sediment clusters in gravel-bed rivers

The effect of geomorphic features and hydraulic conditions on the formation, evolution, and morphology of sediment cluster microforms in an unregulated gravel-bed stream were investigated at field sites on the Entiat River, which drains the eastern slopes of the Cascade Mountains in central Washington state, USA. Sediment clusters were marked, described, and photographed before and after a series of moderate to high discharges over an 18-month period to quantify the evolution of the cluster morphologies. Individual sediment particles were tracked to calculate the range of flows and bed shear stresses that maintain and destroy clusters. Examination of geomorphic settings, channel morphology, and particle size distributions documented the conditions that favor cluster formation. The investigation tested the hypotheses that clusters in this environment delay sediment entrainment and that their morphology and evolution follow a predictable evolution similar to that found in laboratory studies. Clusters formed on gravel bars adjacent to riffles with slopes ≥ 1%, poor to moderate sorting, and mean and maximum particle size values 1.5 times greater than those associated with noncluster bars. Clusters were more stable under the bimodal sediment size distribution contributed by a tributary alluvial fan and were destroyed with and without anchor clast mobilization at sites where sediment size was more uniform. The six cluster morphologies identified in the field were similar to those in flume studies, but did not follow the same evolutionary cycle over multiple flow events. This contrast was attributed to the flow events resetting the cluster cycle, leading to a high percentage of upstream triangles. The dimensionless critical shear stress required to entrain the mean grain size ranged from 0.06 to 0.08. The hydraulic thresholds and geomorphic features that result in stable vs. mobilized cluster microforms in this setting can serve as a model for regulated rivers and restoration projects aimed at sustaining instream flows to maintain natural sediment transport conditions