Deluge and Deposition: Sediment Accumulation in a Large Midwestern River Following a Catastrophic Flood

Sediment traps were placed at ten locations in the lower Grand River, MI, in April 2004. Morphological, land cover, and land use characteristics of this large, entrenched, sand bottom river were measured to determine variables contributing to the mass and particle size distribution of settleable sediment collected in the traps. Beginning in May, heavy rainfall resulted in water levels rising to five meters over baseflow. Results suggest that the mass of sediment accumulated in the traps may be related to the ratio of baseflow discharge to total channel water carrying capacity. Reaches where baseflow discharge filled a greater proportion of the channel's total capacity had more sediment deposited in the traps (p = 0.013, adj. r2 = .62). These reaches also exhibited a lack of herbaceous riparian vegetation (p = 0.029, adj. r2 = .50), suggesting that a larger proportion of flood discharge escaped the channel and scoured these riparian zones. This riparian scouring also points to overland washload as a major source of settleable sediment. The shape of the river channel also appeared to be a significant variable, with deeper and narrower reaches accruing more sediment than shallower and wider reaches (p = 0.002, adj. r2 = .79). Abstract does not necessarily reflect EPA policy.