During floods, small mountainous rivers transport large sediment loads to the coastal ocean. This sediment often deposits near shore, and if conditions allow, various types of gravity flows (including hyperpycnal river plumes) can transport the sediment across the shelf within minutes to months of delivery. The stochastic and energetic nature of small‐mountainous‐river flood dispersal presents challenges to observations of transport processes and flood‐deposit formation during events, however. In this study, the removal of two dams on the Elwha River afforded an opportunity to closely monitor coastal sediment dispersal using boundary‐layer instrument systems during 4 years of intense sediment loading. A flood in March 2014 generated fluvial sediment concentrations of 14–20 g/L, coastal boundary‐layer concentrations of up to 14 g/L, and 23 cm of muddy sand deposition in a few days. Limited evidence exists for a gravity flow lasting <2 hr during slack tide, coinciding with the waning phase of the secondary flood peak. However, rapid sediment deposition over the 4‐day event period was dominated by advection of sediment in the boundary layer and by suspension settling from turbid river water, which never exceeded the well‐established 35–40 g/L required for hyperpycnal river plume formation. Within 3 weeks, the entire sandy flood deposit had been eroded by strong tidal currents. This system highlights the difficulty in forming hyperpycnal river plumes within tidally energetic systems, even from an extremely turbid river, and the conditions that can erase major sediment delivery events from the sedimentary record.