Variability of Particle Distribution Using Optical Measurements within the Columbia River Estuary

Physical, optical, and particle-sensing instruments were profiled at a series of stations through the mouth and into the nearshore in the highly stratified waters of the Columbia River Estuary (CRE). A variety of proxies of particle concentration, size distribution and composition were derived from these measurements and, were found, in general, to agree well with established sedimentological methods, as well as with expectations based on particle dynamics. The advantage of optical measurements is their relatively high spatial resolution, which provided a clear description of particulate mass and biogeochemical properties in the vicinity of the estuarine turbidity maxima.

At an anchor station in CRE, particle and optical properties were affected strongly by proximity to the density front. Observations in CRE demonstrated that the organic-rich river water brings smaller and denser particles to meet with salty ocean water, and then particles aggregate and settle into the salt wedge seaward of the density front. Large tidal currents resuspend mineral-rich, larger and loose aggregates from the seabed, which accumulate at the density front. Suspended particulate mass concentration (SPM) was high in the vicinity of boundary between fresh and salt water in the mouth of the river, because the high bed stresses landward of density front resuspended silts and sands at the river mouth that settled rapidly.