Estuary Turbidity Maxima -- Connections between the Tidal-Fluvial and Estuarine Regimes

Abstract:

An Estuary turbidity maximum or zone (ETM or ETZ) is an area of elevated sediment concentration that often occurs in coastal plain, salt wedge, and river-dominated estuaries. ETMs influence the morphodynamic development, biogeochemical cycling, and contaminant distribution of the many systems in which they occur. In developed estuaries, they are often created or augmented by dredging. Material of either fluvial or marine origin may be trapped, but fluvial supply is dominant in most river-estuary ETMs. An ETM can be described in terms of the type of particle trapping mechanisms that concentrate or trap suspended particulate matter (SPM). Convergent alongchannel SPM fluxes are required to create an ETM, and for a steady-state to pertain, seaward fluxes related to river flow must be balanced by landward mean, tidal or overtide fluxes. Horizontal and vertical salinity and/or sediment gradients often enhance trapping by concentrating SPM near the bed and cause near-bed landward flow and SPM transport. Also, lateral processes can concentrate or disperse SPM, and lags between SPM concentration and velocity are often a dominant factor in systems with fine grained sediment. The settled bed in an ETM may be fine grained, but ETM also occur in sand-bedded systems where no long-term deposition of ETM material occurs. We summarize results of theoretical models that provide a conceptual understanding of how ETM fluxes trap material and how ETM properties vary in response to external forcing. Remote sensing images provide a spatial view of ETM phenomena, and analyses of 15 years of ocean color data for the Columbia River Estuary validate theoretical results. Recent advances in acoustic and optical instrumentation in other environments should facilitate a new generation of ETM measurements, providing better time-space coverage and better flux estimates. Accordingly, we suggest ETM research questions for the coming decade.