Using Satellite Observations to Characterize the Response of Estuarine Turbidity Maxima to External Forcing

This study explores the spatial and temporal character of turbidity maxima in the Columbia River Estuary (CRE) using satellite observations. Surface reflectance data measured by the Moderate Imaging Spectroradiometer (MODIS) were calibrated against in situ measurements of surface turbidity (R-2 = 0.85 for 205 measurements). More than 1500 satellite images from 2000 to 2015 were then conditionally sampled to explore the physical processes that drive the spatial distribution of the turbidity field. We find satellite measurements are able to describe seasonal, spring-neap, and spatial features of the estuarine turbidity maxima (ETM) that are not easily observable by other means. System-wide levels of turbidity are most sensitive to river flow and spring-neap tidal range, with a weaker correlation to wind and waves. Maximum surface turbidity is observed in winter during elevated flow from coastal tributaries and remains elevated during the spring freshet of the main stem Columbia. Two ETM with asymmetric along-channel profiles are observed, one in the North Channel and another in the South Channel. Turbidity distributions migrate downstream as tidal range and river flow increase but appear to become topographically trapped near topographic holes at river kilometers 15-20. Hence, depth-sensitive circulation processes like internal asymmetry and gravitational circulation are likely important mechanisms for trapping particles and determining ETM location. These conclusions confirm the theoretical result that along-channel distributions of turbidity should have an asymmetric distribution and emphasize the role of bottom topography.