Resolution of turbidity patterns from runoff events in a water supply reservoir, and the advantages of in situ beam attenuation measurements

The impact of runoff events on light scattering, or turbidity, levels in a water supply reservoir, and the comparative performance of three surrogate metrics of light scattering, are documented for the spring through fall interval of a high runoff year. The analysis is supported by: (1) frequent (42 d) field measurements of the beam attenuation coefficient at 660 nm (c(660)) and "optical" backscattering (OBS) collected with rapid profiling instrumentation at multiple sites; (2) laboratory measurements of c(660) and turbidity (Tn); and (3) characterizations of inorganic particles with scanning electron microscopy interfaced with automated image and X-ray analyses. Conspicuous increases in light scattering levels are reported following runoff events, as reflected in increases in c(660), OBS and Tn, associated with terrigenous inputs of clay minerals. The extent of this impact is demonstrated to be driven by the magnitude of the runoff event. Terrigenous inputs of turbidity are shown to enter as density currents, which travel the entire length of the reservoir for major runoff events, manifested as peak scattering levels in subsurface layers. Strong longitudinal and lateral differences are documented soon after runoff peaks. Scattering levels and spatial gradients are shown to diminish rapidly, with pre-event conditions approached within a week. Systematically lower c(660) values are reported for laboratory measurements compared to in situ observations, particularly at high scattering levels, consistent with the operation of particle coagulation. In situ measurement of c(660) is identified as the preferred surrogate metric of light scattering.