Research Areas > Stormwater > River Plume Patterns and Dynamics within the Southern California Bight (Bight’03)

Project: River Plume Patterns and Dynamics within the Southern California Bight (Bight’03)

Background and Objectives

Discharge of stormwater to the coastal ocean, through both natural waterways and storm drains, can carry a variety of pollutants from throughout the watershed. This freshwater quickly stratifies into a buoyant plume when it reaches the ocean. Management of stormwater requires an understanding of how these discharge plumes impact coastal water quality. Stormwater plumes from southern California watersheds can extend tens of kilometers from the shoreline into the ocean, though plume size significantly varies temporally and spatially among different coastal watersheds. These river plumes are strongly influenced by the river discharge inertia, i.e., the momentum induced by the mass flux from the river. The plumes are also subject to buoyancy, wind and tidal forcing, which dictate dispersal patterns and dynamics. This multi-institution investigation characterized plume dynamics for the eight major river systems of southern California.

Status

This study was completed in 2007.

Methods

To evaluate the dispersal patterns and dynamics of the freshwater plumes, this study analyzed MODIS satellite remote sensing images in combination with in situ water samples from multi-day cruises, buoy meteorological observations, drifters, and HF radar current measurements. For comparison, in-situ measurements were obtained by shipboard profiling of the plumes with an enhanced CTD system (conductivity, temperature, depth, dissolved oxygen, pH, transmissometer, chlorophyll fluorometer, and CDOM fluorometer). Sampling occurred on regularly spaced grids for each region. River discharge observations were also obtained from USGS and other gauging stations.

Findings

The combined plume observing techniques revealed that plumes commonly detach from the coast and turn to the left, which is the opposite direction of the Coriolis influence. Although initial offshore velocity of the buoyant plumes was ~50 cm/s and was influenced by river discharge inertia (i.e., the direct momentum of the river flux) and buoyancy, subsequent advection of the plumes was largely observed in an alongshore direction and dominated by local winds. Due to the multiple day upwelling wind conditions that commonly follow discharge events, plumes were observed to flow from their respective river mouths to down-coast waters at rates of 20 to 40 km/day.

Suspended sediment concentration and beam-attenuation were poorly correlated with plume salinity across and within the sampled plumes, while colored dissolved organic matter (CDOM) fluorescence was well correlated, suggesting that CDOM may serve as a good tracer of the discharged freshwater in subsequent remote sensing and monitoring efforts of plumes.

Partners

USGS Coastal and Marine Geology Program
NOAA/NESDIS Center for Satellite Applications and Research
Orange County Sanitation District
Department of Biological Sciences, University of Southern California
Institute for Computational Earth System Science, University of California, Santa Barbara
Marine Physical Laboratory, Scripps Institute of Oceanography