The offshore export of sand during exceptional discharge from California
rivers.
Jonathan A. Warrick# and Patrick L. Barnard
U.S. Geological Survey
Pacific Coastal and Marine Science Center
400 Natural Bridges Drive
Santa Cruz, CA 95060 USA
# - corresponding author: jwarrick@usgs.gov, 831-460-7569
ABSTRACT
Two competing hypotheses exist for sand discharge from rivers. The first
hypothesis is that the majority of river sediment discharge will be emplaced within
and immediately “resupply” local littoral cells. The competing hypothesis is that the
infrequent, large floods that discharge the majority of sediment may discharge
water-sediment mixtures within negatively buoyant hyperpycnal plumes that
transport sediment offshore of the littoral cell. Here we summarize pre- and postflood surveys of two river deltas along the California coast following record to nearrecord floods to help evaluate these hypotheses: the 1982-‘83 delta at the San
Lorenzo River mouth and the 2005 delta at the Santa Clara River mouth. Flood
sedimentation at both deltas resulted in several meters of aggradation and 100’s of
meters of offshore displacement of isobaths. One substantial difference between
these deltas was the thick (over 2 m) aggradation of sand on the inner shelf of the
Santa Clara River delta that contained substantial amounts (~50%) of littoral-grade
sediment. Once deposited on the inner shelf, only a fraction (~20%) of this river
sand was observed to migrate toward the beach over the following five years.
Furthermore, simple hypopycnal plume behavior could not explain deposition of
this sand on the inner shelf. Thus, during an exceptional flood a substantial amount
of littoral-grade sand was exported offshore of the littoral system at the Santa Clara
River mouth – likely from hyperpycnal plume processes – and was deposited on the
inner shelf.