U.S. Geological Survey Open-File Report 93-298, 1993.
Ordering information: http://pubs.er.usgs.gov/publication/ofr93298
Map of Seafloor Declivity and Fall Lines on the Continental Slope, Gulf
of the Farallones, Central California
Monty A. Hampton, Florence L. Wong, Robert V. Lugo, and W. Clinton
The accompanying map depicts the seafloor declivity
and fall lines, displayed over a bathymetric
base, on a portion of the continental slope in the Gulf of the Farallones,
central California. The map was prepared as part of an extensive geophysical
and sediment-sampling program whose purpose is to map and interpret the
offshore geology of the Farallones region for application to resource and
environmental concerns. The seafloor in the region is littered with debris
that includes sunken ships and barrels of low-level radioactive waste.
Seafloor declivity is the slope of the seafloor measured from horizontal.
A fall line indicates the direction of steepest slope. One application of
this map is to evaluate the gravitationally induced movement of objects
on the seafloor. Objects in areas of steep declivity are more likely to
move in response to gravity than those in areas of shallow declivity, and
if they do move, they will follow the general path of the fall lines.
The bathymetric contours, seafloor declivity, and fall lines were derived
from National Oceanic and Atmospheric Administration (NOAA) multibeam
data. These data depths are accurate within 1% of true depths and have a
positional accuracy of 75 meters (National Geophysical Data Center, Data
Announcement 92-MGG-03, November 1992). The National Ocean Service (NOS),
Coast and Geodetic Survey, Ocean Mapping Section produced gridded data sets
from these raw data with grid nodes at 250-meter intervals. Computer programs
and techniques were developed within the USGS to store, extract, manipulate,
and display these data. Norman Maher contoured the bathymetry with the
ARC/INFO functions developed for hydrologic modeling were used to determine
declivity and fall lines, using flow- direction and accumulated-flow routines
(Environmental Systems Research Institute, Inc., 1992, Cell-based modeling
with GRID 6.1: Redlands, California, Environmental Systems Research Institute,
Inc., p. 1-1 to 1-13). Values of declivity were calculated for each 250-m
grid node, then color-coded to represent declivity values that fall within
5-degree intervals, except for the shallowest slopes, which were subdivided
into slopes less than 2 degrees and slopes from 2 to 5 degrees. The maximum
declivity in this area is 33.4 degrees.
The network of fall lines produced at the 250-m grid spacing is too dense
to be displayed at the 1:250,000 scale of the map. For this reason, we chose
to initiate fall lines at every fourth grid node (a 2-km grid). The fall
lines were extended iteratively by looking at the surrounding eight nodes
and connecting the reference node with the node having the greatest negative
difference (i.e., the greatest increase in seafloor depth). Thus, the
precision of the fall lines is 45 degrees, the angle between the reference
node and the successive surrounding nodes. The fall lines have no directional
indicators, but direction can be deduced from bathymetric contours near
a particular fall line.
The calculations for the fall lines expose local sinks - cells whose neighbors
are all higher in elevation. Further calculations were not done to fill
the sinks, which causes fall lines to end at midslope in some places.
Flow accumulation refers to the number of upslope cells that flow into a
particular cell. A cell with zero flow accumulation is a topographic high.
A cell with a high value of flow accumulation lies in a major channel. Fall
lines are generated by connecting nodes with progressively greater flow
This map is preliminary and has not been reviewed for conformity with
U.S. Geological Survey editorial standards or with the North American
Code. Any use of trade, product, or firm names in this publication is for
descriptive purposes only and does not imply endorsement by the U.S. Government.
For sale by U.S. Geological Survey, Open-File Reports-ESIC, Box 25286, Federal
Center, Denver, CO 80225-0046. http://pubs.er.usgs.gov/publication/ofr93298
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