Human development during the last century has brought profound changes
to South San Francisco Bay’s estuary and marshes, fueling concern about
the fate of the remaining marshlands.
A monitoring program mandated by the San Francisco Bay Regional Water
Quality Control Board requires the city of San José, Calif., to annually
assess the marshes potentially affected by freshwater discharge from the
San José-Santa Clara Water Pollution Control Plant (WPCP). The studies
use IKONOS 1-meter satellite imagery from GeoEye Inc. (www.geoeye.com)
to document annual changes to plant species in the study area, thereby
determining the distribution and aerial extent of salt, brackish and
freshwater marsh.
Assessing the Study Area
The area’s first large-scale plant community changes were observed in
the 1970s and confirmed by additional studies in 1984. As part of the
Water Quality Control Board’s monitoring program, the city of San José
contracted with H.T. Harvey & Associates to conduct more detailed
studies of the marshes in 1989, 1991, 1994 and annually thereafter.
The data help researchers better understand the relative influences of
environmental and anthropogenic factors affecting changes in marsh type.
In addition, the information will help assess how long-term changes in
marsh composition could potentially affect two endangered salt marsh
wildlife species (see “Monitoring Surveys Protect Endangered Species,”
below). The data also serve as an important baseline for the 15,000-acre
South Bay Salt Pond Restoration Project, the largest wetlands
restoration project on the U.S. West Coast.
The pan-sharpened, multispectral (red, green,
blue and near-infrared bands) satellite imagery has been used for
plant association mapping since 2004. Color-infrared (CIR) aerial
photography was used to monitor the study area for years, but in
2003 it was determined that 1-meter satellite imagery could
significantly reduce costs and still provide needed resolution (see
“Satellite Imagery Provides a Cost-Effective Solution,” below).
The total marsh study area is approximately 2,000 acres. As detailed
in the upper-left figure, the area is divided into the Main Study
Area, a portion of the South San Francisco Bay near and directly
downstream of San José/Santa Clara WPCP discharge, and a Reference
Area located on Alviso Slough, which forms the mouth of the
Guadalupe River.
A GIS-Based Approach
Complete ground-truthing of the preliminary mapping work was
conducted during site visits to the project area during August and
September 2005. Marsh vegetation was observed primarily from areas
directly adjacent to the marshes to maintain consistency with the
methods employed in previous years, as well as to follow U.S. Fish
and Wildlife Service (USFWS) guidelines and regulations.
Acreage -calculations by plant -associations,
dominant species and habitat type maps, as well as acreage tables,
were produced in ArcGIS geographic information system (GIS) software
from ESRI (www.esri.com).
The GIS database was downloaded and backed-up weekly. Digitized
boundaries of habitat areas were reviewed for consistency and
quality, and plant association acreages and color-coded figures for
the entire study area were generated. Comparisons were made between
2005 maps and those created in previous years.
Habitat mapping was completed at a scale of 1:2,400 using IKONOS
imagery acquired between May and June of each study year during low
tide as a base layer prior to the start of fieldwork. The spectral
signatures obtained from the IKONOS imagery identified known and
tentative habitat classifications. Two laptop computers equipped
with ArcGIS, along with the IKONOS imagery, were used to assist in
habitat field verification mapping.
Overall Results
The entire study area has become less saline since 1989. Newly
forming freshwater marsh habitat in both the Reference Area and the
Main Study Area indicates that freshwater influences are affecting
all local marshes.
Between 1989 and 1999, there was greater change in habitat types in
the Reference Area compared to the Main Study Area. However, there
was also a higher rate of new marsh formation in the Main Study
Area. This indicates that the conversion of salt marsh habitats
within the South San Francisco Bay Estuary was likely driven by
large-scale influences affecting the entire system. The overall
gains in salt marsh habitat from 2001 to 2005 highlight the multiple
factors affecting changes in marsh vegetation types in South San
Francisco Bay.
Freshwater discharges from the WPCP clearly influence plant species
distribution within the south portion of the Artesian Slough.
However, the volume of WPCP discharge has been relatively constant
since 1990. During that same period salt marsh conversion further
downstream has fluctuated. Therefore, it’s likely that much of the
interannual variation in habitats within the South Bay marshes is
due to the ongoing resizing of the channels from sedimentation and
reductions in tidal prism, as well as large-scale environmental
factors, such as changes in annual rainfall, changes in bay salinity
due to delta outflows, and local land subsidence or increases in
mean sea level.
Satellite Imagery Benefits
IKONOS satellite imagery has proved to be a reliable, cost-effective
tool for studying marsh vegetation over a large area. Aerial
photography provides higher resolution, but at a greater cost and
with significant limitations to flexibility: aerial overflights have
to be specifically planned and contracted, whereas IKONOS images can
be purchased from archives. Despite a slight reduction in
resolution, the multispectral quality of IKONOS imagery provides an
indication of plant types that is as good or better than that
provided by aerial photographs.
Publisher’s note: For further information on the marsh studies
performed by the city of San José, visit the Marsh Studies Web site
at
www.sanjoseca.gov/esd/marsh-studies.htm.
