A History of Intertidal Flat Area in South 

San Francisco Bay, California: 1858 to 

2005 

By Bruce Jaffe and Amy Foxgrover 

Open-File Report 2006-1262 

U.S. Department of the Interior 

U.S. Geological Survey
 Open-File Report 2006-1262 ii

U.S. Department of the Interior 

Dirk Kempthorne, Secretary 

U.S. Geological Survey

P. Patrick Leahy, Acting Director 

U.S. Geological Survey, Reston, Virginia 2006 

Revised and reprinted: 2006 

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Suggested citation: 

Jaffe, B.E. and Foxgrover, A.C., 2006, A History of intertidal flat area in 

South San Francisco Bay, California: 1858 to 2005, U.S. Geological 

Survey Open-File Report 2006-1262, 32 pp. [URL: 

http://pubs.usgs.gov/of/2006/1262]

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Although this report is in the public domain, permission must be secured 

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material contained within this report. 
 Open-File Report 2006-1262 iii

Front Cover: 

Preliminary map of intertidal flat area (region shaded brown), bathymetry, and tidal 

marsh in South San Francisco Bay from 1858 to 2005. 
 Open-File Report 2006-1262 iv

Contents

Abstract......................................................................................................................... 1 

Background and Introduction ....................................................................................... 1 

Methods......................................................................................................................... 2 

Determination of Intertidal Flat Area ........................................................................ 2 

Intertidal Flat Area Uncertainty................................................................................. 3 

Calculation of Intertidal Flat Area Uncertainty ..................................................... 6 

Estimate of Average Intertidal Flat Width ................................................................ 7 

Results........................................................................................................................... 7 

Change in Intertidal Flat Area for the Portion of South San Francisco Bay Surveyed 

from 1858 to 2005 ..................................................................................................... 9 

Regional Changes in Intertidal Flat Area ................................................................ 10 

Regional Changes in Intertidal Flat Width.............................................................. 14 

Discussion................................................................................................................... 15 

Caveats on Intertidal Flat Area................................................................................ 15 

Trends...................................................................................................................... 15 

Deviations from Trends........................................................................................... 17 

Summary..................................................................................................................... 18 

Acknowledgements..................................................................................................... 18 

References................................................................................................................... 19 

Contact Information.................................................................................................... 21 

Tables.......................................................................................................................... 22 

Appendix I- South San Francisco Bay Intertidal Flat Notes...................................... 26
 Open-File Report 2006-1262 1

A History of Intertidal Flat Area in 

South San Francisco Bay, California: 

1858 to 2005 

By Bruce Jaffe and Amy Foxgrover 

Abstract 

A key question in salt pond restoration in South San Francisco Bay is whether 

sediment sinks created by opening ponds will result in the loss of intertidal flats. 

Analyses of a series of bathymetric surveys of South San Francisco Bay made from

1858 to 2005 reveal changes in intertidal flat area in both space and time that can be 

used to better understand the pre-restoration system. This analysis also documents 

baseline conditions of intertidal flats that may be altered by restoration efforts. From

1858 to 2005, intertidal flat area decreased by about 25% from 69.2 +6.4/-7.6 km2

 to 

51.2 +4.8/-5.8 km2

. Intertidal flats in the north tended to decrease in area during the 

period of this study whereas those south of Dumbarton Bridge were either stable or 

increased in area. From 1983 to 2005, intertidal flats south of Dumbarton Bridge 

increased from 17.6 +1.7/-2.5 km2

 to 24.2 +1.0/-1.8 km2. Intertidal flats along the 

east shore of the bay tended to be more erosional and decreased in area while those 

along the west shore of the bay did not significantly change in area. Loss of intertidal 

flats occurred intermittently along the eastern shore of the bay north of the 

Dumbarton Bridge. There was little or no loss from 1931 to 1956 and from 1983 to 

2005. Predictions of future change in intertidal flat area that do not account for this 

spatial and temporal variability are not likely to be accurate. The causes of the spatial 

and temporal variability in intertidal flat area in South San Francisco Bay are not fully 

understood, but appear related to energy available to erode sediments, sediment 

redistribution from north to south in the bay, and sediment available to deposit on the 

flats. Improved understanding of sediment input to South San Francisco Bay, 

especially from Central Bay, how it is likely to change in the future, the redistribution 

of sediment within the bay, and ultimately its effect on intertidal flat area would aid in 

the management of restoration of South San Francisco Bay salt ponds. 

Background and Introduction 

Intertidal flats, sometimes referred to as mudflats or tidal flats, are relatively 

flat regions submerged at high tide and exposed at low tide. In South San Francisco 

Bay, intertidal flats are primarily composed of mud, especially in the far South Bay 

south of Dumbarton Bridge, but they can be almost entirely sand or shell material, as 

is the case along portions of the east shore. Intertidal flats are an important 

component of the South San Francisco Bay ecosystem, providing many functions 

including supplying a foraging habitat to hundreds of thousands of shore birds each 

year (Stenzel et al. 2002). A key issue in restoration of salt ponds in South San 

Francisco Bay is whether conversion of salt ponds to tidal wetlands will result in loss 

of intertidal flats. This is certainly a possibility if the balance of deposition and 

erosion on intertidal flats is shifted towards erosion through the creation of sediment 
 Open-File Report 2006-1262 2

sinks in subsided ponds opened to tidal exchange. The California Coastal 

Conservancy is in the process of assessing how, to what extent, and at what rate to 

restore salt ponds to optimize beneficial effects to the ecosystem while minimizing 

detrimental impacts such as loss of intertidal flats (South Bay Salt Pond Restoration 

Project, 2006). 

Figure 1. Location of study area showing pond complexes being considered 

for restoration as part of the South Bay Salt Ponds Restoration Project. 

This report presents preliminary data on the history of intertidal flat area in 

South San Francisco Bay from 1858 to 2005. Data is presented for the region north 

of a line connecting Calaveras Point to Guadalupe Slough (where Coyote Creek 

enters the bay) to Hunter’s Point (Fig. 1). The goal of this report is to document and 

present a preliminary analysis of the historical temporal and spatial variability of the 

intertidal flat area in South San Francisco Bay. These data serve as a basis for 

conceptual and quantitative models to predict likely responses of the intertidal flats to 

restoration of salt ponds in South San Francisco Bay. These data can also be used to 

evaluate the cause of intertidal flat change that has occurred during the last 148 years. 

During this time numerous factors and forcings such as wind wave erosion, tidal 

current redistribution of sediment, changes in sediment supply from tributaries and 

Central Bay, and sea level rise, altered the intertidal flat system of South San 

Francisco Bay. These factors and forcings will continue to alter the system as 

restoration proceeds. 

Methods 

Determination of Intertidal Flat Area 

Intertidal flats, as defined in this report, are the region bounded by the Mean 

Lower Low Water (MLLW) contour and the shoreline, which is the Mean High 

Water (MHW) line (Umbach, 1976). Intertidal flat area in South San Francisco Bay 
 Open-File Report 2006-1262 3

was determined using data from hydrographic and topographic surveys dating back to 

1858. From 1858 to 1983, five surveys (1858, 1898, 1931, 1956, 1983) were 

conducted in South San Francisco Bay by the National Ocean Service (NOS) 

(formerly the U.S. Coast and Geodetic Survey, USCGS). The most recent survey of 

the bay was conducted in 2005 by Sea Surveyor, Inc. and relied upon tidal reduction 

schemes and datum conversions supplied by the National Oceanic and Atmospheric 

Administration’s (NOAA) Center for Operational Oceanographic Products and 

Services (CO-OPS), (Jaffe et al., 2005). For all surveys, depth soundings were 

collected in the shoals at or near high water, enabling detailed mapping of the 

intertidal flats. Shorelines were delineated during topographic surveys that were 

performed in conjunction with the hydrographic surveys. 

For surveys from 1858 to 1956, the MLLW contour was digitized from

contours drawn by hand by NOAA (USCGS) mapmakers on the original smooth 

sheets (H-Sheets). For the 1983 and 2005 surveys, we digitized a MLLW contour 

that we drew based on sounding values. For the 1858 to 1983 surveys, shorelines 

were derived from NOAA (USCGS) topographic sheets (1858 and 1983 shorelines 

obtained from the EcoAtlas; SFEI, 1998). The 2005 shoreline was based on EcoAtlas 

data (SFEI, 1998) adjusted using data from May 2004 lidar survey (Foxgrover et al., 

2005). 

Intertidal Flat Area Uncertainty

Uncertainty in intertidal flat area derives from the use of depth soundings to 

determine the location of the MLLW contour and from imprecision in the location of 

the shoreline. By far, the largest uncertainty in intertidal flat area is in the location of

the MLLW contour. 

The location of the MLLW contour is affected by the accuracy of depth 

soundings, the interpretation of bay morphology near MLLW by the mapmaker, and 

the rounding scheme used for plotting soundings on H-sheets. The accuracy criteria 

for depth soundings varied with survey period (Schalowitz, 1964; Adams, 1942) and 

can be estimated at trackline crossings where two independent measures of depth are 

compared. Differences in sounding values at crossings result from both horizontal 

(positioning) and vertical (instrumental and tidal reduction) error. Criteria used by 

NOAA (USCGS) for acceptable trackline crossing error are summarized in Sallenger 

et al. (1975). The first instructions including sounding accuracy criteria were 

presented circa 1860 (Schalowitz, 1964) as “the allowable error at sounding-line 

crossings was not to be more than 3 percent of the depth, with a limiting error of five 

percent”. 

For example, if soundings were taken at a high tide of 2 m above MLLW, the 

3% criteria for depth error is 0.06 m, which results in a shift of the MLLW contour 

approximately 35 m on the very flat slopes (<0.1 degree or 0.017%) typical near 

MLLW in South San Francisco Bay. The resulting difference in intertidal flat area in 

South San Francisco Bay would be approximately 3.5 km2

 (35 m times ~100 km of 

shoreline with intertidal flats) (Table 1). If soundings were taken at lower tides, the 

error would be less. It is also likely that accuracy has improved over time with 

advancements in technology (from leadline and sextant in the earlier surveys, to 

digital echo-sounder and Differential GPS in the last survey). Additionally, this type
 Open-File Report 2006-1262 4

of error should not be biased and would tend to cancel out because a sounding is just 

as likely to be deeper or shallower than the true value. 

It is difficult to assess the uncertainty introduced by variation in approaches to 

drawing the MLLW contour. Because the distance between soundings along 

tracklines was relatively small, from 1 m for the 2005 survey to 150 m for the 1850s 

survey, and the typically simple morphology in the region near MLLW, this is not a 

large source of uncertainty. However, a bias would be created if two mapmakers 

consistently located the MLLW contour 25 m landward or bayward of the other. The 

resulting difference in intertidal flat area in South San Francisco Bay would be 

approximately 2.5 km2

 (25 m times ~100 km of shoreline with intertidal flats). 

Inspection of the smooth sheets reveals consistency in the approach of mapmakers to 

drawing the MLLW contour, so bias is not of concern. The greater data density and 

more precise data for the 1983 and 2005 surveys constrain MLLW better than earlier 

surveys, further decreasing the mapmaker’s interpretation as a source of uncertainty 

for intertidal flat area. 

The rounding of sounding values will result in change in intertidal flat area by 

shifting the MLLW contour bayward or landward. Rounding conventions were not 

the same for all surveys used in this analysis (see Appendix I for notes on rounding 

conventions). Adams (1942) gives instructions for reporting sounding values for 

surveys conducted from the late 1800s to the mid 1900s as, “In order that the location 

of the low-water line may be more precisely delineated on the smooth sheet, the 

soundings in the vicinity of the line should be plotted to the nearest half-foot”. With 

the exception of the 1850s survey (with tidal flat soundings values all shown as 0) 

and the 1980s and 2005 surveys with digital sounding values recorded in meters and 

tenths, all soundings near MLLW were plotted on smooth sheets to the nearest half 

foot. For the 1898, 1930s, and 1956 surveys, rounding could result in depths 0.25 

feet (0.076 m) deeper or shallower than MLLW being reported as MLLW, resulting

in an uncertainty in intertidal flat area. 

Uncertainty in location of the shoreline also contributes to uncertainty in 

intertidal flat area. Morton et al. (2004) estimated an uncertainty of ~10 m or less for 

shoreline positions determined from USCGS and NOS topographic sheets. The 

USCGS standard that “no point shall be more than 2.0 mm (0.075”) from it true 

geographic position” (Swanson, 1949) corresponds to an upper limit uncertainty of 20 

m in shoreline location for the 1:10,000 scale topographic sheets that are typical for 

South San Francisco Bay. A 10-20 m uncertainty in location of the shoreline results 

in approximately 1-2 km2 difference (10- 20 m times ~100 km of shoreline with 

intertidal flats) in intertidal flat area in South San Francisco Bay if the shoreline was 

consistently located either landward or bayward of its true position. However, 

comparison of the topographic sheets from different surveys reveals that hard objects 

(e.g.; levees) do not exhibit consistent offset in locations between sheets. 

It is clear from the above that the uncertainty in intertidal flat area changed for 

different periods as survey techniques, standards, and sounding reporting conventions 

changed. Also, uncertainty is inversely related to the slope of the bay floor near 

MLLW. Taking all of the above into consideration, the conservative estimate of the 

uncertainty in intertidal flat area for South San Francisco Bay that is used in this 

report is based on the assumption that the MLLW contour may be either 0.076 m (1/4 
 Open-File Report 2006-1262 5

foot) too deep or too shallow and results in the translation of its position landward or 

bayward (Table 1). The actual uncertainty is probably less because of stringent 

sounding-error criteria for shallow soundings (Schalowitz, 1964; Sallenger et al, 

1975) and the reduction of the overall error in MLLW contour location resulting from 

the averaging of a large number of estimates. Uncertainty in shoreline location is 

considered small relative to MLLW location uncertainty and is considered to be 

unbiased and not contribute significantly to intertidal flat area uncertainty. 

Because the slope on the bayward side of the MLLW contour tends to be 

steeper than the slope on the landward side, the 0.076 m depth uncertainty results in 

asymmetric area uncertainty with greater uncertainty landward (Fig. 2). A 

consequence of this is a slight tendency for overestimation of intertidal flat area for 

all periods. 

Figure 2. Uncertainty bands for intertidal flat area in 2005 using a ± 0.076 m 

(1/4 foot) depth uncertainty criteria. The MLLW contour is at the boundary of 

the green and red shaded bands. Note that the width of uncertainty bands 

increases as slope decreases on the landward side of the MLLW contour. 

The effect of choosing higher or lower values for depth uncertainty on total 

South San Francisco Bay intertidal flat area for 2005 is shown in Figure 3. Choosing 

a less conservative depth uncertainty value of 0.038 m (1/8 foot) decreases intertidal 

flat area uncertainty by 45% from the 0.076 m (1/4 foot) depth uncertainty adopted 

for this report. Choosing an extremely conservative depth uncertainty value of 0.15 

m (1/2 foot) increases intertidal flat area uncertainty by about 95% for the 2005 

survey. 
 Open-File Report 2006-1262 6

Figure 3. Variation in intertidal flat area uncertainty for South San Francisco 

Bay for 2005 as a function of depth uncertainty. 

Calculation of Intertidal Flat Area Uncertainty 

To calculate the uncertainty in intertidal flat area arising from depth 

uncertainty it was necessary to construct a series of bathymetric models of South San 

Francisco Bay because uncertainty depends on slope. Each model of the bay floor is 

based on thousands to millions of individual depth soundings and depth contours. 

The generation of accurate bathymetric models is a time intensive process involving a 

number of steps (Foxgrover et al., 2004) because the model must honor the data and 

have realistic morphology in area not supported by data. For earlier surveys (pre 

1930s), no digital data existed and the soundings were manually digitized from Mylar 

copies of the original NOS H-sheets. Soundings from more recent surveys were 

obtained in digital format. Depth contours were either traced from H-sheets or added 

based upon sounding values. The digitized soundings and shorelines were entered 

into a GIS and georeferenced to a common horizontal datum. An ArcInfo surface 

modeling module (TopoGrid) was used to generate 50 m resolution bathymetric grids 

for the NOS (USCGS) surveys (Foxgrover et al. 2004) and a 25 m resolution grid for 

the 2005 survey. 

The bathymetric model for the intertidal flat in 1858 is a special case. Prior to 

1860 it was common practice to display zeros for all soundings above the MLLW

plane of reference, without regard to the actual elevations (Schalowitz, 1964). This is 

a result of plotting practices at that time and does not accurately reflect the slope of 

the intertidal flats. To create a more realistic slope in the intertidal, we artificially 

generated contours to mimic the slopes documented in the 1890s surveys in which 

soundings above MLLW were retained. Contours were placed at approximately 1⁄2 

and 1⁄4 of the distance between the shoreline and MLLW (Appendix I). These 

contours were assigned values of 25% and 50%, respectively; of Mean Tide Level 

(MTL) as estimated from surrounding tide stations. 

To augment the bathymetric models, marsh extent was also documented. For 

all periods except 2005, the marsh extent was derived from topographic sheets (1850s 
 Open-File Report 2006-1262 7

and 1980s marsh extent from SFEI EcoAtlas, 1998). The 2005 marsh extent was 

acquired from the National Wetlands Inventory and based upon aerial imagery (this 

data has not yet undergone QA/QC from the U.S. Fish and Wildlife Service). 

Estimate of Average Intertidal Flat Width 

To estimate a metric for change in intertidal flat width over time, we 

calculated an average intertidal flat width by dividing intertidal flat area by an 

estimate of shoreline length. This is only a rough metric because the length of 

shoreline varies with complexity, scale of the map, and distance between digitized 

points (it is fractal) and this length changes over time. For this report, we used a 

straight-line approximation for sections of shorelines with similar orientations, which 

resulted in segments ranging from several kilometers to 5-10 km long. We also 

calculated a width uncertainty by dividing the area uncertainty by the estimate of 

shoreline length. Where this value was less than 0.1 km, we report a value of 0.1 km. 

Results 

The history of intertidal flat area, bathymetry, and tidal marsh for the South 

San Francisco Bay is shown in Figure 4. The pattern of intertidal flat area change is 

complex in detail, although inspection of Figure 4 reveals general trends and coherent 

patterns such as loss over time, similar behavior for large sections of flats, differences 

between the northern and southern flats, and differences between flats along the 

eastern and western shores of the bay. 

 Open-File Report 2006-1262 9

Change in Intertidal Flat Area for the Portion of South San 

Francisco Bay Surveyed from 1858 to 2005 

This section presents data for the portion of South San Francisco Bay 

surveyed six times from 1858 to 2005, which is defined by the region north of a line 

connecting Calaveras Point to Guadalupe Slough (where Coyote Creek enters South 

San Francisco Bay) to the northern extent of the 2005 bathymetric survey (Fig. 4). 

Intertidal flat area decreased by about 25% from 1858 to 2005 from 69.2 +6.4/ -7.6 

km2

 to 51.2 +4.8/-5.8 km2 (Table 2, Figure 5). Between the last two surveys, from

1983 to 2005, intertidal flat area could have been constant when considering the 

uncertainty bounds used in this report (46.4 +5.7/-6.0 km2

 in 1983; 51.2 +4.8/-5.8 

km2 in 2005). Another complexity in behavior of the system is that the rate of 

intertidal flat loss appears to be fairly constant from 1858 to 1956, and then increased 

from 1956 to 1983. This increased rate of loss occurred during a period of erosion 

and removal of sediment from the subtidal of South San Francisco Bay for use in bay 

fill and cement production (Foxgrover et al., 2004). The relative stability in intertidal 

flat area from 1983 to 2005 was accompanied by deposition in the main channel and 

shallow subtidal regions. 

Figure 5. Intertidal flat area in South San Francisco Bay south of the 2005 

survey northern boundary from 1858 to 2005. Error bars indicate change in 

area resulting in ± 0.076 m (1/4 foot) depth uncertainty about MLLW. 

The vast majority of intertidal flat area change was the result of erosion 

or accretion at the bayward edge (MLLW). Reclamation and conversion of 

mudflats to marsh south of San Francisco and Oakland airports resulted in 

approximately 8 km2

 of intertidal flat loss from 1858 to 1983 (Fig. 6). Erosion 

of the shoreline during the same time period created approximately 7 km2

 of 

new intertidal flats, which nearly offsets the loss from reclamation and marsh 

formation. Of the 18 km2

 of loss, only about 1 km2 (6%) was from change 

near the shoreline. 
 Open-File Report 2006-1262 10

Figure 6. Intertidal flat change at the shoreward edge from 1858 to 1983. 

Areas shaded red were intertidal flat in 1858 lost to reclamation and marsh 

formation by 1983. Areas in green were land in 1858 that became intertidal 

flats. 

Erosion of intertidal flats, shown by a landward shift in the MLLW in Figure 

7, occurred between every survey in some portions of the bay (e.g.; the west shore, 

north of Dumbarton Bridge). In contrast, erosion of intertidal flats occurred 

intermittently along the eastern shore of the bay north of the Dumbarton Bridge. 

There, the 1983 and 2005 MLLW contours coincide, but are landward of the 1931 

and 1956 MLLW contours, which also coincide indicating erosion from 1956 to 

1983, but no significant erosion from 1931 to 1956 or from 1983 to 2005. 

Regional Changes in Intertidal Flat Area 

To further examine the spatial and temporal variability of intertidal flat 

change, the study area was divided into eight regions (Fig. 8). Change in intertidal 

flat area was variable (Table 3, Fig. 9), although coherent patterns of change emerge 

when examining north-south trends (Table 4, Fig. 10) and east-west trends (Table 5, 

Fig. 11). 
 Open-File Report 2006-1262 11

Figure 7. Bayward boundary of intertidal flats (MLLW contour) for South San 

Francisco Bay for surveys conducted from 1858 to 2005. The extent of water 

in 1853 and the 1983 shoreline are shown for reference. 

Figure 8. Regions in South San Francisco Bay used in the analysis of spatial 

and temporal variability of intertidal flat area. 
 Open-File Report 2006-1262 12

Figure 9. Change in intertidal flat area from 1858 to 2005 for regions shown 

in Figure 8. Error bars indicate change in area resulting from ± 0.076 m (1/4 

foot) depth uncertainty about MLLW. 
 Open-File Report 2006-1262 13

Figure 10. North-south variation in intertidal flat area in South San Francisco 

Bay south of the 2005 survey northern boundary from 1858 to 2005. Intertidal 

flats in the north tended to decrease in area whereas ones south of 

Dumbarton Bridge were either stable or increased. From 1983 to 2005, 

intertidal flat area south of Dumbarton Bridge increased from 17.6 +1.7/-2.5 

km2

 to 24.2 +1.0/-1.8 km2. Error bars indicate change in area resulting from 

± 0.076 m (1/4 foot) depth uncertainty about MLLW. 

Figure 11. East-west variation in change in intertidal flat area in South San 

Francisco Bay south of the 2005 survey northern boundary from 1858 to 

2005. From 1858 to 1983, intertidal flats along the east shore of the bay 

decreased in area. From 1983 to 2005, area was constant within the 

uncertainty of the analysis. Intertidal flats along the west shore of the bay did 

not significantly increase or decrease in area. Error bars indicate change in 

area resulting in ± 0.076 m (1/4 foot) depth uncertainty about MLLW. 
 Open-File Report 2006-1262 14

The combined north-south and east-west trends are presented in Figure 12. In 

brief, the east shore of the bay had abundant intertidal flats in 1858 that eroded in the 

subsequent 148 years. The area south of Dumbarton, especially along the west shore, 

gained intertidal flat area through the filling of subtidal channels and gained or lost

area though the contraction or expansion of a subtidal basin in its northern section 

(Fig. 4). From 1983 to 2005, intertidal flat area south of Dumbarton Bridge increased 

by about 35% from 17.6 +1.7/-2.5 km2

 to 24.2 +1.0/-1.8 km2 (Table 4). In 2005, 

intertidal flats were the major component of the bay south of Dumbarton Bridge, 

accounting for about 70% of the total area. 

Figure 12. Spatial and temporal variation in intertidal flat area in South San 

Francisco Bay south of the northern boundary of the 2005 survey from 1858 

to 2005. Error bars indicate change in area resulting in ± 0.076 m (1/4 foot) 

depth uncertainty about MLLW. Dashed lines are regions along the east 

shore; solid lines are regions along the west shore. 

Regional Changes in Intertidal Flat Width 

Average intertidal flat width, a rough metric to quantify flats, generally 

decreased throughout the study period for all portions of the bay except south of 

Dumbarton Bridge (Table 6, Figs. 4, 7). In 1858, the widest intertidal flats, 1.8 +0.1 

-0.2 km on average, were on the east shore of the bay between the present locations of 

the Dumbarton and San Mateo Bridges. In 2005, intertidal flats were widest on the 

western shore south of Dumbarton Bridge, 1.4 +0.1 -0.1 km on average, and extended 

more than 2 km into the bay in places (Fig. 4). 
 Open-File Report 2006-1262 15

Discussion 

Caveats on Intertidal Flat Area 

Although we have attempted to quantify intertidal flat uncertainty for each 

survey we have not addressed issues of comparing areas between surveys. 

Hydrographic surveys customarily are conducted over the span of a few years and at 

different times of year. Seasonal or longer variations in intertidal flat area caused by 

cycles in sediment input, wave height, winds, tidal currents or other factors confounds 

the detection of long-term trends. Thomson-Becker and Luoma (1985) found that 

grain size on intertidal flats along the west shore South San Francisco Bay near Palo 

Alto coarsened during seasons with greater tidal velocities and wind speeds and fined 

during periods when precipitation was high (increased runoff delivering sediment to 

the bay). Because the grain size changes with erosion and deposition, intertidal flat 

area may have changed with grain size. Anecdotal evidence of rapid accretion on the 

order of 5 to 10 cm on the flats of South San Francisco Bay supports such seasonal 

changes (pers. comms. Janet Thompson and Fred Nichols). The magnitude of 

seasonal or interannual variation in intertidal flat area in South San Francisco Bay is 

not know and is potentially an important knowledge gap in understanding the natural 

intertidal flat system and the effects of salt pond restoration on intertidal flat area. 

Another factor that would bias the comparison of surveys is the change in the 

method for determining the MLLW sounding datum. For the surveys after 1898, the 

MLLW datum was calculated using the method of simultaneous comparisons 

(Dedrick, 1983). It is possible that the 1898 survey also used simultaneous 

comparison to determine the MLLW datum. Simultaneous comparisons allows a 

short time series of local water levels to be used, in conjunction with a long time 

series from a primary station, to calculate tidal metrics for the standard 19-year tidal 

epoch (Swanson, 1974). A MLLW datum based on a tidal epoch averages out shortterm fluctuations (e.g. effects of the phase of the moon or seasonal steric effects). 

Because the 1858, and possibly the 1898, surveys used a short time series of water 

levels from local tide stations to calculate the MLLW datum, short-term fluctuations 

in MLLW is reflected in the datum. Because of the differences in MLLW datum

determination, additional caution should be used when drawing conclusions based on 

comparisons of intertidal flat areas from the earlier surveys with ones from the 20th

century. 

Trends 

The spatial and temporal trends in intertidal flat area are consistent, in general, 

with our knowledge of the hydrodynamics and the sediment dynamics of South San 

Francisco Bay, but also require new conceptual models to be fully explained. A 

greater rate of loss of intertidal flats along the eastern shore than the western shore 

(Tables 3, 5; Figs. 9, 11, 12) is consistent with wave-induced erosion from the higher 

wave energy there caused by the predominate westerly and northwesterly winds 

(Hayes, 1984). Another contributing factor could be that the sediments, which are 

sandier and contain oyster shells, are more erodable along the bay’s eastern shore 

than in regions where the bay floor is compacted mud. It should be noted that there is 
 Open-File Report 2006-1262 16

also a significant loss of intertidal flats along portions of the west shore that occur in 

the “lower” wave energy regime (Table 3; Figs. 4, 7, 9, 12). 

The stability of the intertidal flats south of Dumbarton Bridge is consistent 

with low wave energy caused by the short fetches in this region and constriction near 

Dumbarton Bridge restricting propagation of waves from the north into the far south 

bay. Stability and growth of intertidal flats south of Dumbarton Bridge is also 

consistent with a reduced tidal prism from loss of tidal marshes, which could decrease 

tidal velocities, leading to filling of subtidal channels. However, filling of subtidal 

channels in this region from 1858 to 1898 predated the largest loss of tidal marshes, 

which occurred from 1898 to 1931. A possible explanation for the filling of subtidal 

channels from 1858 to 1898 is an increased sediment supply to the far south bay, 

either associated with the vast quantities of debris from hydraulic gold mining that 

entered the San Francisco Estuary (Jaffe et al, 1998; Jaffe et al., accepted) or with 

increased load from local tributaries, causing a disequilibrium between 

hydrodynamics and subtidal channel size. 

The stability of intertidal flats south of Dumbarton Bridge from 1931 to 1983 

(Tables 3, 4; Figs. 9, 10), which includes the period with subsidence induced by 

groundwater withdrawal in San Jose from the 1930s to 1960s (Poland and Ireland, 

1982), is evidence of an abundance of sediment supply to this region. There was 

sufficient sediment to maintain the intertidal flats even though there was about 1 m of 

subsidence in some portions of this region (Poland and Ireland, 1982; Foxgrover et 

al., 2004). A conceptual model for the long-term sediment transport pattern of South 

San Francisco Bay is that sediment redistributes from north to south where it is 

available for deposition in the shallows to maintain or grow intertidal flats south of 

Dumbarton Bridge. This conceptual model is supported by the history of intertidal 

flat area and of net volume change in bay sediments; the region south of Dumbarton 

Bridge is the only one with net deposition for all time periods since 1858 (Foxgrover 

et al., 2004). A key unknown in predicting the effects of restoration on intertidal flats 

(and other components of the bay and marsh geomorphic system) is the supply of 

sediment from Central San Francisco Bay. If there is net sediment inflow, then the 

likelihood for successful restoration is increased and impacts related to a paucity of 

sediment, such as loss of intertidal flats, would be minimized. Research to improve 

understanding of sediment exchange between South and Central San Francisco Bays 

would aid in the management of restoration of South San Francisco Bay salt ponds. 

The trend of decreased intertidal flat area over time north of Dumbarton 

Bridge is consistent with the conceptual model of waves eroding the flat’s bayward 

edge and shore-protection measures preventing erosion of the shore, which would 

otherwise compensate for loss bayward. Loss is also occurring where the shoreline is 

prograding (e.g.; north of San Mateo Bridge along the east shore), which indicates a 

shift in intertidal flat morphology, possibly resulting from sediment supply 

constraints or change in the erodibility of sediments. 

Another cause of intertidal flat loss is sea level rise, which creates sediment 

demand to maintain flats. For each centimeter of sea level rise, about 1 million cubic 

meters of sediment (0.01 m times 100 km2

) is required to raise the intertidal flat 

system, which includes a subtidal portion that acts to dissipate wave energy, to keep 

up with sea level. From 1855 to 1999, the trend in mean sea level rise at San 
 Open-File Report 2006-1262 17

Francisco was 0.145 cm/yr (Flick et al., 2003). This corresponds to about 0.15 

million cubic meters of sediment each year to maintain intertidal flats, which is a 

small number in a sediment budget for South San Francisco Bay. An analysis of data 

from 1870 to 2004 from a global network of tide gauges showed sea level rise has 

been accelerating (Church and White, 2006). If this acceleration continues at the 

same rate, sea level in 2100 would be 31 ± 3 cm higher than in 1990, which 

corresponds to a sediment demand of about 31 ± 3 million cubic meters (0.3 million 

cubic meters per year). 

Cayan et al. (submitted) provide a range of future sea level rise estimates for 

California from a set of climate simulations governed by lower, middle-upper, and 

higher green house gas emission scenarios. Projecting sea level rise from the ocean 

warming in global climate models, observational evidence of sea level rise, and 

separate calculations using a simple climate model yields a range of potential sea 

level increases, from 11 cm to 72 cm for California, by the 2070-2099 period 

(Climate Action Team, 2006; Cayen et al., submitted). The projected 0.1 to about 1.0 

cm/yr average sea level rise for the next hundred years corresponds to an average of 

0.1 to 1.0 million cubic meters of sediment each year to maintain the intertidal flats of 

South San Francisco Bay. Sediment required to maintain the intertidal flats of South 

San Francisco Bay to keep up with an accelerated sea level rise is a large term in a 

sediment budget. If the sea level rise is in the higher part of the range of estimates, 

the increased sediment demand for the entire bay system could severely impact 

restoration, especially north of Dumbarton Bridge. 

Additional research to improve understanding of the causes of historic 

intertidal flat loss and the likely effects of accelerated sea level rise would aid in the 

management of restoration of South San Francisco Bay salt ponds. 

Deviations from Trends 

There are significant deviations from the spatial and temporal trends of 

intertidal flat loss in South San Francisco Bay. Consideration of spatial variation in 

wind, wave, and tidal energy; sediment characteristics; human activities, and other 

factors affecting the sediment dynamics redistributing sediment in South San 

Francisco Bay are likely to provide significant insights into the cause of the 

variations. 

Deviations from the general trend of loss of intertidal flat area over time are of 

the utmost importance for understanding causes for intertidal flat change and have 

significant implications for management of restoration of the salt ponds. Pulses of 

sediment to South San Francisco Bay are evident in the intermittent change in the 

MLLW (Fig. 7) and intertidal flat area (Tables 3, 4; Figs. 9, 12). When a sediment 

pulse enters, the MLLW contour maintains its position, as is the case for the periods 

from 1931 to 1956 and from 1983 to 2005 (Fig. 7). Intertidal flat loss is either slowed 

or reversed during these periods of large sediment input (Fig. 9). The sediment pulse 

periods correspond with deposition or decreased erosion in the subtidal (Foxgrover et 

al., 2004). To increase the certainty of predictions for the effects of salt pond 

restoration on intertidal flats and other components of the bay and marsh system, we 

need to improve the understanding of the cause and likelihood of continued sediment 

pulse events in the future. 
 Open-File Report 2006-1262 18

Summary 

A preliminary analysis of bathymetric surveys to document the history of 

intertidal flat area in South San Francisco Bay from 1858 to 2005 revealed changes in 

both space and time. Major changes included: 

• Intertidal flat area in South San Francisco Bay decreased by about 25% from 69.2 

+6.4/-7.6 km2 in 1858 to 51.2 +4.8/ -5.8 km2 in 2005. 

• Intertidal flats in the north tended to decrease in area whereas those south of 

Dumbarton Bridge were either stable or increased in area. From 1983 to 2005, 

intertidal flat area south of Dumbarton Bridge increased from 17.6 +1.7/-2.5 km2

to 24.2 +1.0/-1.8 km2

. 

• Intertidal flats along the east shore of the bay decreased in area while those along 

the west shore of the bay did not significantly change in area. 

• Loss of intertidal flat area occurred stepwise along the eastern shore of the bay 

north of the Dumbarton Bridge. There was little or no loss from 1931 to 1956 and 

from 1983 to 2005. Other time periods had significant loss of intertidal flats. 

• The causes for the spatial and temporal changes in intertidal flat area in South San 

Francisco Bay are not fully understood. A key knowledge gap is the sediment 

input to the system. Improved understanding of sediment input, especially from

Central Bay, how it is likely to change in the future, and it redistribution and 

effect on intertidal flat area would aid in the management of restoration of South 

San Francisco Bay salt ponds. 

Acknowledgements 

The California Coastal Conservancy, U.S. Geological Survey Priority 

Ecosystem Study of San Francisco Bay, and the San Francisco Bay Regional Water 

Quality Control Board supported this research. Melissa Ingraca digitized historical 

hydrographic and topographic surveys. Shawn Higgins contributed to georeferencing 

H-sheets and modeling of the bay surface. This report was improved by reviews from

Jessica Lacy and Theresa Fregoso. 
 Open-File Report 2006-1262 19

References 

Adams, K.T., 1942, Hydrographic manual (revised ed.). U.S. Dept. of Commerce, 

Coast and Geodetic Survey, Special Publication 143, revised edition, 407 pp. 

Cayan, D. R., Bromirski, P.D., Hayhoe, K., Tyree, M., Dettinger, M.D., and Flick, 

R.E., submitted, Climate change projections of sea level extremes along the 

California coast, Climate Change. 

Church, J.A. and White, N.J., 2006, A 20th century acceleration of global sea-level 

rise, Geophysical Research Letters, v. 33, L01602, 

doi:10.1029/2005GL024826. 

Climate Action Team, 2006, Climate Action Team report to Governor 

Schwarzenegger and the Legislature, 110 p. [URL: 

http://www.climatechange.ca.gov/climate_action_team/reports/2006-04-

03_FINAL_CAT_REPORT.PDF] 

Dedrick, K.G., 1983, Use of Early Hydrographic Surveys in Studies of California 

Estuaries. Coastal Zone ’83, v. III, p. 2294-2316. 

Flick, R.E., J.F. Murray and L.C. Ewing, 2003, Trends in United States tidal datum

statistics and tide range, Journal of Waterway, Port, Coastal and Ocean 

Engineering, American Society of Civil Engineers, v. 129, no. 4, p. 155-164. 

Foxgrover, A.C., Higgins, S.A., Ingraca, M.K., Jaffe, B.E., and Smith, R.E., 2004, 

Deposition, erosion, and bathymetric change in South San Francisco Bay: 

1858-1983: U.S. Geological Survey Open-File Report 2004-1192, 25 p. 

[URL: http://pubs.usgs.gov/of/2004/1192] 

Foxgrover, A.C. and Jaffe, B.E., 2005, South San Francisco Bay 2004 topographic 

lidar survey: Data overview and preliminary quality assessment, U.S. 

Geological Survey Open-File Report OFR-2005-1284, 57 p. [URL: 

http://pubs.usgs.gov/of/2005/1284/] 

Hayes, T. P., Kinney, J. J., and Wheeler, N. J., 1984, California surface wind 

climatology, California Air Resources Board, Aerometric Data Division, 107 

pp. 

Jaffe, B. E., Smith, R. E., and Torresan, L. Z., 1998, Sedimentation and bathymetric 

change in San Pablo Bay: 1856 to 1983: U.S. Geological Survey Open-File 

Report 98-759 [URL: http://wrgis.wr.usgs.gov/open-file/of98-759/]. 

Jaffe, B., Hutzel, A., Foxgrover, A., Takekawa, J., Athearn, N., Hubbard, J., Samant, 

M., Martin, C., Hovis, G., Sullivan, S., Vickers, C., and Newby, S., 2005, 

Integrated LIDAR and bathymetric surveys of the South San Francisco Bay 

region: New data for salt pond restoration planning, (abs.) 7th Biennial State 

of the Estuary Conference, Oakland, CA, p.110. 

Jaffe, B. E., Smith, R. E., and Foxgrover, A., accepted with revision, Anthropogenic 

Influence on Sedimentation and Intertidal Mudflat Change in San Pablo Bay, 

California: 1856 to 1983, Estuarine Coastal and Shelf Science 

Morton, R.A., Miller, T.L., and Moore, L,J., 2004, National assessment of shoreline 

change: Part 1 Historical shoreline changes and associated coastal land loss 

along the U.S. Gulf of Mexico, U.S. Geological Survey Open File Report 

2004-1043 [URL: http://pubs.usgs.gov/of/2004/1043]. 
 Open-File Report 2006-1262 20

Poland, J.F., and Ireland, R.L. 1988, Land Subsidence in the Santa Clara Valley, 

California, as of 1982: U.S. Geological Survey Professional Paper 497-F, 61 

p. 

Sallenger, A.H., Goldsmith, V., and Sutton, C.H., 1975, Bathymetric comparisons: A 

manual of methodology, error criteria and techniques. Special Report in 

Applied Marine Science and Ocean Engineering (SRAMSOE) 66, 34 pp. 

San Francisco Estuary EcoAtlas, 1998, Version 1.50b4, Compact Disc. 

Schalowitz, A.L., 1964, Shore and sea boundaries: U.S. Coast and Geodetic Survey 

Publication 10-1, v. 2, 749 pp. 

South Bay Salt Pond Restoration Project, 2006, [URL: 

http://www.southbayrestoration.org] 

Stenzel, L. E., C. M. Hickey, J. E. Kjelmyr, and G. W. Page, 2002, Abundance and 

distribution of shorebirds in the San Francisco Bay area. Western Birds, v. 33, 

p. 69-98. 

Swanson, L.W., 1949, Topographic Manual Part II Photogrammetry. US Coast and 

Geodetic Survey Special Publication No. 249, 580 pp. [URL: 

http://unicorn.csc.noaa.gov/docs/rarebooks/00000005/doc.pdf] 

Swanson, R.L., 1974, Variability of tidal datums and accuracy in determining datums

from short series of observations. National Oceanic and Atmospheric 

Administration Technical Report 64, 41 pp. 

Thomson-Becker, E.A. and Luoma, S.N., 1985, Temporal fluctuations in grain size, 

organic materials and iron concentrations in intertidal surface sediment of San 

Francisco Bay, Hydrobiologia, v. 129, p. 91-107. 

Umbach, M.J., 1976, Hydrographic Manual Fourth Edition, U.S. Dept. of 

Commerce, Coast and Geodetic Survey, 400 pp. [URL: 

http://www.thsoa.org/pdf/hm1976/hmall.pdf] 
 Open-File Report 2006-1262 21

Contact Information 

Bruce Jaffe: bjaffe@usgs.gov

Amy Foxgrover: afoxgrover@usgs.gov

For details on the South Bay Salt Pond Restoration Project visit: 

http://www.southbayrestoration.org/ 
 Open-File Report 2006-1262 22

Tables 

Table 1. Terms contributing to intertidal flat area uncertainty for South San 

Francisco Bay. A slope of 0.1 degrees (0.017%) and 100 km of shoreline 

with intertidal flats were used to convert depth uncertainty to area uncertainty. 

Estimate of Uncertainty

Depth 

Near 

MLLW 

(m) 

MLLW 

Contour 

Position 

(m) 

Intertidal 

Flat Area 

 (km2

) Notes

Soundings near 

MLLW 

< 0.06 < 35 < 3.5 Not biased, tend to cancel out

Drawing MLLW 

Contour 

- < 25 < 2.5 Not biased, tend to cancel out

Shoreline Location - 10-20 < 1-2 Not biased, tend to cancel out

Sounding Rounding < 0.076 < 45 < 4.5 Could be biased, varies with

survey, < for 1983, 2005 

surveys 

Used in this Report 0.076 varies < 7.6 Depends on survey, calculated 

from actual slopes near MLLW

Table 2. History of intertidal flat area in South San Francisco Bay south of the 

northern boundary of the 2005 survey. Entries for uncertainly above or below 

MLLW is the change in intertidal flat area caused by the landward (above 

MLLW) or bayward (below MLLW) shift in the MLLW contour resulting from a 

± 0.076 m (1/4 foot) change in depth. 

Year Intertidal Flat 

Area (km2

) 

Uncertainty

Above MLLW (km2) 

Uncertainty

Below MLLW (km2) 

1858 69.2 7.6 6.4

1898 64.1 6.8 6.0

1931 60.8 6.7 5.8

1956 58.1 7.1 6.3

1983 46.4 6.0 5.7

2005 51.2 5.8 4.8
 Open-File Report 2006-1262 23

Table 3. History of variation in intertidal flat area in South San Francisco Bay 

in regions defined in Figure 8. Entries for uncertainly above or below MLLW 

is the change in intertidal flat area caused by the landward (above MLLW) or 

bayward (below MLLW) shift in the MLLW contour resulting from a ± 0.076 m 

(1/4 foot) change in depth. 

Intertidal Flat Area (km2

) 

1858 1898 1920 1956 1983 2005

NNSM-W 10.0 5.5 5.0 5.9 3.6 not surveyed

uncertainty above MLLW 1.8 0.9 1.0 0.9 0.3 not surveyed

uncertainty below MLLW 1.3 1.0 0.8 0.9 0.4 not surveyed

NNSM-E 4.3 2.3 4.2 2.0 0.6 not surveyed

uncertainty above MLLW 0.3 0.4 2.0 0.2 0.1 not surveyed

uncertainty below MLLW 0.4 0.4 1.8 0.3 0.2 not surveyed

1858 1898 1931 1956 1983 2005

NSM-W 3.3 3.2 2.2 2.2 1.3 1.2 

uncertainty above MLLW 0.4 0.6 0.4 0.4 0.2 0.1

uncertainty below MLLW 0.4 0.8 0.4 0.5 0.3 0.1

NSM-E 13.3 12.1 10.5 9.0 5.4 5.1 

uncertainty above MLLW 0.9 1.4 2.1 1.5 0.4 0.7

uncertainty below MLLW 0.7 1.1 1.7 1.2 0.8 0.7

ND-W 13.6 11.2 12.7 11.6 10.1 8.7 

uncertainty above MLLW 0.5 0.9 1.1 1.5 1.3 1.5

uncertainty below MLLW 0.8 0.8 0.9 1.1 1.2 1.5

ND-E 23.6 18.7 16.2 15.5 12.0 12.0 

uncertainty above MLLW 2.1 2.6 1.8 2.3 1.6 1.6

uncertainty below MLLW 1.5 2.3 1.5 2.4 1.6 1.4

SD-W 8.5 12.5 12.9 13.4 11.0 16.3 

uncertainty above MLLW 3.5 1.2 1.1 1.3 2.0 1.7

uncertainty below MLLW 2.7 0.8 1.1 1.0 1.5 0.9

SD-E 7.0 6.4 6.4 6.4 6.7 7.9 

uncertainty above MLLW 0.2 0.2 0.2 0.1 0.5 0.2

uncertainty below MLLW 0.3 0.2 0.2 0.1 0.3 0.1
 Open-File Report 2006-1262 24

Table 4. History of north-south variation in intertidal flat area in South San 

Francisco Bay in regions defined in Figure 8. Entries for uncertainly above or 

below MLLW is the change in intertidal flat area caused by the landward 

(above MLLW) or bayward (below MLLW) shift in the MLLW contour resulting 

from a ± 0.076 m (1/4 foot) change in depth. 

 Intertidal Flat Area (km2

) 

1858 1898 1920 1956 1983 2005

NNSM 14.4 7.8 9.2 8.0 4.2 not surveyed

uncertainty above MLLW 2.1 1.3 3.0 1.2 0.3 not surveyed

uncertainty below MLLW 1.7 1.5 2.6 1.1 0.6 not surveyed

1858 1898 1931 1956 1983 2005

NSM 16.6 15.2 12.8 11.3 6.7 6.3 

uncertainty above MLLW 1.3 2.0 2.4 1.9 0.6 0.8

uncertainty below MLLW 1.2 1.9 2.1 1.7 1.1 0.8

ND 37.2 29.9 28.9 27.1 22.1 20.7 

uncertainty above MLLW 2.7 3.5 2.9 3.8 2.9 3.2

uncertainty below MLLW 2.3 3.1 2.4 3.5 2.9 2.9

SD 15.4 19.0 19.2 19.8 17.6 24.2 

uncertainty above MLLW 3.7 1.4 1.3 1.4 2.5 1.8

uncertainty below MLLW 2.9 0.9 1.3 1.1 1.7 1.0

Table 5. History of east-west variation in intertidal flat area in South San Francisco 

Bay in regions defined in Figure 8. Entries for uncertainly above or below MLLW is

the change in intertidal flat area caused by the landward (above MLLW) or bayward 

(below MLLW) shift in the MLLW contour resulting from a ± 0.076 m (1/4 foot) 

change in depth. 

Intertidal Flat Area (km2) 

1858 1898 1931 1956 1983 2005

West Intertidal Flats 25.4 26.9 27.8 27.2 22.4 26.2 

uncertainty above MLLW 4.4 2.6 2.6 3.2 3.5 3.3

uncertainty below MLLW 3.8 2.4 2.3 2.6 3.0 2.6

East Intertidal Flats 43.9 37.2 33.1 30.9 24.1 24.9 

uncertainty above MLLW 3.3 4.2 4.1 3.9 2.5 2.5

uncertainty below MLLW 2.5 3.6 3.5 3.6 2.7 2.2
 Open-File Report 2006-1262 25

Table 6. History of variation in average intertidal flat width in South San Francisco 

Bay in regions defined in Figure 8. This metric is calculated by dividing intertidal 

flat area by an estimate of shoreline length. Shoreline lengths are straight-line 

approximations for sections of shorelines with similar orientations, which resulted in 

segments ranging from several kilometers to 5-10 km long. Shoreline lengths used 

were NNSM-W 23 km, NNSM-E 8 km, NSM-W 8 km, NSM-E 10 km, ND-W 17 

km, ND-E 13 km, SD-W 12 km, SD-E 9 km. Width uncertainty was calculated by 

dividing the area uncertainty (Table 2) by the estimate of shoreline length. Where 

this value was less than 0.1 km, we report a value of 0.1 km. 

Average Intertidal Flat Width (km)

1858 1898 1920 1956 1983 2005

NNSM-W 0.4 0.2 0.2 0.3 0.2 not surveyed

uncertainty above MLLW 0.1 0.1 0.1 0.1 0.1 not surveyed

uncertainty below MLLW 0.1 0.1 0.1 0.1 0.1 not surveyed

NNSM-E 0.5 0.3 0.5 0.3 0.1 not surveyed

uncertainty above MLLW 0.1 0.1 0.2 0.1 0.1 not surveyed

uncertainty below MLLW 0.1 0.1 0.2 0.1 0.1 not surveyed

1858 1898 1931 1956 1983 2005

NSM-W 0.4 0.4 0.3 0.3 0.2 0.2 

uncertainty above MLLW 0.1 0.1 0.1 0.1 0.1 0.1

uncertainty below MLLW 0.1 0.1 0.1 0.1 0.1 0.1

NSM-E 1.3 1.2 1.1 0.9 0.5 0.5 

uncertainty above MLLW 0.1 0.1 0.2 0.1 0.1 0.1

uncertainty below MLLW 0.1 0.1 0.2 0.1 0.1 0.1

ND-W 0.8 0.7 0.7 0.7 0.6 0.5 

uncertainty above MLLW 0.1 0.1 0.1 0.1 0.1 0.1

uncertainty below MLLW 0.1 0.1 0.1 0.1 0.1 0.1

ND-E 1.8 1.4 1.2 1.2 0.9 0.9 

uncertainty above MLLW 0.2 0.2 0.1 0.2 0.1 0.1

uncertainty below MLLW 0.1 0.2 0.1 0.2 0.1 0.1

SD-W 0.7 1.0 1.1 1.1 0.9 1.4 

uncertainty above MLLW 0.3 0.1 0.1 0.1 0.2 0.1

uncertainty below MLLW 0.2 0.1 0.1 0.1 0.1 0.1

SD-E 0.8 0.7 0.7 0.7 0.7 0.9 

uncertainty above MLLW 0.1 0.1 0.1 0.1 0.1 0.1

uncertainty below MLLW 0.1 0.1 0.1 0.1 0.1 0.1
 Open-File Report 2006-1262 26

Appendix I- South San Francisco Bay Intertidal Flat 

Notes 

Data Source and Sounding Precision and Rounding

1850s 

• Soundings plotted in integral feet in shallows (or 1⁄4 fathom in main channel). 

• Soundings above MLLW all replaced with zeros. 

• Original sounding books record depth to tenths of feet. Maps obtained from 

the State Lands Commission (the work of Kent Dedrick) have the zero 

soundings above MLLW substituted with actual values in feet and tenths, as 

recorded in the sounding books. These maps cover the majority of South Bay 

on the eastern shore and south of the Dumbarton Bridge and will be 

incorporated into this work in the future. 

• MLLW contour digitized from H-sheets. 

1890s 

• About half of the H-sheets (H2304, H2315, H2412) show soundings plotted in 

1⁄4 foot increments. 

• H2411, H2413, H2414 and H2415 are plotted 1⁄2 foot increments. 

• MLLW contour digitized from H-sheets. 

1930s 

• Northernmost H-sheet (H4137) soundings are in integral feet, for all other 

sheets shallow soundings are plottted in 1⁄2 foot increments, and soundings in 

the main channel in integral feet. 

• H4137 was digitized by us, for the 6 other sheets soundings were downloaded 

off of GEODAS. 

• GEODAS header states that smooth sheets were “digitized for NOS under 

Ashville Contract, lead line assumed, non-acoustic depth measurement (in

feet)”. Soundings in GEODAS are in meters and tenths, but otherwise are 

identical to values on H-sheets. 

• Depth contours are hard to distinguish on scanned H-sheets (especially in the 

shallows), therefore some of the contours are based upon sounding values. 

• MLLW contour digitized from H-sheets and soundings. 

1950s 

• Select intertidal soundings (+/- 0.7 feet MLLW) plotted in 1⁄2 foot increments 

while other soundings plotted in integral feet. Limited number of 3.5 and 6.5 

ft soundings displayed (presumably for more accurate placement of contour 

lines), all other soundings plotted in integral feet. 

• Soundings downloaded from GEODAS. GEODAS headers state that “smooth 

sheets were digitized for NOS under Ashville Contract. Digital echo sounder 
 Open-File Report 2006-1262 27

with graphical record assumed, units in feet.” Soundings in GEODAS are in 

meters and tenths, but otherwise are identical to values on H-sheets. 

• MLLW contour digitized from H-sheets and soundings. 

1980s 

• Soundings downloaded from GEODAS. Digital soundings from GEODAS 

are the reduced soundings in meters and tenths. 

• GEODAS headers state that all sheets were originally recorded in feet and 

tenths except H10132 which claims meters and tenths (not likely) and 

H10102, H10158, and H9927 just say feet (not tenths). Headers also state that 

digital echo sounder with graphical record assumed. 

• On H-sheets select shallow soundings (+/- 0.7 ft MLLW) plotted in 1⁄2 foot 

increments. All other soundings plotted in integral feet with a shoal based 

rounding scheme applied. Therefore soundings downloaded from GEODAS 

do not identically match the H-sheets. 

• Contours digitized based upon sounding values, not H-sheets. 

• Because of rounding, placement of the MLLW contour is different when 

compared to H-sheets. 

• Descriptive report states that values for H09984 and H10070 do not have 

appropriate tide corrector values. It is unknown whether or not the soundings 

in GEODAS have been corrected. 

Estimating 1850s Intertidal Flat Slope Where Only 0 Sounding Values Exist

A new coverage was created to which contours were added based on our 

estimate of intertidal flat slope. The first contour was drawn at 1⁄2 the distance 

between MHW and MLLW paralleling the shoreline. The second contour was drawn 

at 1⁄2 the distance between the first contour and MHW. 

The first contour (1/2 the distance between MHW and MLLW) was assigned 

an elevation of: MTL / 4. The second contour (1/2 the distance between MHW and 

MLLW), was assigned an elevation of: MTL / 2. These contours, when combined 

with the shoreline (MHW) and shoreline buffer (see section 9), produce a tidal flat 

profile that compares to the slope observed from more recent surveys (see figure 

below). 

We also added supplementary soundings (values estimated from surrounding 

soundings) to the narrow channels crossing the tidal flats at steam outlets to prevent 

discontinuity of channels as a result of sparse soundings. 
 Open-File Report 2006-1262 28

25% of MTL

50% of MTL

Buffer (MTL) 

 Profile of modified 1858 tidal flat. 

Descriptive Report Notes

1850s 

• H628 – the reducers for H-sheets 421, 628, 629, & 636 appear to be referred 

to the plane of MLLW within the allowable error of 0.5ft. 

• H637 has some pts within 0.5ft error, some may be in error up to a foot due to 

limited tide data. 

1890s 

• H2304 – a number of shell banks were found which do not show on the old 

chart. The shoreline seems to be growing out into the bay 

• H2315 – some changes in mud flats and shell banks found where not 

previously shown on chart. 

• H2411, H2412, H2413, H2414, and H2415 have no notes in the Descriptive 

Reports. 

1930s 

• H4137 refers to using simultaneous comparison for reducing soundings.