Historical Sedimentation in the San Francisco Estuary Bruce Jaffe 1, Theresa Fregoso 1, Amy...

Historical Sedimentation in the San Francisco Estuary

Bruce Jaffe1, Theresa Fregoso1,

Amy Foxgrover1, Shawn Higgins2

1 United States Geological Survey2 Kamman Hydrology and Engineering

San Pablo Bay

Outline

• The Estuary in the past 150 years

• Sediment connectivity of sub-Sediment connectivity of sub-embaymentsembayments

• Sediment delivery control on Sediment delivery control on tidal flat changetidal flat change

Outline

• The Estuary in the past 150 years

• Sediment connectivity of sub-Sediment connectivity of sub-embaymentsembayments

• Sediment delivery control on Sediment delivery control on tidal flat changetidal flat change

Jaffe et al., 1998; Capiella et al., 1999; Foxgrover et al., 2004; Jaffe and Foxgrover, 2006a; Jaffe and Foxgrover, 2006b; Jaffe et al., 2007; Fregoso et al., 2008; Jaffe, 2009

Outline

• The Estuary in the past 150 years

• Sediment connectivity of sub-Sediment connectivity of sub-embaymentsembayments

• Sediment delivery control on Sediment delivery control on tidal flat changetidal flat change

The Data

Image obtained from NOAA photo library (www.photolib.noaa.gov)

Sample Data- South San Francisco Bay

Generation of Bathymetric Models

• Digitize soundings, contours, and marsh extent

• Georeference data• Error checking • Generate 25 or 50m bathymetric grid using TopoGrid

San Francisco Estuary

in the 1980s

• Area: ~1200 km2 • Average depth: 7 m• Median depth: 3 - 4 m• Deepest point: ~120 m

(Golden Gate)

The morphology of the estuary has changed

1850s 1890s 1920s

1950s 1980s

Generation of Sedimentation Grids

• Correct to a common vertical datum

• Adjust for subsidence (as needed)

• Difference the grids

Difference Grids

Net sediment gain of ~250 million cubic meters from 1850s - 1980s

Suisun Bay

Loss

52 Mcm

San Pablo Bay

Gain

339 Mcm

Central Bay

Gain

71 Mcm

( loss of 48 Mcm from borrow areas)

South Bay

Loss

90 Mcm

( loss of 38 Mcm from borrow areas)

The patterns of sediment gain and loss are complex

1850s to 1890s

1890s to 1920s

1920s to 1950s

1950s to 1980s

1850s to 1890s

1890s to 1920s

1920s to 1950s

1950s to 1980s

1850s to 1890s

1890s to 1920s

1920s to 1950s

1950s to 1980s

1850s to 1890s

1890s to 1950s

1950s to 1980s

San Pablo Bay

Suisun Bay

South Bay

Central Bay

300

Mcm

0

200

100

-100

300

Mcm

0

200

100

-100

300

Mcm

0

200

100

-100

300

Mcm

0

200

100

-100

1850s to 1890s- gain of ~290 Mcm

Hydraulic mining

1850s to 1890s

1890s to 1920s

1920s to 1950s

1950s to 1980s

1850s to 1890s

1890s to 1920s

1920s to 1950s

1950s to 1980s

1850s to 1890s

1890s to 1920s

1920s to 1950s

1950s to 1980s

1850s to 1890s

1950s to 1980s

San Pablo BaySuisun Bay

1890s to 1950s

South Bay

Central Bay

300

Mcm

0

200

100

-100

300

Mcm

0

200

100

-100

300

Mcm

0

200

100

-100

300

Mcm

0

200

100

-100

After hydraulic mining

1890s to 1920s- loss of ~100 Mcm

1850s to 1890s

1890s to 1920s

1920s to 1950s

1950s to 1980s

1850s to 1890s

1890s to 1920s

1920s to 1950s

1950s to 1980s

1850s to 1890s

1890s to 1920s

1920s to 1950s

1950s to 1980s

1850s to 1890s

1950s to 1980s

San Pablo Bay

1890s to 1950s

Suisun Bay

South Bay

Central Bay

300

Mcm

0

200

100

-100

300

Mcm

0

200

100

-100

300

Mcm

0

200

100

-100

300

Mcm

0

200

100

-100

1920s to 1950s- gain of ~100 Mcm

SF Bay Area population increase

1850s to 1890s

1890s to 1920s

1920s to 1950s

1950s to 1980s

1850s to 1890s

1890s to 1920s

1920s to 1950s

1950s to 1980s

1850s to 1890s

1890s to 1920s

1920s to 1950s

1950s to 1980s

1850s to 1890s

1950s to 1980s

San Pablo Bay

Suisun Bay

1890s to 1950s

South Bay

Central Bay

300

Mcm

0

200

100

-100

300

Mcm

0

200

100

-100

300

Mcm

0

200

100

-100

300

Mcm

0

200

100

-100

1950s to 1980s- loss of ~200 Mcm

Water projects

From the 1850s-1980s

Circular bays near the center of the Estuary bays (San Pablo and Central) had net deposition

Elongate bays at ends of estuary (Suisun and

South) had net erosion

From 1950s-1980s

All bays were erosional

-100

0

100

200

300

400

500

1850 1900 1950 2000

Suisun BaySan Pablo BayCentral BaySouth Bay

Cum

ulat

ive

Sed

imen

tatio

n (1

0 6

x m

3)

Year

Erosion

DepostionNet

Net

Question:

Will the bays continue to erode?

-100

0

100

200

300

400

500

1850 1900 1950 2000

Suisun BaySan Pablo BayCentral BaySouth Bay

Cum

ulat

ive

Sed

imen

tatio

n (1

0 6

x m

3)

Year

Erosion

Depostion

?

?

??

-100

0

100

200

300

400

500

1850 1900 1950 2000

Suisun BaySan Pablo BayCentral BaySouth Bay

Cum

ulat

ive

Sed

imen

tatio

n (1

0 6

x m

3)

Year

Erosion

Depostion

?

?

?

Partial answer:

Not necessarily….

South San Francisco Bay had net deposition from 1983 to 2005

No data for other bays

Outline

• The Estuary in the past 150 years

• Sediment connectivity of sub-embayments

• Sediment delivery control on Sediment delivery control on tidal flat changetidal flat change

Sediment Connectivity

Sub-embayment#1

Sub-embayment#2

Sediment Exchange

Determining the degree of sediment connectivity

• Modeling studies

• Tracers (natural and anthropogenic)

• Patterns and quantities of erosion and deposition

Determining the degree of sediment connectivity

• Patterns and quantities of erosion and deposition– Similar behavior (erosion and deposition occur at sub-embayments at the same time)

– Opposite behavior of subembayments with similar magnitude (sediment eroded from one sub-embayment transported and deposited in another embayment)

– Quantity of deposition greater than local tributary supply (sediment coming from somewhere else)

Net

Net

South SF Bay example of similar behavior (inferred sediment

connectivity)

Outline

• The Estuary in the past 150 years

• Sediment connectivity of sub-Sediment connectivity of sub-embaymentsembayments

• Sediment delivery control on tidal flat change

Tidal flats of the estuary have changed

1850s 1890s 1920s

1950s 1980s

San Pablo Bay

Central Bay

Suisun Bay

South Bay

1850s1890s1920s1950s1980s

0

20

40

60

80

100

km2

0

20

40

60

80

100

km2

0

20

40

60

80

100

km2

0

20

40

60

80

100

km2Tidal flat area decrease >50%

Intertidal mudflat area responded to changes

in sediment supply to San Pablo Bay

Mudflats widened where sediment was abundant

Jaffe et al., 2007

Summary- Historical Sedimentation in the San Francisco Estuary

• The estuary in the past 150 years– The patterns of gain and loss of sediment are complex

– There were periods with net gain (e.g., hydraulic mining in mid-1800s) and periods with net loss (e.g., water projects in mid- to late 1900s)

• Sediment connectivity of sub-embayments– Patterns and quantities of erosion and deposition point towards a connected system

• Sediment supply controls on tidal flats– In San Pablo Bay, increased tidal flat area during the hydraulic mining period corresponded to increased sediment supply. Decrease in tidal flat area during the late 1900s corresponded with a decrease in sediment supply.

Sediment gains and losses- 1950s to 1980s

Sediment removal from 1950s to 1980s

Areas outlined in green are where sediment was removed

> 50 Mcm of sediment removed from the system

~60% of sediment loss from Central Bay was from human activities

1850s1890s1920s1950s1980s

Central Bay

South Baykm2

km2

Marsh area decrease >90%