Environmental Impact Statement - Sonoma Land Trust

California Department of Fish and Game
U.S. Fish and Wildlife Service
Section 3.2. Surface-Water Hydrology,
Tidal Hydraulics, and Sedimentation
The morphology of San Pablo Bay is characterized by extensive mudflat and
subtidal mud surfaces and a primary 30- to 40-foot-deep channel (the Pinole
Shoal Channel) extending from Carquinez Strait to San Pablo Strait. The Pinole
Shoal Channel is dredged annually by the USACE for deep draft navigation to
the ports of Richmond, Mare Island, Pittsburg, Antioch, Stockton, and
Sacramento. A smaller channel, the Petaluma Across the Flats Channel, traverses
the mudflats from the mouth of the Petaluma River to the primary channel. The
mudflats outside of these channels slope gently upwards through the tidal range
to San Pablo Bay’s shoreline. Average depths are less than 6 feet over much of
the mudflat and subtidal mud surfaces. The shoreline fringe is primarily tidal
marsh, whose width varies from zero to several hundred feet along the Bay’s
northern shoreline.
Tides
Tides in San Pablo Bay follow a mixed semidiurnal cycle, with 2 high tides of
unequal elevation and 2 low tides of unequal elevation per day. The average of
the higher high tide elevation values are referred to as mean higher high water
(MHHW) and the average of all the high water heights observed is referred to as
mean high water (MHW). Similarly, low tide peaks are referred to as mean low
water (MLW) and mean lower low water (MLLW). The highest tide level water
will reach in any one year is referred to as extreme high tide (EHT).
Because of geographic and hydrodynamic conditions, tidal characteristics –
including the elevations of average high, low, and mean tides – differ
substantially throughout the San Francisco Bay–San Pablo Bay system. Tide
cycles in San Pablo Bay typically lag behind those at the Golden Gate by
approximately 90 minutes (National Oceanic & Atmospheric
Administration/National Ocean Survey 2005a and 2005b). Table 3.2-1 shows
statistical tidal information near the project site, obtained from measurements
made by the National Oceanic & Atmospheric Administration/National Ocean
Survey (NOAA/NOS) at the mouth of the Petaluma River (Tide Gage #941
5252) (National Oceanic & Atmospheric Administration/National Ocean Survey
2005c). Spring tides are periods of increased tidal range which occur around
dates of the full and new moon.
Sears Point Wetland and Watershed Restoration
Project Final Environmental Impact
Report/Environmental Impact Statement
3.2-6
April 2012