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
Climate
The project site and surrounding area are characterized by a Mediterranean
climate with warm, dry summers and cool, wet winters. The climate is strongly
influenced by conditions in San Francisco Bay and, to a lesser extent, the Pacific
Ocean. Data from a climate station at the San Rafael Civic Center, located
approximately 10 miles southeast of the project site, indicate that July is typically
the warmest month, with a mean daytime temperature of approximately 82°F.
January is the coldest month, with a mean daytime temperature of approximately
56° F. Minimum and maximum daily temperatures range from approximately 50
to 82 o F in the summer months and 41 to 69°F in the winter (Western Regional
Climate Center 2007).
Annual precipitation (rainfall) is approximately 36 inches per year, with
approximately 90% falling between the months of November and April (Western
Regional Climate Center 2007). Wind-direction frequency plots show a uniform
directional distribution. The highest mean wind speeds originate from the
northwest (10.4 miles per hour [mph]) and southeast (8.8 mph) (Jones & Stokes
1998). Wind-generated waves develop on San Pablo Bay in response to the wind
patterns. Resultant wave periods of 2–5 seconds are reported as typical for
conditions in San Pablo Bay.
Regional Hydrology
San Pablo Bay is the northernmost embayment of the San Francisco Bay. At high
tide, the surface area of San Pablo Bay is approximately 64,000 acres. Tidal
circulation in San Pablo Bay is determined by its connection with the Delta to the
east and the Central Bay and Pacific Ocean to the south and west. Circulation
patterns in San Pablo Bay are dominated by tidal circulation, river discharge, and
winds. The current pattern is generally in a clockwise direction from Point San
Pedro towards the Petaluma River, resulting from both tidal and fluvial forces.
Water currents are greatest in the deeper portions of the bay, such as the Pinole
Shoal Channel.
More than 90% of the freshwater inflow to San Pablo Bay arises from the
Sacramento and San Joaquin River systems and enters through Carquinez Strait.
The combined flow of these rivers averages approximately 32,000 cubic feet per
second (cfs) during the winter months and averages approximately 6,000 cfs
during the summer months (California Department of Water Resources 1993).
Other minor sources of freshwater inflow include the Petaluma River, the Napa
River and Sonoma Creek/Second Napa Slough. Freshwater inflow primarily
occurs during winter rains, spring snowmelt runoff, and reservoir releases. This
freshwater inflow has an extensive influence on current patterns, vertical mixing,
and constituent transport patterns within San Pablo Bay. During periods of high
inflow, the bay becomes well mixed, and salinity stratification and intrusion are
diminished.
Sears Point Wetland and Watershed Restoration
Project Final Environmental Impact
Report/Environmental Impact Statement
3.2-5
April 2012