Oceanography and Geology of the Gulf of the Farallones 



Current Patterns Over the Continental Shelf and Slope 



Marlene A. Noble 



The waters of the Gulf of the Farallones 

 extend along the central California coast 

 from Point Reyes southeastward to Ano 

 Nuevo. This unique region of coastal 

 ocean contains valuable biological, rec- 

 reational, commercial, and educational 

 resources. Sandy beaches provide living 

 space for a wide variety of organisms. 

 Seabirds nest along the beaches, in the 

 rocky cliffs above them, and on the Far- 

 allon Islands. Animals ranging from the 

 small anemones found in tide pools to the 

 elephant seals off Ano Nuevo live in these 

 waters. This coastal region is also a play- 

 ground for people, Californians and visi- 

 tors alike. In addition, fishing and com- 

 mercial shipping activities are an integral 

 part of the economy in the region. 



Tides are the most familiar ocean phe- 

 nomenon. They are easily seen at the sea 

 shore; beaches are covered and exposed 

 twice a day. Tidal currents, the largest cur- 

 rents in San Francisco Bay, move water 

 in and out of the estuary. At the Golden 

 Gate, ebb-current velocities during spring 

 tide can reach 7 miles per hour. Small sail- 

 boats bucking a strong tide can have trou- 

 ble just getting back in through the Golden 

 Gate. Timing of the return is critical. Out- 

 side the Golden Gate, in the coastal ocean, 

 tidal currents are strong near the coastline. 



They diminish offshore, becoming over- 

 whelmed by steadier currents as water 

 depth increases. 



Tidal currents and waves are important 

 in the coastal ocean. They mix the water 

 column, allowing nutrients near the seabed 

 to reach plants growing in the lighted sur- 

 face regions. Tides move nutrients and 

 other suspended materials vertically and 

 back and forth, but they generally do not 

 transport these materials large distances. 



On the Continental Shelf and Slope 

 in the Gulf of the Farallones oceanic cur- 

 rents flow through the area transporting sus- 

 pended sediment, nutrients that allow plants 

 to grow, and possible pollutants. Until 

 recently, however, not much was known 

 about how strong the currents are, in what 

 direction they flow, or how rapidly flow pat- 

 terns change with time or location. Even 

 less was known about how current patterns 

 affect the many creatures that live in the 

 coastal ocean, how currents modify the nat- 

 ural sediment on the sea floor, or about the 

 eventual fate of natural sediment or mate- 

 rials dumped in the gulf. Knowledge is 

 needed about these important factors so that 

 people can make reasonable decisions about 

 how to manage the coastal waters, ensuring 

 that recreational and commercial activities 

 do not harm the environment. 



During the 1 990's, several programs 

 were undertaken by the U.S. Geological 

 Survey and other organizations to gather 

 information about how currents, nutrients, 

 and suspended material move through the 

 Gulf of the Farallones. The area studied by 

 the USGS covers about 1 ,000 square nauti- 

 cal miles of the gulf and ranges in water 

 depth from 660 to 10,500 feet. These stud- 

 ies showed that the general features of the 

 complex current patterns in the area are 

 similar to those observed elsewhere along 

 the central and northern California conti- 

 nental margin. 



Currents over the Continental Shelf 

 tend to flow southeastward and slightly 

 offshore in summer, causing nutrient-rich 

 cool waters to upwell onto the shelf. Shelf 

 currents flow mostly northwestward in 

 winter. Tidal currents are strong over the 

 shelf and tend to be the dominant features 

 in flow patterns near the shoreline or 

 within estuaries. The strong waves that 

 occur during winter storms commonly 

 cause sediment on the sea floor to be 

 resuspended and carried both along and 

 off the shelf. 



The currents on the Continental Slope 

 flow dominately northward in all seasons. 

 Tidal currents over the slope are weak 

 except for regions near the seabed or 



within the conspicuous submarine canyons 

 that cut into the continental margin. 



However, many of the current patterns 

 in the Gulf of the Farallones are altered by 

 the region's unique sea-floor topography; 

 therefore, the local characteristics of flow, 

 such as the amplitude of currents, their 

 detailed response to winds, and the 

 strength of the summer upwelling, are spe- 

 cific to an area. In summer, the promon- 

 tory of Point Reyes causes shelf currents 

 to turn offshore and flow over the slope. 

 The abrupt steepening of the slope in 

 the northern part of the area studied also 

 causes northwestward-flowing slope cur- 

 rents to turn toward the deep ocean. Both 

 of these features enhance the exchange 

 of water, nutrients, and other suspended 

 materials among the shelf, slope, and deep 

 ocean relative to what happens along the 

 simple, straight shelf more common north 

 of the gulf. 



The complex current patterns in the 

 Gulf of the Farallones help to make the 

 coastal waters of the area a truly unique 

 resource. Knowledge of these patterns is 

 essential if competing demands on this 

 resource are to be balanced. 



20 Oceanography and Geology of the Gulf of the Farallones 



