The Science of Sustainability

Sand Waves and the Golden Gate

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Seafloor under the Golden Gate Bridge, USGS

Seafloor mapping of the San Francisco Bay and surrounding areas by marine scientists at the U.S. Geological Survey Pacific Coastal & Marine Science Center and Cal State University at Monterey Bay's Sea Floor Mapping Lab (SFML) is important for keeping shipping lanes safe, understanding pollution dispersal, mapping habitats, and much more.

Mapping of the underwater topography (called bathymetry) reveals landscapes fundamental to understanding the Bay Area's unique geology. The Golden Gate strait connects the San Francisco Bay to the open Pacific Ocean and is only one mile across.

Although the tidal range in the Bay Area is not incredibly large, the narrowness of the Golden Gate creates a funneling effect:

"Large volumes of water move into and out of San Francisco Bay as the tidal level of the Pacific Ocean just outside the Golden Gate changes each day. When the tide is changing from low to high levels, a flooding current moves water inland from (and through) the Golden Gate. When the tide is changing from high to low levels, an ebbing current moves water from inside the Bay toward (and through) the Golden Gate."

The image above (from Cal State Monterey Bay) is a perspective image looking towards the east. The colors represent water depth with the reds and yellows as shallower water and the blue and purple deeper water. The prominent patterns in the foreground might look like ripple marks on the beach, but these sand waves are similar in scale to some sand dunes seen in deserts — up to 30 feet tall and more than 700 feet from crest to crest.

The vigorous currents funneled through the Golden Gate continuously move sediment from underneath the bridge where it accumulates as these sand waves. But remember, this is a static snapshot of very dynamic systems. I'd love to see multiple repeat surveys that show how the field of sand waves change over time.

Learn more about the processes that move this sediment from this USGS article and in this SF Chronicle piece from 2006, along with this QUEST story on 3-D seafloor mapping.

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Images: (1) From USGS Scientific Investigations Map 2917, Under the Golden Gate Bridge — Views of the Seafloor Near the Entrance to San Francisco Bay; (2) Cal State Monterey Bay Sea Foor Mapping Lab

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Category: Geology

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Brian Romans

About the Author ()

Brian Romans is the author the popular geoscience blog Clastic Detritus where he writes about topics in the field of sedimentary and marine geology and shares photographs of geologic field work from around the world. He is fascinated by the dynamic processes that shape our planet and the science of reconstructing ancient landscapes preserved in the geologic record. Brian came to the Bay Area in 2003 and completed a Ph.D. in geology at Stanford University in 2008. He lives in Berkeley with his wife, a high school science teacher, and is currently working as a research scientist in the energy industry. Follow him on Twitter.
  • Dean Heller

    One day while browsing Google Earth I came across some very
    odd repeating lines in the southern end of Africa (around Ngamiland). I have
    spent weeks trying to find any information on what they could be, but to no
    avail. They cover and area in access of 100,000 sq mi. are about 1 to 1 1/2
    miles apart and are orientated east to west. They appear to be bands of
    alternating areas where there is more soil for plant life and areas that have
    much less. I could be wrong, but they look to me like they could be old sand
    waves from a continent wide flood. Just use Goggle Earth and jump to these
    coordinates.

    19 30'55.13"S 21 31'39.96E

    It would tickle me pink if someone could throw some light on this for me.