The Corte Madera Creek Watershed

How the Ross Valley has changed over the past 400,000 years or so.

Repeated glacial periods on the Earth over the past 400,000 years have caused the sealevel to rise and fall.  The graph shown below shows only two of the four major glaciations. We are currently in an interglacial period that followed the end of the Wisconsin Glacial Period.
The impacts of these lowerings and risings of sea level have left their traces in the sediments beneath the bay as shown in the following diagram sketched by Brian Atwater of the U.S. Geological Survey from studies that he did in the late 70's.  Alluvium refers to stream deposition. The "Bay Muds" are sediments that are being, or were, deposited during previous  high sea level periods.

    18,000 years ago a cross-section of the Ross Valley was a more v-shaped valley with a maximum bottom depth 75 or so feet below the present surface of the valley.

    The sides of the valley can be seen in the hills of Kentwoodlands and the slopes of the upper College of Marin campus. There was no San Francisco Bay and Corte Madera Creek joined the Sacramento River and flowed out through the Golden Gate to the shoreline beyond the Farallone Islands off of San Francisco.

    Between 18,000 to 6,000 the cross-section changed significantly due to the melting of the Wisconsin ice age's glaciers.  The rise of sealevel, formation of the San Francisco Bay and the flooding of the Ross Valley. 

    At that time the flat valley upon which the College of Marin Science Center sits would have been an open embayment on the edge of San Francisco Bay proper. A present day example, but on a larger scale, would be the Richardson Bay between Strawberry and Sausalito/ Mill Valley.

    Following the sealevel rise, the cross section changed once more, this time being filled by sediments contributed from the bay and from stream and flood plain sediments carried down Corte Madera Creek off Mt Tamalpais and the surrrounding hills. Since the 1800's extensive development on the flood plain and salt marshes has occured throughout the bay area. 

    The red area on this map portrays just how much the open water area of San Francisco Bay has been decreased from the initial flooding of the bay (some natural changes and some man-made changes).

    Human occupation period:

      The oldest archeological sites in the Bay Area have a maximum age of 6,000 years even though we know that humans occupied North America long before that time. Were there no humans in this area before 6,000 years ago or is the evidence of their presence in this area now simply underwater and accessible only by marine archeologists?
Maps showing the extent of human produced changes in the Ross Valley

These progressive changes created, by the 1940's, a natural flood plain that was occupied by truck gardens and a charming, meandering stream channel that ran through the campus and was lined with large trees (a typical riparian environment).  Snipets of evidence can still be seen by educated eyes.  The stream also created "natural levees" that were only overtopped by water during flood periods onto the natural flood plain. By 1971, as a result of property owners demanding property protection from floods on the natural flood plain, the Corps of Engineers designed and constructed the current concrete channel.  Because of the potential of continued flooding in the valley home and property owners have called for more protection.  Their appeals (2001-03) appear to be leading, once again, to attempts to protect the flood plain from the stream doing what it wants to do.

At the end of the concrete channel, the rip-rap lining at the end of the channel was placed there to prevent the softer, channel margins from being eroding by the physical turbulance at the joining of strong concrete and weak muds. If unprotected problems with the stability of the concrete portion of the channel might have resulted. A common strategy where the shore is being threatened by erosion, big piles of rock on a shoreline should tell you that erosion was going on and someone wanted to avoid or stop it.  The rocks in this rip-rap are called "greywacke" and because of their unique types allows us to identify them as coming from the quarry that was where the apartments, condos, and Courtyard Hotel in Larkspur Landing are today. See discussion of greywacke.

Plants on the margins of the bay including those of the natural salt marsh.  Alkali Bull Rush (Scirpus californicus), is not a common salt marsh plant and is seen most commonly near the mouth of the concrete channel and decreasingly down channel, unless there is a good source of fresh water.  Scirpus happens to be one of the most common plants in the Sacramento-San Joaquin Delta and it is an important plant that supports the western migration of ducks. They eat the seeds from the large brown clusters that are obvious at certain times of the year.

There is a distinct and contrasting seasonal "greening" and "browning" of our salt marshes vs. the upland grasses. The following image is from near China Camp and taken during the winter.

Four major native plants of the salt marshes of S.F. Bay...the sticky gum plant (Grindelia stricta, a legal marker for the edge of SF Bay), the salt grass (Distichlis spicata), the pickleweed (Salicornia), and the cord grass (Spartina foliosa). Idealized they would range from highest to lowest growing in the order above. 

The citizens of Kentfield, attempting to reverse the shrinkage of San Francisco Bay by developers, purchased and restored a local, proposed development site. 

While the restoration of the diked land succeeded a problematic "negative" aspect to the restoration was the introduction of the seeds of a South American cord grass that has now moved out into the channel and is moving up and down the creek (2003), forcing itself into a zone between the cord grass and the pickleweed, decreasing the amount of each of the natural salt marsh plants. We have yet to understand the full implications of this intrusion of a foreign organism into our local natural environment, but we know that it is not uncommon in S.F. Bay since more than 50% of the organisms in the bay were not here when Europeans came to California. A report on the extent of invasive plant species in San Francisco Bay can be found here.

TSAW Home Page
TSAW Home Page
To Geologic Column