History, Landforms, and Vegetation of the Estuary's Tidal Marshes
Brian F. Atwater, U.S. Geological Survey
Susan G. Conard, University of California, Davis
James N. Dowden, California State Lands Commission
Charles W. Hedel, U.S. Geological Survey
Roderick L. MacDonald, University of California, Davis
Wayne Savage, San Jose State University

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ABSTRACT:

Around 8,000 to 10,000 years ago, sharply rising sea levels nursed a newborn San Francisco Bay estuary whose tidal marshes probably covered less area than open water. Thereafter the rate of submergence decreased about 10-fold, and by 6,000 years ago sediment began to maintain marshes that later spread across marginal parts San Francisco Bay. By thus counteracting or overtaking submergence, sedimentation created marshes that, as of 1850, covered about 2200 km², nearly twice as much area as the bays. People have leveed or filled all but approximately 85 km² of these marshes during the past 125 years. Concurrently, human activities have caused the delivery of enormous quantities of sediment to the bays and the slackening of tidal currents in sloughs, thereby contributing to the creation of nearly 75 km² of marsh, about half of which remains pristine. Plains situated near high-tide levels are the most extensive landforms of both historic and modern marshes. Tides rather than upland tributaries created most sloughs around the bays, but riverine floods erected natural levees that confined tidal water in the Delta. Tidal marshes around San Francisco Bay typically contain 13 or 14 species of vascular plants characteristic of salt marshes and are dominated by common pickleweed (Salicornia pacifica) and California cordgrass (Spartinafoliosa). In the Delta, tidal marshes support about 40 species characteristic of fresh-water marshes and are dominated by tules and bulrushes (Scirpus spp.), cat-tails (Typha spp.), and common reed (Phragraites communis). These contrasting communities overlap around San Pablo Bay, Carquinez Strait, and Suisun Bay. Damage to tules and bulrushes during the drought of 1976-1977 confirms that intolerance of salt causes these plants to disappear toward San Francisco Bay. The disappearance of California cordgrass and common pickleweed toward the Delta, alternatively, may result from unsuccessful competition against tules, bulrushes, and other species. If export equals one quarter of net above-ground productivity, then vascular plants of the tidal marshes collectively contribute about 10 billion grams of carbon per year to other parts of the estuary.

 

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