CHAPTER 1 

 THE PHYSIOGRAPHIC SETTING 



The Pacific coastline is noted for 

 its nearby mountains and steep bluffs 

 which provide overviews of rocky shores, 

 sandy beaches, and occasional river 

 outlets. The rugged topography that meets 

 the Pacific Ocean contrasts sharply with 

 the broad, flat coastal plains of the 

 Atlantic and Gulf of Mexico coasts. 

 Because of California's position on the 

 leading edge of the continent, its history 

 of recent uplift, and its continuing 

 seismic activity, only limited areas of 

 flat topography have formed to support 

 bays, estuaries, lagoons, and marshes. 



But the Pacific coastline is not 

 uniform throughout its length. Several 

 features make the southern California 

 region unique. The coastal mountain 

 ranges of California bend eastward near 

 Point Conception (3U-1/2°N lat.), forming 

 the Transverse Ranges of southern 

 California (Figure 2). The coastline 

 likewise bends toward the southeast, where 

 there are a number of submerged basins and 

 troughs which parallel the various 

 submerged mountain ridges. The submarine 

 topography is as rugged as the exposed 

 land and depths drop quickly to 500 m 

 (1,640 ft), leaving a very narrow 

 continental shelf. Isolated peaks to the 

 west form the Channel Islands, where the 

 topography is too steep to support marsh 

 communities . 



Point Conception has long been 

 recognized as a transition area for marine 

 biota, many of whose northern or southern 

 boundaries coincide with this landmark and 

 the nearby shifts in ocean currents and 

 surface temperatures. Along northern and 

 central California, the California Current 

 parallels the shore. At Point Conception 

 it continues southward and creates a 

 countercurrent which curves back toward 

 the north after becoming warmed along the 

 coast of Mexico. The nearshore water off 

 southern California is often 50c (AIT) 



warmer than that of the California 

 Current. While such a difference is 

 expected to influence the distributions of 

 marine organisms along the exposed coast, 

 it may be much less important to marsh 

 plants and associated animals found within 

 protected embayments. The point is a 

 convenient boundary for describing 

 southern California marshes. While 

 marshes between Mexico and Point 

 Conception have many similarities with 

 ones further north and south, a single 

 "community profile" would not be valid for 

 wetlands spanning such a broad latitude. 



1.1 GEOLOGICAL HISTORY 



Coastal areas of southern California 

 have a dynamic geological history, and the 

 area continues to be influenced by seismic 

 activity, by subsidence, and by 

 catastrophic flooding with its associated 

 impacts of scouring and accretion of 

 sediments. Intertidal organisms are very 

 sensitive even to small changes in 

 elevation; hence, it is important to 

 consider how their habitat has changed in 

 the past in order to interpret their 

 modern distribution and composition. The 

 relative instability of this coastline 

 suggests that marsh habitats may have 

 undergone quite recent changes and that 

 current species distributions have not yet 

 stabilized. 



The present location of the southern 

 California coastline is between the 

 maximum and minimum elevations of sea 

 level known for recent geological history 

 (Figure 3). Thick marine deposits were 

 laid down along the coast in the Pliocene 

 and Pleistocene, when waters were at times 

 several hundred meters deep over the 

 present marsh habitats. More recently, 

 sea level fell to about 120 m (400 ft) 

 below the present level, reaching its 

 minimum about 18,000 years ago (Vedder and 



