Howell 1980, Nardin et al. 1981). The 

 subsequent rise in sea level averaged 

 about 12 m (40 ft) per 1000 years until 

 about 5000 years ago, when the level was 

 perhaps within 5 m (16 ft) of its current 

 elevation. In recent centuries, sea level 

 appears to be relatively stable with 

 respect to the land surface (Isaacs 1979). 

 Topographic changes today are largely due 

 to accretion and erosion of sediments, 

 which are greatly influenced by activities 

 of man. 



121° 



120° 



113° 



118° 



iir 



Figure 3. Changes in sea level (from 

 Vedder and Howell 1980). Inner boundary 

 is the maximum extent of the sea during 

 the last half million years; outer 

 boundary is the probable minlmun, 17 to 

 18 thousand years ago. 



Mudie and Byrne (1980) reviewed data 

 from marsh cores which have been carbon 

 dated and cores for which Foraminiferan 

 assemblages have been analyzed. They 

 estimated a sedimentation rate of 10 

 cm/100 years for the period from 2500 B. 

 P. until the late 19th century. Then, 

 based on pollen analysis (using introduced 

 species as a marker for the arrival of 

 European man), they concluded that 

 accretion increased to 50 cm during the 

 last century, following changes in 

 agricultural practices which no doubt 

 enhanced erosion. Their conclusions agree 

 with Macdonald's (1969) statement that 

 Mission Bay doubled its marsh acreage 

 between 1859 and 1933 and with the 



conclusion that some present-day marshes 

 are of quite recent origin. Lohmar et al. 

 (1980) documented sedimentary filling of 

 Goleta Slough following flooding in the 

 I860's and subsequent replacement of 

 channels and intertidal flats by 

 saltmarsh. Stevenson and Emery (1958) 

 likewise concluded that the Newport Bay 

 marsh was only 90 to 130 years old at the 

 time of their study. 



Some of the sedimentation may have 

 slowed following the construction of 

 upstream dams, however. Nordstrom and 

 Inman (1973) estimated that 33% of 

 incoming sediment loads are being trapped 

 by 10 dams within the watersheds between 

 San Clemente and La Jolla, while 72% of 

 the sediment normally flowing down the 

 Tijuana River was blocked behind its 

 upstream dams. These decreased sediment 

 inputs are widely visible as losses in 

 sandy beaches along the southern 

 California coast (Nordstrom and Inman 

 1973). 



Still, local point sources of 

 sediment can be seen next to slopes where 

 prolonged disturbance occurs, such as near 

 storm sewers draining housing developments 

 (as at Los Penasquitos Lagoon) or next to 

 agricultural fields (e.g. at Mugu Lagoon 

 and also documented by Dickert et al. 1981 

 for Elkhorn Slough in central California). 

 Observations during the flood years of 

 1978 to 1980 suggest that catastrophic 

 sedimentation is limited to periods of 

 unusual flooding, and that coastal marshes 

 undergo alternating periods of stable 

 elevation and accretion. 



1.2 TIDAL CIRCULATION 



Tidal circulation is extremely 

 important to coastal marshes in southern 

 California because of the low, seasonal 

 precipitation, low runoff and frequent 

 droughts. Sea water provides most of the 

 soil moisture for intertidal wetlands. 

 Southern California tides are of the mixed 

 semidiurnal type, that is, the two daily 

 high tides are of different height, as are 

 the two daily low tides. The mean tidal 



