161 



detrital material from the land to contribute to the turbidity. 

 There are occasions, however, when plankton or other algal 

 blooms in the offshore area increase the turbidity to the 

 extent that the transparency may drop to less than 20 feet. 

 This is a normal occurrence in all coastal waters along 

 southern California. One notes from this chart that in areas 

 well removed from any point of sewage discharge the trans- 

 parency nearshore is much lower than that offshore. This is 

 completely normal, for along amy shore bordered by a low 

 coastal plain or by hills or mountains composed of soft shales 

 and sandstones, as are the Palos Verdes Hills and the Santa 

 Monica mountains, the nearshore waters are turbid to a certain 

 extent due to silt and clay contributed by streams and kept 

 in suspension by wave action. Thus, the direct contribution 

 by sewage to the turbidity of the water is confined to a 

 relatively small area in the immediate vicinity of the outfall 

 site. 



Santa Monica Bay, then, has waters that are naturally 

 more turbid than those along the Laguna coast to the south 

 or around Catalina Island, because of the geology of the 

 adjacent hinterland and the nature of the circulation. Where- 

 as along these latter shores a Secchi disc reading of ,60 to 

 70 feet is not uncommon within a mile of shore, or closer, 

 such transparencies have never been observed within 12 miles 

 of shore in Santa Monica Bay, Any additional contribution of 

 detrital or organic material into these inshore waters will, 

 therefore, decrease the transparency to an even greater extent. 



