direction) and wave period from these printouts are shown in Figure 8. The data for the two 

 stations show that there are extended periods when the longshore current prevailed in one 

 direction. For example, from February to June 1969, the longshore current at Stinson 

 Beach was generally in a southeast direction with a reversal occurring on only about 15 days 

 during the 5 months of record. (Johnson, 1970.) 



4. Sedimentation Processes— Bolinas Bay. Sedimentation processes in Bolinas Bay cause 

 both short- and long-term effects. The long-term effects are the slow natural changes which 

 may be detected only by a close comparison of bottom and beach surveys which have been 

 repeated over the years. The only sources of reliable information on bottom changes are the 

 USC&GS hydrographic surveys which have been repeated at several intervals since 1854. A 

 comparison of these surveys was made, and it was concluded that the Stinson Beach spit has 

 been stable in geometric configuration since the first survey in 1854; that is, the shoreline of 

 Bolinas Bay is in dynamic equilibrium. (Gilroy, 1970a, App. No. 3.) 



a. Bottom Changes. To obtain a measure of the seasonal bottom changes in the area 

 offshore from the inlet, each of the five surveys made between 17 May 1968 and 16 May 

 1969 for the Bolinas Harbor District was compared with the next following survey. The 

 depth of scour or fill was noted on an overlay. (See Figure 4 for general area covered by the 

 bottom surveys.) These overlays were then contoured to give areas and depth of scour and 

 fill occurring between the two dates. Thus, four comparison maps resulted, and a 

 comparison map for 1 year from the initial survey on 17 May 1968 to the final survey on 

 16 May 1969. (Gilroy, 1970a, App. No. 3; Johnson, 1970.) An example of these maps is 

 presented in Figure 9 which shows a comparison between the survey of 21 August 1968 and 

 that of 19 December 1968. An area of scour occurred near the entrance of Bolinas Lagoon, 

 but a general fill occurred over the remainder of the area surveyed. The largest fill occurred 

 near the surf zone— probably such material was removed from the beach face and deposited 

 immediately offshore by early winter storms. As a result of this general fill the. position of 

 the 30-foot deptli contour in December was further offshore than in August. 



That sand may be shifted laterally within Bolinas Bay is shown in Figure 10 which 

 compares positions of several depth contours as they existed on 17 May 1968 and on 

 16 May 1969. This figure shows that the area opposite the entrance to Bolinas Lagoon in 

 May 1969 had been generally filled compared with May 1968; that is, over most of the area 

 the May 1969 contours are seaward of the 1968 contours. On the other hand, at the east 

 side of Figure 10, a region of scour has occurred; that is, the 1969 contours are generally 

 shoreward of the 1968 positions. 



This indicates that, between the 1968 and 1969 surveys, sand appears to have been 

 shifted northward from Stinson Beach into the north end of Bolinas Bay. Wave conditions 

 and tidal currents in the following year couid possibly be such that the material would be 

 shifted back in an east and southerly direction. In the comparison maps discussed above, the 

 actual depths of scour and fill are subject to some sounding error, but the areas of scour and 

 fill show no randomness and are consistent with seasonal changes generally observed 

 elsewhere. 



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