Although the inlet has always been in the same general location, it is probably in a 

 continuous state of change in both cross-section area and planform because of the seasonal 

 variation in the intensity of wave attack and strength of tidal currents. This is readily 

 apparent in the aerial photographs shown in Figures 13 through 20, which cover a 25-year 

 period from 1945 to 1969. These photographs are all reproduced to about the same scale 

 for comparative purposes. Because the photographs were taken at various tide stages, some 

 differences in appearance are obvious; shoal areas are uncovered at low tide, but covered at 

 high tide. Hence, the appearance of the channel across the offshore bar may vary 

 considerably between low and high tide. The aerial photographs (Figs. 13—20) also show 

 that the waves generally break on the offshore bar so that little wave action appears at the 

 gorge or narrowest section of the inlet— except possibly during periods of extreme high 

 tides. 



Although the aerial photographs of the Bolinas Lagoon inlet (Figs. 13—20), indicate that 

 changes in the configuration of the channel must be occurring frequently, few bottom 

 surveys of the inlet and adjacent area are available to document the nature and extent of 

 such changes, probably because of the difficulties and hazards in making accurate surveys 

 especially during heavy wave action and strong tidal currents. Some limited information on 

 the changes that occur at the throat section is available from the three USGS discharge 

 measurements in 1967-68. Plots of these cross sections are shown in Figure 21. (J. R. Putter, 

 10 May 1971, private communication.) Cross-section areas of these sections below 

 mean -sea-level follow: 



A c = 1330 square feet on 22 June 1967 

 A c = 1198 square feet on 24 Oct. 1967 

 A c = 1378 square feet on 17 May 1968 



Some quantitative information on changes at the Bolinas Lagoon inlet was obtained by 

 the USGS (Beck, 1971) by time-lapse photography during the period from January 1968 

 until July 1969. A camera, located on the Bolinas bluff, was focused on the mouth of 

 Bolinas Lagoon and the end of Stinson Beach spit to observe the high and low tides. 

 6. Sediment Transport by Tidal Currents. Suspended-sediment discharge in or near the 

 Bolinas Lagoon inlet was measured during seven ebbtides and six floodtides in June and 

 October 1967. (Ritter, 1969.) The measured suspended load for the ebbtides ranges from 3 

 to 1,150 tons and for the floodtides from 49 to 152 tons. The suspended load discharge was 

 directly related to average velocity. (Ritter, 1972.) Most of the transported sediment was 

 sand, the concentration of which had considerable temporal, lateral, and vertical variation. 

 The concentration of suspended sediment finer than sand remained relatively constant. 



No data are available on the net amount of sand (moved inward and outward at the inlet 

 with each change of tide) that either remains permanently in the lagoon or is carried out of 

 the lagoon system to deposit offshore. Most of the sediments of sand size which are carried 

 into the lagoon by streams from the watershed probably remain in the lagoon and 



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