Coasts 



25 



the coast are backed by lowlands, mostly 

 marshes or mud flats, such as those of Fig- 

 ure 6. These are the only truly depositional 

 coasts of the region, ones which have pro- 

 graded over a long period of years. Accord- 

 ing to Handin (1951, p. 6) one of them, the 

 Santa Clara River delta near Ventura, pro- 

 graded 580 feet between 1855 and 1933. 

 Most large changes of other sand beaches 

 during the last hundred years appear to have 

 resulted from man's activities. The remain- 

 ing 70 per cent of the sand beaches are at the 

 base of sea cliffs in coastal areas that must 

 be classed as of erosional origin over a long 

 time span but depositional over shorter time 

 spans when a wide sand beach is present. 

 About a sixth of the latter occupies small 

 coves in irregular rocky coasts; many exam- 

 ples exist along the shores of the Palos Ver- 

 des Hills, Laguna Beach, La Jolla, and most 

 of the islands. These beaches consist largely 

 of sediment derived locally from streams and 

 sea cliff's, as commonly shown by great dif- 

 ferences in texture and mineralogy of sand 

 in adjacent coves. Movement of sand out 

 of the coves is difficult because waves which 

 enter the coves are refracted so that their 

 crests are convex landward. The lagging 

 ends produce longshore currents at both 

 sides that drive sand toward the center of 

 the cove. As a result, the coves are com- 

 monly lined with boulder beaches at the en- 

 trance that grade through gravel to sand 

 beaches in the center. Because of their ap- 

 parent isolation from other beaches, these 

 areas are sometimes known as pocket 

 beaches. Most beaches at the base of sea 

 cliffs, however, are more or less continuous 

 with those that are backed by lowlands. 

 They also are of similar grain size and min- 

 eralogy; thus they are part of large units of 

 sand having related sources and are in equi- 

 librium with related forces at least through- 

 out much of the year. 



Sand reaches the beaches of southern 

 California chiefly from streams and to a 

 much lesser extent from erosion of sea cliff's 

 and the sea ffoor (Fig. 26). Most of the 

 sediment comes during large infrequent 

 floods; for example, more than 7,303,000 

 cubic yards of sand was deposited at the 



mouth of the Santa Clara River and more 

 than 6,000,000 cubic yards at the mouth of 

 the Los Angeles River during the floods of 

 March 2, 1938, according to surveys reported 

 by Handin (1951, pp. 36, 44). The quantity 

 of finer sediment carried seaward past these 

 temporary deltas is unknown but was prob- 

 ably much greater than the quantity of sand 

 deposited. The contribution by streams dur- 

 ing years of ordinary rainfall is even less well 

 known, no reliable figures for load near the 

 mouths of streams being available. Existing 

 information on stream contribution is based 

 on rate of erosion of watersheds which, in 

 turn, is inferred from the rate of deposition 

 in reservoirs and flood debris basins in up- 

 stream areas. The finest material escapes 

 these traps, and there is little reason to as- 

 sume that all the coarse sediment which is 

 trapped would have crossed the lowlands 

 between the traps and the ocean. 



A second method of estimating stream 

 contribution has been that of measuring the 

 rate of accretion behind breakwaters near 

 the stream mouths, but of course little sedi- 

 ment finer than sand is trapped, and it is 

 virtually impossible to separate the contribu- 

 tion of littoral drift from that of nearby 

 streams. Although the amount is not well 

 estabhshed, it is estimated that several mil- 

 lion cubic yards of sediment are contributed 

 annually by streams in the area of Chart L 

 It also seems evident that the greatest rate 

 of contribution occurs in the middle part of 

 the broad coastal embayment in the area 

 reached by the largest rivers, the Santa Clara, 

 Los Angeles, San Gabriel, and Santa Ana. 

 These rivers together drain about half the 

 total drainage area of 12,000 square miles 

 (31,000 sq km) that borders the shore of 

 southern California (Fig. 27). 



Once on the beach, the sediment comes 

 under the influence of waves. The constant 

 movement eventually breaks up the grains, 

 and the fine debris is carried seaward. Be- 

 fore being lost to the sea, the sand is moved 

 along the shore for greater or lesser distances 

 by waves. Where waves strike the beach 

 diagonally, they run up the beach and down 

 again at a point a few feet farther along the 

 beach. As a result of this net longshore 



