238 



Sediments 



sampled during the heavy rain of March 

 1939 ranged between 1 and 50 microns but 

 that the sediments of the larger streams were 

 more uniform, ranging mostly between 20 

 and 35 microns. During a more typical 

 moderate rainfall in early 1957 samples of 

 sediment collected from just below the sur- 

 face of the approximately 3-meter-deep San 

 Gabriel, Los Angeles, and Dominguez Rivers 

 had median diameters of 13, 12, and 5 

 microns, respectively. Similar samples col- 

 lected in early 1958 at the San Gabriel, Los 

 Angeles, and Santa Ana Rivers yielded 

 median diameters of 5, 14, and 54 microns, 

 respectively. Lindsay (1951b) found a 

 unique method of determining the grain size 

 of sediment discharged to the ocean by a 

 small intermittent stream near Ventura. 

 After the first major rain of the 1956-1957 

 season the stream discharged water and 

 sediment atop a berm that lay in front of 

 the stream mouth. Since no channel was 

 cut through the sand, the water was trapped 

 behind the crest of the berm and much of it 

 seeped out through the sand, leaving behind 

 a filter cake of sediment. Eventually a 

 wedge-shaped layer of stream-contributed 

 sediment having a maximum thickness of 

 60 cm accumulated, but wind-blown sand 

 contaminated the top portion which re- 

 mained soupy for several weeks. Five 

 samples from the lower uncontaminated 

 half of the deposit had median diameters 

 ranging from 1.7 to 43 microns and averag- 

 ing 19 microns. 



On the basis of supply of sediment by 

 streams Revelle and Shepard (1939) pre- 

 pared an estimate of the rate of deposition 

 in basins. They assumed that about 10 per 

 cent of the regional average of 50 cm of 

 rain (in a drainage area of 31,000 sq km) 

 reached the ocean with an average concen- 

 tration of 0.5 per cent sediment. This esti- 

 mate corresponds to 7.8 million tons per 

 year. A diff'erent kind of computation by 

 Handin (1951) was based on the rate of ac- 

 cumulation of sediment in artificial reser- 

 voirs and debris basins. He obtained a fig- 

 ure of 1.2 million cu meters of sand con- 

 tributed to the ocean by streams between 

 Point Conception and San Pedro. For the 



entire coast this might be extrapolated to 

 about 2.4 million cu meters, or 5 million 

 tons of sand annually. If the average 

 stream-discharged sediment consists of one 

 part of sand to three parts of silt and clay, 

 the total discharge of sediment would 

 amount to about 20 million tons annually. 



Neither Revelle and Shepard's nor Hand- 

 in's estimate can be very firm, owing to lack 

 of precision of the necessary basic data. 

 Clearly, a different method is needed. This 

 has recently been provided by radiocarbon 

 methods of age determination of sediments 

 that will be discussed in more detail later. 

 For the present, it is sufficient to give only 

 a brief summary. For each basin the rate 

 of deposition of inorganic detrital sediments 

 ranges between 6.3 and 117 mg/sq cm/yr; 

 for the basin slopes and other deep irregu- 

 lar areas, 4.0 mg/sq cm/yr; for the conti- 

 nental slope, 3.0 mg/sq cm/yr; and for the 

 abyssal sea floor of Chart I, 3.3 mg/sq 

 cm/yr. Using these rates weighted accord- 

 ing to the area of each unit (Tables 1 and 5), 

 we find that about 5 million tons of land- 

 derived sediment are deposited annually in 

 the basins. To this must be added an esti- 

 mated 5 million tons for the basin slopes 

 and other deep irregular areas, the conti- 

 nental slope, and the abyssal sea floor. The 

 total is 10 million tons per year. This is 

 close to the other two estimates but is based 

 on much firmer data. It seems probable 

 therefore that the streams contribute more 

 than ten times as much sediment to the 

 ocean as do wind and sea cliff erosion 

 combined. 



Sediments of organic origin are domi- 

 nated by calcium carbonate, but they also 

 include siliceous tests, frustules, spicules, 

 and organic matter. Calcium carbonate, 

 mostly as recognizable foraminiferal tests, 

 averages about 24 per cent of total sedi- 

 ment on the basin floors with the highest 

 values in the basins having the lowest rate 

 of deposition of inorganic detrital sediments. 

 Radiocarbon measurements of age at the 

 bottoms of basin cores show that the average 

 rate of deposition of calcium carbonate in 

 the basins is about 3.5 mg/sq cm/yr. This, 

 of course, is the total for calcium carbonate 



