MOVEMENTS OF VEGETATION IN THE SALTON SINK. 121 



and plankton, were not seriously touched upon, except in so far as the work of Dr. Brannon 

 upon the bacteria contributes to the matter. 



A number of the above subjects are considered in full in this section, while others 

 are treated in separate chapters, as indicated in the table of contents. 



The chief features of the phenomena of revegetation of the beaches may be best pre- 

 sented by a reduction of the field notes to a history of the strands or zones emersed during 

 each year of the recession of the lake. Any intelligent consideration of such descriptions 

 must be made in the light of the following conditions affecting the strands: 



1. The observational areas selected represent widely different habitats as to soil composition and 



other environic factors or components. 



2. The lake rose quickly to its maximum level and receded rapidly. 



3. The infiltration and leaching of the soil varied year by year as affected by the concentration of 



the water on one hand and the time of submergence on the other. 



4. The salt content of the water was least during 1907 and increased about 18 to 20 per cent in 



each succeeding year. 



5. Every emersed strand would therefore be saturated with a soil-solution resulting from the 



infiltration of the lake water of the concentration and composition prevalent in the period 

 preceding emergence. 



6. The desiccation of the emersed strands would proceed at a rate determined by the character of 



the soil and by the composition of the infiltrated water. 



7. The rising water of the lake picked up seeds lying on the surface, and their survival constituted 



a means of revegetation, chiefly of the strand bared in 1907. 



8. The rates of evaporation and of recession of the lake varied with the season, being most rapid 



in June to August and slowest in December and January. The possible total may be estimated 

 at 116 inches per year. 



9. The rainfall data of the Sink, obtained from the records of the U. S. Weather Bureau made at 



Indio, which is located near the extreme northwestern end of the Sink, only a few feet below 

 sea-level, show an average of 2.74 inches per year. 



10. Rapid recession of the water would result in separating stranded seeds quickly from the margin 



of the water, with consequent rapid desiccation of the surface layer of soil, which would be 

 unfavorable to germination and survival. 



11. The shallow water lying on wide mud flats fringing the shores was raised to a much higher tem- 



perature (15° to 20° F.) than the body of the lake during even the winter season, thereby 

 greatly increasing its toxicity for seeds, plantlets, and propagating bodies. The greater 

 number of the seeds falling into the lake would be subjected to this action. The muddy flats 

 fringing the shores at all stages of the lake must, therefore, be considered as a barrier of some 

 magnitude which would be crossed by a plant carried out into the lake and again when de- 

 posited on a beach. (See also p. 140.) 



REOCCUPATION OF THE STRANDS OF 1907. 



The level of the lake reached its highest point about February 10, 1907. A record 

 quoted, by the writer (Desert Basins of the Colorado Delta, Bull. Amer. Geog. Soc, December 

 1907) is to the effect that the rise in the water continued until in March, but the numerous 

 observations on the beaches late in February showed that a fall had taken place beyond 

 doubt and the record for March may be assumed as an instrumental error. The immediate 

 and rapid rate of fall of the water-level consequent upon the diminution of the inflow 

 which was brought about on February 10 may be ascribed to the enormous evaporation 

 and infiltration. This was sufficient to lower the level by a foot at a date early in June 

 1907, and in one year from the time of cessation of the rise the fall was between 40 and 42 

 inches. 



The record of the change in level of the lake and the varying area covered are shown 

 by figure 3, page 138, reproduced from Mr. Cory's records (see fig. 31, Trans. Amer. Soc. 

 Civil Engineers, vol. xxxvin, 1912). 



The slope lying above the observational area at Imperial Junction beach showed a 

 surface of adobe containing such an amount of dissolved salts that it bore a distinctly 



