MOVEMENTS OF VEGETATION IN THE SALTON SINK. 139 



plex, Heliotropium (?), Sesuvium, and a grass appeared. Measurements, of the level of 

 the lake, besides those compiled into the curve shown in Fig. 4, were now received, from 

 which it appeared that the surface had fallen 9.6 inches in August, 7.2 inches in Septem- 

 ber, 3.6 inches in October, 2.4 inches in November, and 1.2 inches in December 1912. 

 The next measurement (made on February 1, about the time of the first observation in 

 1913) showed that the level of the water had been raised 2.4 inches during January, clearly 

 due to the lessened evaporation and the increased inflow, and that the level had fallen an 

 equal amount during February, so that the level on March 1 was the same as at the begin- 

 ning of the year. A fall of 3.6 inches took place during March and 2.4 inches during April. 

 The recession was probably at a slightly higher rate during May. On May 24 the rank 

 of seedlings at the base of the bank included Atriplex, Cyperus, Heliotropium, Prosopis 

 pubescens, Salix, Populus, and a grass. The experiments described on page 162 would indi- 

 cate that the Salix and Populus seedlings were from wind-borne fruits trapped by the bank, 

 and that the Prosopis was from plantlets germinated in the water. The last-named, how- 

 ever, was found only among thick bunches of twigs and other buoyant debris, and it is 

 possible that the pods or seeds did not float independently (see Plate 26). 



INFLUENCE OF THE LAKE UPON THE VEGETATION OF THE DRY SLOPES 



ABOVE ITS LEVEL 



The flood waters of the Colorado River which flowed down the slopes of the Cahuilla 

 Basin into the Salton Sink caused an enormous amount of erosion, as described elsewhere 

 in this volume. With the widening of the channels, bars and sand-spits were formed on 

 which a vegetation characteristic of a river margin was established. The tops of the banks 

 and the gentler slopes of the same were seen to support species of a halophytic character, 

 the presence of the stream having but little effect on the plants indigenous to the region. 



More marked effects were to be seen on the bajadas or detrital slopes leading down 

 from the mountains directly toward the lake. The waters of the lake were gently pushed 

 up over these slopes and doubtless penetrated slowly to some depth. The infiltrating 

 water would meet the slow underflow from the mountains with the result that the latter 

 would be dammed and would be brought nearer the surface. A more luxuriant growth of 

 halophytic and spinose forms on the steep slopes contiguous to the Travertine wash on 

 the westward may be ascribed to this cause rather than to any increase in the relative 

 humidity. Although every effort was made to find effects due simply to the influence of 

 increased humidity nothing of the kind was seen, and all such appearances above the level 

 of the strands were in places in which the structure was such as to present conditions for 

 retarding the underflow. The amount of water evaporating from the lake was compara- 

 tively enormous, yet the vapor formed was carried away so rapidly that it would have 

 but little effect in checking the transpiratory activities of land vegetation and thus increas- 

 ing the physiological value of the scanty supply of soil moisture. The relative humidity 

 with no wind may show an increase for a few hundred yards away from the shore of the 

 lake, but such conditions prevail only during comparatively brief periods. Some observa- 

 tions on this matter were made near the western shore of the lake in February 1907, and 

 the following extract is taken from my notebook. 



"On February 11, the wet and dry bulbs at 40 feet from the shore read 73 and 62 at 2 h 25 m 

 p. m., indicating a relative humidity of 68 per cent and at 3 h 34 m , 71H and 67, indicating a relative 

 humidity of 87 per cent. In order to ascertain the extent of this humid zone the instruments were 

 taken directly west, three-eighths of a mile from the shore, where a reading of 82 and 61 was obtained, 

 indicating a relative humidity of 49 per cent at 3 h 30 m p. m., and 15 minutes later a second reading 

 showed 81 and 60J4i indicating a relative humidity of about the same. It is thus to be seen that 

 the zone of greater saturation is a comparatively narrow one, as on returning to one-fourth mile 

 from the shore at 4 p. m. the readings were 80 and 733^, indicating a relative humidity of 81 per cent. 



