34 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
of men now living the trees have moved southward toward the Gulf a 
distance of 8 or 10 miles, indicative of the extension of the delta conditions 
by accretions to the deposits around the mouth of the river (plate 31). 
The river cuts directly into the gravelly plain or mesa of the Sonoran 
Desert at four points on the eastern margin of the delta (plate 32). At 
these places may be found within a compass of 100 feet the most vivid 
contrasts of rank swamp vegetation and water-loving plants having broad 
leaves and delicate tissues with the toughened, spinose and hairy xero- 
phytic forms of the desert. The presence of the moist area of the delta 
has but little effect upon the climate of contiguous arid regions, although 
a popular supposition to the contrary promises fairly to be immortal. 
The relative humidity here is often as low as 17 per cent within 50 feet 
of the margin of the water. During the recent visit to this region, how- 
ever, clouds of mosquitoes from the delta had been blown for many miles 
across the sandy plains, and these pestiferous swarms among the creosote 
and sage bushes gave a deceptive but unalluring appearance of altera- 
tion of climatic factors. 
A shallow depression extends up from the eastern side of the Gulf of 
California to the main channel of the Colorado River about the head of 
tidewater, but its upper end is above the ordinary leveloftheriver. Seep- 
age water gathers in the channel at a point within a few miles of the river 
and finds its way to the sea, gathering salt from the soil and from the tides 
that push up into it, the whole being designated as the Santa Clara Slough. 
Here are found interesting combinations of xerophytic plants on the sand- 
dunes near the water, and of the halophytes or fleshy, salt-loving plants 
that border the water and cover the mud-flats adjacent to it. During 
some explorations in this region in 1905 the flood-waters of the Colorado 
River were seen to be making their way down through this channel to 
the Gulf, and a slight cutting action of the current might readily make 
this the main outlet. 
Late in 1907 it was found that the main current of the river was 
actually making its way to the Gulf through this channel, even at the 
low-water stage, a change which might have the profoundest influence on 
all life in several hundred square miles of the delta. 
THE SONORAN DESERT. 
The region east of the delta of the Colorado River and extending south- 
ward along the Gulf of California consists principally of a series of sandy, | 
gravelly plains near the delta and the coast, with moving dunes or “‘sables”’ 
in places, while in some localities these are replaced with mounds a few 
yards in height bearing Ephedra, Covillea, and Prosopis. (See plate 33.) 
In addition to the few herbaceous annuals which arise during the 
season favorable for growth, the principal types are perennials with 
spinose branches and reduced deciduous leaves, although a few species 
