4 : The Delta of the Rio Colorado. 
these exposures. Small rivulets of sand and gravel trickle continu- 
ously down the steep slopes of the cliffs, and occasionally large masses 
are detached and fall into the water with no little commotion. 
At these places may be found within a compass of a hundred 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, xerophytic 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 seventeen per cent. within fifty feet of the margin 
of the water. During the recent visit to this region however, 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 onenys but unalluring appearance of alter- 
ation of climatic factors. 
The above conditions are wel illustrated by the following data, 
obtained in 1904. For purposes of comparison, observations were 
made on the alluvial plain among the leafless willows and poplars 
during the dry season, and the results wire. — ¥j 
January 28. 4:30 P.M...... 17% rel. hum. 
January 28. 6:30 P.M 55. Fs. 45% rel. hum. 
January 28. 8:00 P.M...... 51° F. 18% rel. hum. 
\ Jaitiary: 205° ISAS A Me cua. 32° F. 41% rel. hum. 
Jaary 20. 5220 As 30° F. 50% rel. hum. 
Jattiary. 20::. 0:00 Ae: 29° F. 66% rel. hum. 
January 29. 6:25 A.M...... 27° F. 70% rel. hum. 
January 29. 7:00 A.M...... 38° F. 60% rel. hum. 
January 29. 7:40(Sunoninstrument) 36% rel. hum. 
Observations were also made on the “Upper Mesa” or “Line 
Mesa” at a distance of less than a hundred and fifty feet from the 
margin of the bank, among typical desert shrubs which were scattered 
over the gravel plain in characteristic fashion. The following data 
- were obtained :— 
January 30. 
January 30. 
January 30. 
January 30. 
January 31. 
January 31. 
4:40 P.M... 
A PM, 
7 PE i 
8730 P.M... 
ae e ere 
19% rel. 
16% rel. 
15% rel, 
277 rel. 
29% rel. 
30% rel. 
The high level of the March flood, being fairly uniform, is very _ 
hum. 
So eee tres tee erie 
i eee 
