Erosion in Colorado Desert. (33 
slowly subsided. After 33 hours it was still about 13 meters (5 
feet) deep and running swiftly, and it had not entirely ceased three 
days later. 
Two days afterward (August 16), when at the Moki Pueblo of 
Oraibi, a furious rain set in about 4 p.m., and lasted more than an 
hour, flooding the house-tops and streets and parts of the valley 
below. And yet the desert was as parched next day asif it had 
never been wet. 
The heaviest and most extended rain-fall observed by us occurred 
September 20, on which date Mr. Bailey and I set out from Little 
Spring for Moencopie. Heavy laden clouds began scurrying over 
the mountain toward the northeast early in the morning, and by 
noon the entire sky was overcast and had a most ominous appear- 
ance. Soon the rain began falling in torrents, and the storm 
moved steadily eastward from the edge of the lava-beds to the 
Little Colorado, and thence across the desert to the high mesas 
beyond. Such a delnge I never saw, and we afterwards learned 
that it extended 160 kilometers (nearly 100 miles) to the south. 
The gulck in the edge of the lava-beds, about 23 kilometers (14 
miles) east of Black Tank, was full to overflowing; the flat upon 
which it empties was 14 meters (5 feet) under water; great lakes 
appeared in various parts of the desert, and the Little Colorado 
bottom was completely flooded. And vet all this vast volume of 
water disappeared in a few hours. A red, sirupy, alkaline mud 
filled the bed of the Little Colorado for a few days, and pools of 
similar mud were occasionally found in depressions in the sand- 
rock all the way to Moencopis. The whole desert, from the San 
Francisco lava-beds on the west to Echo Cliffs on the east, showed 
that it had been recently deluged, as if by the breakage of some 
mighty dam, but the water had disappeared. 
From the scanty data available, and from the experience of 
residents of the region, it is safe to infer that the rain-fall was 
unusually heavy in the Plateau region during the summer of 1889. 
In No. 5 of the “ North American Fauna,” Dr. Merriam also 
makes the following observations on the effects of water-courses on 
the geographic distribution of species : 
Mountain streams, in passing down into the plains, exert a two- 
fold influence on the distribution of animals and plants. By their 
constant efforts to reach base level, these streams are continually 
cutting down and lengthening the valleys in such a way as to pro- 
duce gradually sloping bottom lands, which penetrate the highlands 
from the plain below, carrying with them narrow prolongations or 
tongues of the fauna and flora of lower levels, which follow the 
the contour lines in a general way. 
The second effect mentioned is of an exactly opposite character. 
The low temperature of the water, coming from melting snow-banks 
or cold springs in the mountains, lowers the temperature of the soil 
supporting the vegetation on its immediate banks, while the eva- 
poration from its surface cools the air to which the foliage of such 
vegetation is exposed, thus bringing the northern or higher fauna 
and flora down along the immediate course of the stream. : 
The length of the stream and steepness of the slope determine 
whether the first or second effect is most pronounced. Rivers 
having long courses over the plains, such as the Missouri and 
6 
