PLANT ASSOCIATIONS AND HABITATS. ! 
for a moment fail to recognize this fact, especially as just beyond its 
banks there is growing on every hand the mesquite, the everywhere- 
present species of the Lower Sonoran zone. 
Thus even an abundant water-supply, with the strong growth of hygro- 
phytes determined by it, altogether fails to reproduce here, except in the 
most superficial way, the plant associations of rivers in the eastern United 
States. There may be an approach to a mesophytic forest, where willows 
and cottonwoods grow thick along the banks of the Santa Cruz, but not 
a mesophytic forest of the East or North. Other factors than water- 
supply, however potent this may be, are to be reckoned with in attempt- 
ing to account for the wide differences of plant and animal life that are 
here observed. 
(3) THE FLOOD-PLAIN; MESQUITE FOREST ASSOCIATION. 
The flood-plain of the Santa Cruz River is essentially the same in its 
physical characteristics as those of other rivers of the Southwest. A 
deep, fine alluvium, closely resembling the Maricopa sandy loam of the 
Gila and Salt River Valleys, the product of a long period of erosion and 
deposition, fills the valley from the river-bank to the mesa-like slopes at 
the foot of the mountains. It approaches adobe in texture, and bricks 
of a rather inferior quality are manufactured from it, but it is of a high 
degree of fertility, as evidenced by the crops of grain, fruits, and vege- 
tables which it produces. 
As already stated, the flood-plain in the immediate vicinity of the 
river suffers from erosion when the stream is high. Plate 3 shows the 
condition of affairs at a point less than a mile south of Tucson, where 
the roots of mesquite and other plants have been exposed along the deeply 
cut channel. 
According to statements of residents, this extensive erosion is of recent 
date. Previous to the advent of cattlemen some 20 years ago, and the 
destructive effects of over-pasturing, the valley of the Santa Cruz had a 
luxuriant growth of saccaton and other vegetation, which prevented 
the cutting of cha1nels, and the water spread out over the whole valley 
instead of flowing through the deep cuts it has since made; tules grew 
thickly in the springy places, and a fine forest of mesquite covered the 
ground.’ At present the effects of such erosion are seen most plainly 
from the point below Tucson already indicated to one about 2 miles 
above the city. It is inevitable that such changes, where they occur, 
should be followed by a lowering of the water-table of the flood-plain, 

1] have given the commonly received version of the cause of the cutting of oe 
nels of the Santa Cruz, but have since been told by Mr. Herbert Brown, of the see 
Post, that about 20 years ago, certain old settlers undertook to poe water” a 
a point about 2 miles down the river, where there were springs, and in e ex £0 ae 
plish this most easily cut a channel for a little distance, expecting the river w = 
it rose to do the rest. “Their expectations were fully realized, for the river scoure 
out the cut and kept on with its work, as already indicated. 
