136 
JOURNAL OF MAMMALOGY 
RODENTS AND RECLAMATION IN THE IMPERIAL VALLEY^ 
By Joseph Dixon 
Human occupancy of a region always results in great changes in the 
animal life therein. Species of mammals, for instance, that afford 
man meat, hides, or fur are almost sure to decrease in number, whereas 
other species, including some that are harmful to man’s interests, 
frequently increase. In the case now under discussion, both classes of 
animals are represented. 
The recent reclamation of the major part of the Imperial Valley, 
which occupies the southern portion of the Colorado Desert in the 
southeastern corner of California, is of unusual interest in many ways, 
but particularly so in connection with its widespread influence on the 
distribution and abundance of the rodents of that region. The area 
involved consists of the entire delta of the Colorado River. This delta 
is one hundred miles in length and has an average width of thirty 
miles; it extends from Salton Sea on the north to the Gulf of Cali- 
fornia on the south, and from the Cocopah Mountains on the west to 
Arizona and Sonora on the east. About one half of the delta lies 
within the United States, whereas the other half lies south of the 
Mexican boundary in Lower California and Sonora. 
That part of the delta region lying within the Imperial Valley in the 
United States is practically all below sea level. From surveys made 
in 1915 the elevation of the Imperial Valley was found to vary from sea 
level at Calexico to 234 feet below sea level at Salton Sea. In Lower 
California nearly all of the delta is above sea level. The crest or 
divide of the delta lies several miles south of the International Boundary. 
The Colorado Desert, including the delta region, is characterized 
by marked aridity and extremely high summer temperature. Exten- 
sive areas on both the east and west margin of the Imperial Valley 
are occupied by hundreds of sand dunes. The rich alluvial soil of 
the delta is over one hundred feet in depth for hundreds of square 
miles. Previous to the construction of the irrigation system the 
Alamo and New Rivers, both being overflow channels of the Colorado 
River, were the only waterways of any size and importance traversing 
the northern half of the delta. Both of these were intermittent rather 
than permanent streams. 
^ Contribution from the University of California Museum of Vertebrate 
Zoology. 
