DIXON- — KODENTS IN IMPERIAL VALLEY 
137 
Under the original desert conditions only true desert vegetation 
flourished in Imperial Valley and much of the best land of the delta 
was covered with creosote-bush, arrow-weed and quail-brush (Atriptex). 
Mesquite was most common in sandy areas where it helped in the 
formation of sand dunes. Cottonwood, willow, cat-tails, cane and 
other riparian trees and plants were restricted to sloughs along the 
overflow channels. Under true desert conditions in the Colorado 
Delta certain rodents were closely associated with certain plants or 
certain types of ground. For example, the pallid muskrat (Ondatra 
zibethica pallida) and Sonora beaver (Castor canadensis frondator) were 
confined to the river, ponds, and other permanent waterways. The 
western desert cottonrat (Sigmodon hispidus eremicus) and desert 
harvest mouse (Reithrodontomys megalotis megalotis) were typical 
inhabitants of the sedge or tule along the banks of streams. The 
Colorado Valley woodrat (Neotoma alhiguld venusta) was a character- 
istic dweller of mesquite thickets. The Yuma round-tailed ground 
squirrel (Citellus tereticaudus) and allied kangaroo rat (Dipodomys 
merriami simiolUs) were two typical inhabitants of the dry sand 
dunes, while the Imperial Valley gopher (Thomomys perpallidus alhatus) 
was found sparingly in sandy places where sufficient food could be had. 
The gopher, being unable to forage about for any great distance above 
ground, had the most restricted distribution of any of the mammals. 
Having ascertained conditions as they existed prior to the reclamation 
of Imperial Valley, the changes that have taken place there during 
the past fifteen years may now be noted. One of the first and most 
important changes was the construction of a network of irrigation 
canals. These vary in width from four to one hundred and sixty feet; 
their aggregate length runs into hundreds of miles, there being over 
three hundred miles of main canals to say nothing of the laterals. The 
removal of the native vegetation and the breaking up and cultivation 
of hundreds of thousands of acres soon followed. The levelling of 
the land so that all parts of a field might be flooded during the frequent 
summer irrigation came next. Alfalfa and cotton fields now mark 
the former site of extensive stretches of creosote ' and other desert 
vegetation. Man’s occupancy of the region also resulted in the destruc- 
tion of many of nature’s best rodent destroyers. This reduction, by 
man, in the numbers of snakes, coyotes, wild cats, kit foxes, coons, 
badgers, hawks, and owls has especially assisted certain rodents, such 
as gophers, in their rapid increase, since it removed one of their most 
important natural checks. 
