AZ vs. CA:04 California Exhibits, Vol. IV; Nos. 151-192

page 27

THE ALL-AMERICAN CANAL. 27 river is not alone the sole dependency for irrigation water for large areas in both the United States and Mexico, but its floods have become a menance to property interests in both countries by reason of the fact that these floods threaten extensive areas with permanent destruction. In the case of most regions which are subject to overflow from rivers the occasional flood is held in check by levees, which if they break cause more or less inundation with inconvenience and loss of property and perhaps of lives, but .the water recedes in the course of a week or a month as the case may be. On the lower Colorado, however, the situation is entirely different. The river from the head of its delta to the Volcano Lake region is from 300 to 400 feet higher than the lowest portions of the Imperial Valley. The river in fact flows on the southeasterly rim of this valley-if a very broad flat delta ridge may be regarded as delimiting the valley in this direction-at an elevation above sea level from low to high water in round numbers of 100 to 120 feet near Pilot Knob at the -head of the delta and at an elevation of 30 to over 40 feet in the Volcano Lake region. These elevations of the river are to be compared with sea level as the general elevation of the ground near Calexico on the boundary between the United States and Mexico, 50 feet below sea level at El Centro, 110 feet below sea level at Brawley, and over 280 feet below sea level at the deepest points of the bed of Salton Sea. On the present route of the lower Colorado, practically along the top of the delta ridge to Volcano Lake, any overtopping of the river's west and north or right bank, if not restrained by artificial barriers, would send water inland down a slope into the Salton Basin. This basin to the southernmost portion of which the name Imperial Valley has been given, has a surface extent of about 2,000 square miles. Being a depressed basin, there can be no return flow therefrom to the river until the basin is full. A large part of the north central portion of the river delta-that is to say, land near and to the northward of the present Abe j as section of the river-has for some years past escaped submersion. The result has been that vegetation has died out to a large extent, thus leaving this region more exposed to erosion at flood if barriers are broken than was the case under natural conditions. Any flood waters of the river which go over bank toward the north must, therefore, at all costs be returned to the river and not allowed to flow down the northern slope of the delta cone. This is being done by the maintenance of levees judiciously placed on high ground, frequently some miles distant from the present river channel. From the head of the river delta at Pilot Knob to its outfall into the Gulf of California the right or west bank of Colorado River is in Mexico. Consequently all of the levee work for the protection of the