These images show the Imperial Valley, on the border of California and Mexico. The international border is plain in the images because of the different intensity of vegetation, shown in bright green. These images also show the Salton Sea and the growing cities of El Centro, Calexico, and Mexicali.
This valley, also known as the Salton Sink, the Salton Basin, and the Salton Trough, is actually an extension of the Gulf of California, cut off from the Gulf by the Colorado River’s delta fan. The valley was renamed Imperial by turn-of-the-century land investors. The area south of the U.S.-Mexico border is known as the Mexicali Valley.
At the bottom of the sink lies the Salton Sea, the largest lake in California. It lacks an outlet to the ocean and lies 70 m below sea level. About 85% of the sea’s inflows come from agricultural runoff, and its waters are 37% saltier than the Pacific Ocean.
For thousands of years, the Colorado River flowed above and to the east of the valley, on its way to the nearby Gulf of California. The river was higher than the valley, but it was hemmed in by its own natural levees land barriers on either bank built up over years from silt left behind by floods. With each flood, these levees grew a bit higher and harder to breach. But once in a while, the Colorado would break out and pour down into the Salton Basin, partly filling it. Then the levee break would fill with silt, the river would revert to its normal channel, and the basin would dry up again. Dams and channels on the river now prevent these cycles.
European American settlers saw that the Imperial Valley had good soils for agriculture, except for being extremely dry. In 1901, the Colorado Development Company began diverting Colorado River water into the valley for irrigation, similar to what the Colorado had done naturally thousands of times.
In 1905, the company lost control of the river during a flood, and the Colorado broke through the half-finished headgate of an irrigation ditch. The river kept widening the ditch, until almost the entire river was flowing into the sink rather than toward the Gulf of California. It took engineers and work crews until 1907 to return the river to its proper course, by which time a considerable lake had formed.
The All-American Canal (visible as the dark line in these close-ups) is now the Imperial Valley's main source of water. It carries 26,155 cubic feet of water per second from the Colorado River to allow irrigation of more than 2,000 km2 of agricultural fields.
Also noticeable in these two images is Interstate 8, which runs along the canal and occasionally crosses it. In the 2011 image, a subtle change is a road that runs along the U.S.-Mexico border—the road is part of border fence construction efforts. One clear change between these two images is an area of new irrigated fields in Mexico (green circles and rectangles in the lower left).
Several cities are visible within the area of irrigated agriculture in the valley, which is surrounded by the natural desert. The close-up images show the growth of El Centro, California, and the urban area of Mexicali/Calexico on the border. These cities' populations have grown rapidly over the last few decades:
- El Centro grew from a population of 19,272 in 1970 to 42,598 in 2010, a 121% increase.
- Calexico grew from a population of 10,625 in 1970 to 38,572 in 2010, a 263% increase.
- Mexicali, Mexico, grew from 458,877 people in 1990 to 689,775 in 2010, a 50% increase.
The Salton Sea's resources include fish, migratory birds, and recreation, and several endangered species rely on the sea for habitat. These resources are threatened by how the sea may change in the future. Because the sea has no outlet (other than evaporation), dissolved salts are left behind and salinity gradually rises each year. If water levels drop, air quality could be affected; the exposed lakebed could increase the intensity of dust storms. This area already has the highest childhood asthma rate in California (U.S. Department of the Interior, 2007).
Furthermore, recent water transfer agreements will reduce agricultural inflows into the sea by an estimated 30% by 2018, which will cause the Salton Sea to recede. To address these problems, evaporation basins are under consideration to extract some of the salt. Wetland impoundments are proposed to maintain wildlife habitats. Pumping saltwater to the Gulf of California might also be attempted.