When the original total allocation of the river was set in the 1920s, it was far above regional consumption. But it was also more than the river could supply in the long term. The river was divided based on an estimated annual flow of roughly 21 billion cubic meters per year. More recent studies have indicated that the 20th century, and especially the 1920s, was a time of above-normal flows. These studies indicate that the long-term average of flow is closer to 18 billion cubic meters, with yearly flows ranging anywhere from roughly 6 billion cubic meters to nearly 25 billion cubic meters. As utilization has increased, the deficit between flow and allocation has become more apparent.
Total allocations of river resources for the Upper and Lower basins and Mexico plus water lost to evaporation adds up to more than 21 billion cubic meters per year. Currently, the Upper Basin does not use the full portion of its allocation, and large reservoirs along the river can help meet the demand of the Lower Basin. Populations in the region are expected to increase; in some states, the population could double by 2030. A study released at the end of 2012 by the U.S. Bureau of Reclamation predicted a possible shortage of 3 billion cubic meters by 2035.
The Colorado River provides water for irrigation of roughly 15 percent of the crops in the United States, including vegetables, fruits, cotton, alfalfa and hay. It also provides municipal water supplies for large cities, such as Phoenix, Tucson, Los Angeles, San Diego and Las Vegas, accounting for more than half of the water supply in many of these areas. Minute 319, signed in November 2012, gives Mexico a small amount of additional water in an attempt to restore the delta region. However, the macroeconomic impact on Mexico is minimal, since agriculture accounts for the majority of the river’s use in Mexico but only about 3 percent of the gross domestic product of the Baja Norte province.
There is an imbalance of power along the international border. The United States controls the headwaters of the Colorado River and also has a greater macroeconomic interest in maintaining the supply of water from the river. This can make individual amendments of the 1944 Treaty somewhat misleading. Because of the erratic nature of the river, the treaty effectively promises more water than the river can provide each year. Cooperation in conservation efforts and in finding alternative water sources on the U.S. side of the border, not treaty amendments, will become increasingly important as regional water use increases over the coming decades.