The Colorado River serves as one of the most vital water sources in the United States, providing water to nearly 40 million people in the West.
Numerous resources are dependent on the river, which has been under drought conditions since 2000. According to the U.S. Department of the Bureau of Reclamation, several resources that depend on a healthy river system include hydroelectric power generation, fish and wildlife, as well as water for municipal, industrial and agricultural use.
As drought conditions grip the western part of the country, states are reinforcing preparations they've made years ago in the possibility of a water shortage.
Officials are taking steps to monitor current water levels and analyze future scenarios for potential shortages. As part of its 24-month study, the Bureau of Reclamation issues a study every month based on computer modeling of lake elevations, trying to give water managers solid data to plan for future water reserves.
The river is divided into two districts-the upper basin and the lower basin-with Arizona, Nevada and California making up the lower basin, while Colorado, New Mexico, Wyoming and Utah encompass the upper basin. Mexico is also allocated 1.5 million acre-feet from the river.
As part of the Colorado River Compact instituted in 1922, the two basins were each allotted 7.5 million acre-feet per year. One acre-foot is equivalent to 325,851 gallons of water according to the Southern Nevada Water Authority (SNWA).
Lake Mead, the largest reservoir in the U.S. and supplier of nearly 90 percent of southern Nevada's water supply, is dwindling to near-record low levels. It also holds water for California, Arizona and Mexico. As of the end of June, the lake sat at an elevation of 1,082 feet above sea level.
"The levels of Lake Mead are solely dependent on runoff from the Colorado River," said SNWA Spokesman Bronson Mack.
The first intake sits at elevation 1,050 feet above sea level and the second intake resides at elevation 1,000 feet. Currently the SNWA is building a third intake, which is expected to be completed in 2015, that will sit at an elevation of 860 feet to keep supplying the region with Colorado River water.
Another reservoir along the river is Lake Powell, which straddles the Utah-Arizona border.
Lake Powell, which acts as a savings account for the upper basin similarly to Lake Mead in the lower basin, is operated in tandem with Lake Mead, with the average release of water from Powell to Mead typically around 8.23 million acre-feet per year.
"That allows Lake Powell to, in good, wet years, release more water to Mead and in dry years, it also allows them to release less water to Mead," Mack said.
In the past 14 years, there have been only three years where the river flows have either been normal or slightly above normal. The last above-average year was 2011, when inflows in Lake Powell were 130 percent of average, Mack said.
Currently, the state of Arizona is not under any water shortages and still receives its full 2.85 million acre-feet allotment. However, by 2017, officials said there is a greater-than-50-percent probability of a shortage, according to the aforementioned computer models.
The shortage time-frame was delayed by one year after predictions for this coming water year (starting Oct. 1) indicated that the flow from Lake Powell to Lake Mead will be 9 million acre-feet, due to a strong snowpack in the watershed, said Tom Buschatzke, the assistant director of the Water Planning Division for the Arizona Department of Water Resources.
In Arizona, there are priorities in place for Colorado River users. The senior users are those in the Yuma region, an essential agricultural area in the state. The more junior users get their water provided to them by the Central Arizona Project (CAP), a 336-mile-long system of aqueducts, tunnels pumping plants and pipelines.
The CAP receives about 23 percent of the states Colorado River allotment and diverts its water from Lake Havasu, which unlike Lake Mead, doesn't see its levels fluctuate.
Buschatzke said there are water reserves throughout the state of Arizona in case a shortage takes effect.
Established in 1996, the Arizona Water Banking Authority is a state agency that stores about 3.2 million acre-feet of excess Colorado River water in aquifers underground to be used in times of shortage. There are also individual providers that can store water and receive long-term credits. About 5 million acre-feet of water is stored by those providers.
Storing water has allowed water officials plenty of time to prepare for a shortage situation.
Pamela Pickard, president of the CAP board, recently wrote that "CAP is likely to experience shortage by 2017, but that shortage will primarily affect central Arizona agriculture which is already preparing for planned reductions in their CAP supplies."
Pickard also wrote that the CAP shortages are not expected to impact Arizona cities for 10 to 15 years, due to the fact that cities have the highest priority within the CAP system.
In the short term, over the next five to 15 years or so, upper basin states are in a better position for water supply than the lower basin due to the Law of the River, Buschatzke said.
The greater concern for those states would be the loss of power production from Lake Powell and the Glen Canyon Dam, which forms the reservoir, since they rely more heavily on hydroelectricity than the lower basin states, Buschatzke said.
According to a recent article from Circle of Blue, the upper basin states want to keep the surface level of Lake Powell above 3,490 feet because below that point hydropower generation from Glen Canyon Dam would stop.
The revenues that come from the hydropower have surcharges that can create funding for programs such as the Endangered Species Act that help the upper basin continue to use its water, Buschatzke said.
Mack added that the effects of the drought on the river have been more visible in the lower basin, in particular because of the dwindling of levels at Lake Mead.
Because the river is such a vital resource, many regulatory guidelines, federal laws, court decisions, decrees and compacts are in place. Collectively they are known as the "Law of the River."
If a shortage were to take effect, some states could be in better shape than others.
For instance, the Colorado River Basin Project Act of 1968, which authorized the development of the CAP, among other projects in the upper basin, states that water supply for the CAP would be subordinate to California's apportionment.
California's reliance on the river (4.4 million acre-feet) is primarily for irrigation of several huge agricultural areas such as the Coachella and Imperial Valleys, that are extremely important to the food supply to the entire United States, especially during the winter months.
In the event of a shortage, Nevada and Arizona would have to curtail their usage while California would not.
Mack said this is because the Colorado River was initially divided up based on how much agriculture or potential agriculture each state supplied.
While short-term priorities are taking effect, the long-term picture is less clear.
In 2012, the Bureau of Reclamation, along with agencies representing the seven basin states, completed the Colorado River Basin Study.
According to the study, "the amount of water available and changes in the demand for water throughout the Basin over the next 50 years are highly uncertain and dependent upon a number of factors. The potential impacts of future climate variability and climate change further contribute to these uncertainties."