The "Law of the River" is a complex collection of interstate compacts, federal laws, court decisions, and other documents that governs the allocation of Colorado River water and operation of the river's dams. But this esoteric body of law is now making the news due to the Southwest's shrinking water supply.

Due to a multi-year drought coupled with rapid development in the Southwest, the water level in Nevada's Lake Mead, an almost 10 trillion-gallon reservoir created by Hoover Dam, has dropped to historic lows: hitting 1083.18 ft. below sea level on Sunday, Oct. 17, then falling further to 1083.09 ft. by Monday morning. Lake Mead water levels are the trigger for incremental rationing under a 2007 agreement between the Upper Basin states of Colorado, Utah, and Wyoming, and the thirstier Lower Basin states of Arizona, California, and Nevada. If Lake Mead drops another eight feet to hit 1075 ft. above sea level, then never-before-imposed water restrictions will kick in for Arizona and Nevada, affecting agricultural users first. Southwest water managers, including the federal Bureau of Reclamation (BOR), may be able to avoid or delay this result, however, by transferring water from Utah’s Lake Powell upriver. In its plans for next year, the BOR calls for a 40 percent increase in water deliveries from Lake Powell to Lake Mead.

Read more here. Time has an informative article here.