<P> In recent years, the compact has become the focus of even sharper criticism, in the wake of a protracted decrease in precipitation in the region . Specifically, the amount of water allocated was based on an expectation that the river's average flow was 16,400,000 acre feet (20.2 km) per year (641 m3 / s). Subsequent tree ring studies, however, have concluded that the long - term average water flow of the Colorado is significantly less . Estimates have included 13,200,000 acre feet (16.3 km) per year (516 m3 / s), 13,500,000 acre feet (16.7 km) per year (528 m / s), and 14,300,000 acre feet (17.6 km) per year (559 m / s). Many analysts have concluded that when the compact was negotiated, the period used as the basis for "average" flow of the river (1905--1922) included periods of abnormally high precipitation, and that the recent drought in the region is in fact a return to historically typical patterns . The decrease in precipitation has led to widespread dropping of reservoir levels in the region, in particular at Lake Powell, created by the Glen Canyon Dam in 1963, where the exposure of long - inundated canyons has prompted calls for the reservoir to be permanently drained and decommissioned . </P> <P> In December 2007, a set of interim guidelines on how to allocate Colorado River water in the event of shortages was signed by the Secretary of the Interior . The guidelines are described as interim because they extend through 2026, and, "acknowledging the potential for impacts due to climate change and increased hydrologic variability," interim guidelines provide "the opportunity to gain valuable operating experience for the management of Lake Powell and Lake Mead, particularly for low reservoir conditions...whether during the interim period or thereafter ." The agreement specifies three levels of shortage conditions, depending on the level of Lake Mead: </P> <Ul> <Li> Light shortage . When the surface elevation at Lake Mead is below 1,075 feet (328 m) relative to mean sea level but above 1,050 feet (320 m), the Lower Basin states will receive 7,167,000 acre feet (8.840 km) per year: 4,400,000 acre feet (5.4 km) to California, 2,480,000 acre feet (3.06 km) to Arizona, and 287,000 acre feet (0.354 km) to Nevada . </Li> <Li> Heavy shortage . When the surface elevation of Lake Mead is below 1,050 feet (320 m) but above 1,025 feet (312 m), 7,083,000 acre feet (8.737 km) per year will be delivered to the Lower Basin states: 4,400,000 acre feet (5.4 km) to California, 2,400,000 acre feet (3.0 km) to Arizona, and 283,000 acre feet (0.349 km) to Nevada . </Li> <Li> Extreme shortage . The most severe shortage considered in the interim guidelines is when the level of Lake Mead drops below 1,025 feet (312 m), in which event 7,000,000 acre feet (8.6 km) per year will be delivered to the Lower Basin states: 4,000,000 acre feet (4.9 km) to California, 2,320,000 acre feet (2.86 km) to Arizona, and 280,000 acre feet (0.35 km) to Nevada . </Li> </Ul> <Li> Light shortage . When the surface elevation at Lake Mead is below 1,075 feet (328 m) relative to mean sea level but above 1,050 feet (320 m), the Lower Basin states will receive 7,167,000 acre feet (8.840 km) per year: 4,400,000 acre feet (5.4 km) to California, 2,480,000 acre feet (3.06 km) to Arizona, and 287,000 acre feet (0.354 km) to Nevada . </Li>

Who decides where the water in the river system goes